Strong acid–strong base reactions

WEBVTT
Kind: captions
Language: en

00:00:00.210 --> 00:00:02.020
- [Instructor] Hydrochloric
acid is an example

00:00:02.020 --> 00:00:03.900
of a strong acid,

00:00:03.900 --> 00:00:08.170
and sodium hydroxide is an
example of a strong base.

00:00:08.170 --> 00:00:10.600
When an aqueous solution
of hydrochloric acid reacts

00:00:10.600 --> 00:00:12.940
with an aqueous solution
of sodium hydroxide,

00:00:12.940 --> 00:00:14.650
the products are an aqueous solution

00:00:14.650 --> 00:00:16.970
of sodium chloride and water.

00:00:16.970 --> 00:00:18.920
Since the reaction goes to completion,

00:00:18.920 --> 00:00:20.530
instead of an equilibrium arrow,

00:00:20.530 --> 00:00:22.780
there's just an arrow going to the right.

00:00:22.780 --> 00:00:26.360
And this is called an acid-base
neutralization reaction.

00:00:26.360 --> 00:00:28.590
Let's think about the
ions that are involved

00:00:28.590 --> 00:00:31.360
in this acid-base neutralization reaction.

00:00:31.360 --> 00:00:34.060
Since hydrochloric acid is a strong acid,

00:00:34.060 --> 00:00:37.400
HCl ionizes 100% in solution,

00:00:37.400 --> 00:00:38.950
and therefore, in solution,

00:00:38.950 --> 00:00:43.950
HCl consists of H plus
ions and Cl minus anions.

00:00:44.460 --> 00:00:46.640
Sodium hydroxide is a strong base,

00:00:46.640 --> 00:00:50.350
and strong bases dissociate
100% in solution.

00:00:50.350 --> 00:00:53.290
Therefore, an aqueous solution
of sodium hydroxide consists

00:00:53.290 --> 00:00:58.290
of sodium ions, Na plus, and
hydroxide anions, OH minus.

00:00:59.510 --> 00:01:00.343
For our products,

00:01:00.343 --> 00:01:02.160
sodium chloride is a soluble salt

00:01:02.160 --> 00:01:04.460
and therefore forms an aqueous solution.

00:01:04.460 --> 00:01:07.770
So in solution, we would
have sodium cations, Na plus,

00:01:07.770 --> 00:01:11.090
and chloride anions, Cl minus.

00:01:11.090 --> 00:01:14.170
Since water ionizes only
to a very small extent,

00:01:14.170 --> 00:01:15.960
we don't write it as the ions,

00:01:15.960 --> 00:01:17.197
we simply write H2O.

00:01:19.160 --> 00:01:20.170
To save some time,

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I went ahead and added
in aqueous subscripts,

00:01:22.670 --> 00:01:24.370
and for water, it'd be liquid,

00:01:24.370 --> 00:01:27.100
and some plus signs
and the reaction arrow.

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This is called the overall ionic equation

00:01:30.660 --> 00:01:33.850
for our strong acid-strong base reaction.

00:01:33.850 --> 00:01:36.580
Instead of calling it an
overall ionic equation,

00:01:36.580 --> 00:01:40.560
we could also call it a
complete ionic equation.

00:01:40.560 --> 00:01:43.160
And we can use the overall ionic equation

00:01:43.160 --> 00:01:45.510
to determine the net ionic equation

00:01:45.510 --> 00:01:48.230
for this strong acid-strong base reaction.

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To figure out the net ionic equation,

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we first need to determine
the spectator ions.

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Remember, spectator ions don't participate

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in the chemical reaction.

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Notice how we have a sodium
cation on the reactant side

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and a sodium cation on the product side,

00:02:02.880 --> 00:02:05.560
so we can cancel out the sodium cation.

00:02:05.560 --> 00:02:08.353
We also have a chloride
anion on the reactant side

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and a chloride anion on the product side.

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So we can cross that ion out as well.

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So the sodium cation
and the chloride anion

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are the spectator ions.

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And once we take out our spectator ions,

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we're left with the net ionic equation.

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So for the net ionic equation,
we would have H plus,

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plus OH minus yields H2O.

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So this is one way of writing
our net ionic equation.

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However, remember that H plus and H3O plus

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are used interchangeably in chemistry.

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So instead of writing H plus
in our net ionic equation,

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we could have also written H3O plus,

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and when H3O plus, or the hydronium ion,

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reacts with the hydroxide anion,

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we still form water.

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But notice, for this version
of the net ionic equation,

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we need to put a two as a
coefficient to balance everything.

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Next, let's think about a situation

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where we have equal
moles of our strong acid

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and strong base.

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For example,

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let's say we have one mole of HCl

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and one mole of NaOH.

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To help us think about what's
happening in this situation,

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we're gonna use an ICF table,

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where I stands for the
initial number of moles,

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C stands for the change in moles,

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and F stands for the
final amount of moles.

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The reason why we're using an ICF table

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instead of an ICE table
is in an ICE table,

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the E stands for equilibrium,

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and here we assume the
reaction goes to completion.

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Therefore, instead of E,

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we're writing F for final amount of moles.

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Let's fill out the rest of our ICF table.

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We have the initial amount
of moles of HCL and NaOH,

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and if we assume the
reaction hasn't happened yet,

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the initial amount of moles
of NaCl would be zero.

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Looking at the balanced equation,

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one mole of HCl reacts with
one mole of sodium hydroxide.

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And since the reaction goes to completion,

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we can go ahead and write,
under the change part here,

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we're gonna react one mole of HCl,

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so we're gonna lose that one mole of HCl.

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And since it's a one-to-one ratio,

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we're gonna lose one mole of NaOH.

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And since there's a one as a
coefficient in front of NaCl,

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under the change part on our ICF table,

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we would write plus one mole for NaCl.

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Since we started with one mole
of HCl, and we lost one mole,

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the final amount of moles
for HCl would be zero.

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Same thing for sodium hydroxide,

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the final amount of moles would be zero,

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because it's one minus one.

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For NaCl, we started off
with zero and we gained one.

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So we're ending up with
a final amount of moles

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of one mole of sodium chloride.

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So when the reaction goes to completion,

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the strong acid and the strong base

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have completely neutralized each other.

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And essentially, we
have an aqueous solution

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of sodium chloride.

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At 25 degrees Celsius,
the pH of water is seven,

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which is neutral,

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and the sodium cations
and the chloride anions

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do not react with water,

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so they don't affect the pH.

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Therefore, for the situation
where you have equal amount

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of moles of a strong
acid and a strong base,

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the pH of the resulting
solution will be seven.

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Let's do another strong
acid-strong base calculation.

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However, this time,

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the moles of strong acid and
strong base are not equal.

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So let's say we react 300 milliliters

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of a 1.0 molar solution of HCl

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with 100 milliliters
of a 1.0 molar solution

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of sodium hydroxide.

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And our goal is to calculate the pH

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of the resulting solution.

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And I should point out
these are aqueous solutions

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of HCl and NaOH.

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The first step is to
calculate moles of strong acid

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and moles of strong base.

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And the equation for molarity

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is equal to moles over liters.

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So for our strong acid,
HCl, the molarity is 1.0,

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and the volume is 300 milliliters,

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which is equal to .300 liters.

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So solving for x, we find that x

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is equal to .30 moles of HCl.

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For our strong base, sodium hydroxide,

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the concentration is one molar,

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and the volume is 100 milliliters,

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which is equal to .100 liters.

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So solving for x,

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we find that x is equal
to .10 moles of NaOH.

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Going back to HCl,

00:06:21.080 --> 00:06:24.830
since HCl is strong
acid that ionizes 100%,

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if we have .30 moles of HCl,

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we have .30 moles of H plus ions,

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or we could say .30 moles
of hydronium ions, H3O plus.

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And for sodium hydroxide,

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since sodium hydroxide is a strong base

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that dissociates 100%,

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if we have .10 moles of sodium hydroxide,

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we have .10 moles of sodium ions

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and also .10 moles of
hydroxide ions, OH minus.

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And if we think about
our net ionic equation,

00:06:58.570 --> 00:07:02.410
the .30 moles of hydronium
ions will react with

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the .10 moles of hydroxide ions.

00:07:05.890 --> 00:07:08.710
So here's one way to write
our net ionic equation.

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The hydronium ion plus the
hydroxide ion forms 2H2O.

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And to help us find the pH,

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we're gonna use another ICF table.

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The initial moles of hydronium ions,

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we calculated to be .30,

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and the initial moles of hydroxide ions,

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we calculated to be .10.

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Because we don't have
equal amounts of moles

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and the mole ratio is one to one.

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In this case,

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we're gonna have a limiting
reactant and an excess reactant.

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Since we only have .10
moles of hydroxide ions,

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all of the hydroxide ions will react.

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And since it's a one-to-one mole ratio,

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.10 moles of hydroxide ions
will react with .10 moles

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of hydronium ions,

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so we can write minus .10
under hydronium as well.

00:07:56.320 --> 00:07:58.950
Therefore, when the
reaction goes to completion,

00:07:58.950 --> 00:08:00.210
for the hydroxide ions,

00:08:00.210 --> 00:08:01.450
we'll have used all of them up.

00:08:01.450 --> 00:08:04.660
We started with .10
and we're using up .10,

00:08:04.660 --> 00:08:06.960
therefore, we'll have zero moles.

00:08:06.960 --> 00:08:08.220
For the hydronium ion,

00:08:08.220 --> 00:08:13.220
we started with .30 moles
minus .10 gives us .20 moles.

00:08:14.670 --> 00:08:16.943
So the hydroxide ions were
our limiting reactant,

00:08:16.943 --> 00:08:21.180
and the hydronium ions
where our excess reactant.

00:08:21.180 --> 00:08:24.760
So the strong base neutralized
some of the strong acid

00:08:24.760 --> 00:08:25.800
that was present,

00:08:25.800 --> 00:08:29.170
and we can calculate the pH
of the resulting solution

00:08:29.170 --> 00:08:33.070
by the number of moles
of excess hydronium ions.

00:08:33.070 --> 00:08:35.080
Next, we calculate the concentration

00:08:35.080 --> 00:08:37.450
of hydronium ions in solution.

00:08:37.450 --> 00:08:40.530
Since the number of moles
of hydronium ions is .20,

00:08:40.530 --> 00:08:42.450
we plug that into our
equation for molarity,

00:08:42.450 --> 00:08:45.110
so molarity is equal to moles over liters,

00:08:45.110 --> 00:08:47.450
and the total volume of our solution

00:08:47.450 --> 00:08:50.110
will be 300 milliliters
plus 100 milliliters,

00:08:50.110 --> 00:08:54.700
which is 400 milliliters, or .400 liters.

00:08:54.700 --> 00:08:55.900
So let me go and write this in here.

00:08:55.900 --> 00:09:00.480
So .20 moles divided by .400 liters

00:09:00.480 --> 00:09:05.470
is equal to a concentration of .50 molar.

00:09:05.470 --> 00:09:08.040
And because the pH is
equal to the negative log

00:09:08.040 --> 00:09:10.220
of the concentration of hydronium ions,

00:09:10.220 --> 00:09:13.920
if we plug in our concentration
of hydronium ions of .50,

00:09:13.920 --> 00:09:16.640
the pH is equal to the
negative log of .50,

00:09:16.640 --> 00:09:18.263
which is equal to .30.

00:09:19.900 --> 00:09:22.180
Having a low pH for our resulting solution

00:09:22.180 --> 00:09:25.000
at 25 degrees Celsius
makes a lot of sense,

00:09:25.000 --> 00:09:28.033
because we ended up
with an excess of acid.


Sal's back to school 2021 message

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- Hi, everyone, Sal Khan
here from Khan Academy,

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and we're entering into
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compared to all others.

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We hope that we're finally going to get

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to some level of normalcy

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as we see the light at
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with the pandemic, at
least in some places.

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I know a lot of parts of
the world are still going

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through some of the worst
impacts of the pandemic.

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But as we hopefully get to that
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I just wanted to say how excited we are

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to partner with y'all.

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The pandemic has been tough.

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It's been tough on many
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but especially education.

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As I always say,

00:00:38.620 --> 00:00:40.770
and people are surprised
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if I had to pick between an
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I would pick the amazing
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Now, I don't think there
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I think the best is the
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But you can imagine, people
haven't had the benefit

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But the real work is
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00:01:13.930 --> 00:01:15.810
as we go into back to school.

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A lot of folks have been talking
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or learning loss.

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And every summer, there's
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00:01:22.230 --> 00:01:25.010
Kids forget things, and
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00:01:25.010 --> 00:01:27.410
But we can imagine with a year,

00:01:27.410 --> 00:01:30.540
year and a half of pandemic schooling,

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some of the kids who
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actually fell off the radar.

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And many, many other kids
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00:01:37.110 --> 00:01:39.070
and even the kids and the
learners, I should say,

00:01:39.070 --> 00:01:41.200
of all ages who were able to learn,

00:01:41.200 --> 00:01:42.980
there's other things
that fell by the wayside,

00:01:42.980 --> 00:01:44.520
the socialization, the connection,

00:01:44.520 --> 00:01:46.520
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00:01:46.520 --> 00:01:51.520
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00:01:53.470 --> 00:01:56.760
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00:01:56.760 --> 00:01:59.530
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00:01:59.530 --> 00:02:02.170
it's not gonna be easy,
but if we work together,

00:02:02.170 --> 00:02:04.720
figure out ways to finish
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00:02:04.720 --> 00:02:06.680
there might be a post-pandemic world

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where we can accelerate learning
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00:02:10.830 --> 00:02:12.520
So I look forward to
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00:02:12.520 --> 00:02:13.870
and welcome back to school.


LearnStorm 2021 - Khan Academy LearnStorm

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00:00:00.090 --> 00:00:01.240
- Hello teachers.

00:00:01.240 --> 00:00:04.800
I'm Sal Khan, founder of the
not-for-profit Khan Academy.

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And I'm here to announce a nationwide

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back to school learning
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LearnStorm is an exciting way
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around learning activities.

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It's designed to motivate your students

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and to help build their growth mindset.

00:00:22.390 --> 00:00:24.500
- This year was amazing with LearnStorm.

00:00:24.500 --> 00:00:27.140
I had a group of students
that couldn't add, subtract,

00:00:27.140 --> 00:00:29.070
multiply, or divide fractions.

00:00:29.070 --> 00:00:31.370
They avoided it at all costs.

00:00:31.370 --> 00:00:33.170
But since it was an assignment,

00:00:33.170 --> 00:00:36.650
they would try it and they
would eventually do much better.

00:00:36.650 --> 00:00:40.100
And what was amazing was that
the lower achieving students

00:00:40.100 --> 00:00:44.300
were able to see that they
were part of something huge.

00:00:44.300 --> 00:00:46.040
They were able to contribute,

00:00:46.040 --> 00:00:49.390
and that I thought boosted
their self-confidence,

00:00:49.390 --> 00:00:50.550
their self-esteem,

00:00:50.550 --> 00:00:52.590
and I didn't realize that would happen

00:00:52.590 --> 00:00:53.840
after doing LearnStorm.

00:00:53.840 --> 00:00:56.423
(upbeat music)

00:00:59.014 --> 00:01:03.060
- LearnStorm is a good
way to differentiate,

00:01:03.060 --> 00:01:07.500
and as well as motivate, and
with the growth mindset pieces,

00:01:07.500 --> 00:01:09.420
you can also educate them

00:01:09.420 --> 00:01:11.910
about why it's important to learn,

00:01:11.910 --> 00:01:12.970
how they learn,

00:01:12.970 --> 00:01:15.140
how their body is set up to learn,

00:01:15.140 --> 00:01:17.450
what strategies help them learn.

00:01:17.450 --> 00:01:18.450
It's amazing.

00:01:18.450 --> 00:01:19.995
It's amazing, and it's free.

00:01:19.995 --> 00:01:21.251
(laughing)

00:01:21.251 --> 00:01:22.720
It's free!

00:01:22.720 --> 00:01:25.180
- We think this is really
going to motivate your students

00:01:25.180 --> 00:01:26.940
to understand, to appreciate,

00:01:26.940 --> 00:01:29.760
that they truly can learn anything.

00:01:29.760 --> 00:01:31.870
The activities have been designed

00:01:31.870 --> 00:01:35.200
to have measurable gains
in learning attitudes.

00:01:35.200 --> 00:01:37.840
- One thing I learned from
the Growth Mindset Activities

00:01:37.840 --> 00:01:40.340
is that your intelligence can grow

00:01:40.340 --> 00:01:41.700
just like your muscles can grow.

00:01:41.700 --> 00:01:43.990
And that's something that
really stuck with me,

00:01:43.990 --> 00:01:46.490
'cause I didn't know that before.

00:01:46.490 --> 00:01:49.170
- I did over 300 lessons in one week.

00:01:49.170 --> 00:01:52.230
I felt proud of myself
getting all these good grades.

00:01:52.230 --> 00:01:54.720
- It helps the students tremendously.

00:01:54.720 --> 00:01:56.240
Their confidence is higher.

00:01:56.240 --> 00:01:57.190
They're able to do it.

00:01:57.190 --> 00:01:59.970
And it warms my heart that
they think that way now.

00:01:59.970 --> 00:02:02.100
And they know that they can keep going,

00:02:02.100 --> 00:02:05.850
and keep growing, and nothing
will stand in their way.

00:02:05.850 --> 00:02:08.530
- So together as we
jumpstart this school year,

00:02:08.530 --> 00:02:12.440
I look forward for all of us
living the LearnStorm motto,

00:02:12.440 --> 00:02:15.609
to keep going and to keep growing.

00:02:15.609 --> 00:02:18.276
(kids chanting)


Adaptation and environmental change

WEBVTT
Kind: captions
Language: en

00:00:00.170 --> 00:00:01.070
- [Dr. Sammy] Hi, everybody.

00:00:01.070 --> 00:00:03.680
Dr. Sammy here, your friendly
neighborhood entomologist

00:00:03.680 --> 00:00:06.390
here to talk to you about how adaptation,

00:00:06.390 --> 00:00:08.570
which is dependent on the environment,

00:00:08.570 --> 00:00:11.880
responds in contexts of
environmental change.

00:00:11.880 --> 00:00:15.040
Natural selection promotes
adaptation in populations.

00:00:15.040 --> 00:00:17.600
It encourages populations
to develop traits

00:00:17.600 --> 00:00:21.560
that better allow individuals
to survive and reproduce.

00:00:21.560 --> 00:00:23.740
Those adaptations are thus linked

00:00:23.740 --> 00:00:26.610
to the environment in
which they were forged.

00:00:26.610 --> 00:00:28.490
You can look at a lot of organisms

00:00:28.490 --> 00:00:31.950
and based on their physical
traits and/or their behavior,

00:00:31.950 --> 00:00:33.910
you can tell what sort of environment

00:00:33.910 --> 00:00:36.950
their population likely evolved in.

00:00:36.950 --> 00:00:40.000
So adaptations are products of,

00:00:40.000 --> 00:00:43.820
and are inextricably
connected to, the environment.

00:00:43.820 --> 00:00:45.860
But what happens when the environment

00:00:45.860 --> 00:00:49.040
which forges a specific set of adaptations

00:00:49.040 --> 00:00:51.660
no longer exists in that form?

00:00:51.660 --> 00:00:53.510
How do populations respond?

00:00:53.510 --> 00:00:55.630
Well, this happens to actually be

00:00:55.630 --> 00:00:57.640
a pretty common occurrence on Earth.

00:00:57.640 --> 00:01:00.860
I wouldn't be surprised if
the saying "things change"

00:01:00.860 --> 00:01:03.450
was first coined by a paleontologist.

00:01:03.450 --> 00:01:06.190
Studying the history of life on Earth

00:01:06.190 --> 00:01:07.940
means you'd be well-acquainted

00:01:07.940 --> 00:01:10.690
with the dogged persistence of change.

00:01:10.690 --> 00:01:12.660
Well, the good news is the forces

00:01:12.660 --> 00:01:14.430
that drive natural selection

00:01:14.430 --> 00:01:17.560
don't just disappear when
an environment changes.

00:01:17.560 --> 00:01:20.230
Because of the persistence
of heritable diversity

00:01:20.230 --> 00:01:22.800
and differential survivorship
and reproduction,

00:01:22.800 --> 00:01:26.340
natural selection is still
there to promote new adaptations

00:01:26.340 --> 00:01:29.360
suited to the new
environmental conditions.

00:01:29.360 --> 00:01:31.790
This means that when
the environment changes,

00:01:31.790 --> 00:01:34.010
the distribution of
traits in the population

00:01:34.010 --> 00:01:36.550
will often change as well.

00:01:36.550 --> 00:01:38.890
What those changes will ultimately be

00:01:38.890 --> 00:01:40.340
is based on the adaptations

00:01:40.340 --> 00:01:42.320
promoted by the previous environment

00:01:42.320 --> 00:01:45.070
and the pressures
presented by the new one.

00:01:45.070 --> 00:01:46.450
So let's look at some examples

00:01:46.450 --> 00:01:49.010
so I can make sure you're
picking up what I'm putting down.

00:01:49.010 --> 00:01:52.290
Okay, so in evolutionary biology,

00:01:52.290 --> 00:01:56.070
we like to discuss three
primary shift patterns

00:01:56.070 --> 00:02:00.090
that can be observed as a
result of environmental change:

00:02:00.090 --> 00:02:02.173
directional selection,

00:02:03.210 --> 00:02:05.543
disruptive selection,

00:02:06.960 --> 00:02:10.433
and stabilizing selection.

00:02:12.020 --> 00:02:13.400
For each of these,

00:02:13.400 --> 00:02:16.330
I'll show you a graph and tell you a story

00:02:16.330 --> 00:02:20.320
of the stubbornness
and resilience of life,

00:02:20.320 --> 00:02:23.160
because every graph has a story.

00:02:23.160 --> 00:02:24.640
And the first of these stories

00:02:24.640 --> 00:02:26.750
is that of directional selection.

00:02:26.750 --> 00:02:30.790
The snow vole is a mouse-like
mammal that even I,

00:02:30.790 --> 00:02:33.780
as an admittedly bug-biased scientist,

00:02:33.780 --> 00:02:36.060
have to admit is pretty cute.

00:02:36.060 --> 00:02:39.010
These rodents are born at high
elevations in the Swiss Alps

00:02:39.010 --> 00:02:40.870
and researchers have observed something

00:02:40.870 --> 00:02:43.470
that may seem obvious on its face.

00:02:43.470 --> 00:02:46.810
Large adult snow voles
have better survivorship

00:02:46.810 --> 00:02:49.050
than smaller adult snow voles.

00:02:49.050 --> 00:02:50.810
They're more difficult for predators

00:02:50.810 --> 00:02:53.540
to capture, overpower, and consume.

00:02:53.540 --> 00:02:55.680
They have greater temperature tolerance

00:02:55.680 --> 00:02:58.500
and typically more fat
stores for a long winter.

00:02:58.500 --> 00:03:01.710
So historically, that
distribution of snow bowls...

00:03:01.710 --> 00:03:02.543
Bowls.

00:03:02.543 --> 00:03:05.270
Snow voles (laughing) has been shifted

00:03:05.270 --> 00:03:08.363
toward the higher end of
the adult weight spectrum.

00:03:09.810 --> 00:03:12.570
However, climate change has led to changes

00:03:12.570 --> 00:03:14.450
in the snow vole's environment.

00:03:14.450 --> 00:03:17.390
Snow has been falling earlier
and earlier in the Alps,

00:03:17.390 --> 00:03:20.560
giving the voles less
time to actually develop.

00:03:20.560 --> 00:03:22.370
If the voles haven't reached maturity

00:03:22.370 --> 00:03:23.970
by the time the winter begins,

00:03:23.970 --> 00:03:26.770
they typically can't
continue their lifecycle.

00:03:26.770 --> 00:03:30.820
And because larger body sizes
require more time to develop,

00:03:30.820 --> 00:03:33.920
there's a strong, selective
pressure against the genes

00:03:33.920 --> 00:03:36.000
underlying larger body sizes

00:03:36.000 --> 00:03:39.410
as the environment shifts
towards earlier winter onset.

00:03:39.410 --> 00:03:41.030
This also means that the genes

00:03:41.030 --> 00:03:44.080
underlying smaller body sizes are favored.

00:03:44.080 --> 00:03:47.210
So the smallest individuals
in the original population

00:03:47.210 --> 00:03:49.000
likely had the most offspring,

00:03:49.000 --> 00:03:51.080
and the variation in those offspring

00:03:51.080 --> 00:03:53.740
likely included some
that were even smaller

00:03:53.740 --> 00:03:56.000
than in the original population.

00:03:56.000 --> 00:03:58.550
This means that as the
smallest individuals

00:03:58.550 --> 00:04:01.460
in each generation continue to be favored

00:04:01.460 --> 00:04:03.100
and have the most offspring,

00:04:03.100 --> 00:04:04.870
the distribution of the population

00:04:04.870 --> 00:04:07.840
will likely continue to march to the left

00:04:07.840 --> 00:04:10.190
until it reaches a point
where the individuals

00:04:10.190 --> 00:04:14.060
are too small to do well given
the environmental conditions.

00:04:14.060 --> 00:04:16.460
This shift can be represented by a graph

00:04:16.460 --> 00:04:19.350
that shows directional selection.

00:04:19.350 --> 00:04:21.580
Now let's go over the graph one more time

00:04:21.580 --> 00:04:23.380
to make sure that everything is clear.

00:04:23.380 --> 00:04:24.950
Here on the X-axis,

00:04:24.950 --> 00:04:28.090
you'll see the spectrum
of average adult body size

00:04:28.090 --> 00:04:31.330
ranging from the smallest
to the largest individuals.

00:04:31.330 --> 00:04:33.490
Traits are quite frequently on a spectrum,

00:04:33.490 --> 00:04:35.360
such that some individuals possess

00:04:35.360 --> 00:04:37.200
a greater or lesser expression

00:04:37.200 --> 00:04:39.450
and are thus differentially impacted

00:04:39.450 --> 00:04:41.330
by environmental change.

00:04:41.330 --> 00:04:43.420
On the Y-axis is the percent

00:04:43.420 --> 00:04:45.200
of individuals in the population,

00:04:45.200 --> 00:04:48.730
which allows you to see
proportionally how individuals fare

00:04:48.730 --> 00:04:51.940
by comparison to those of
greater or lesser size.

00:04:51.940 --> 00:04:54.580
The red arrow represents
selection pressure,

00:04:54.580 --> 00:04:56.440
which in this case is imposed

00:04:56.440 --> 00:04:58.790
by the abiotic factors of weather.

00:04:58.790 --> 00:05:00.430
When looking at these graphs,

00:05:00.430 --> 00:05:02.780
you wanna consider who is least able

00:05:02.780 --> 00:05:04.850
to handle the pressure imposed.

00:05:04.850 --> 00:05:07.970
It's typically the area
of the distribution

00:05:07.970 --> 00:05:09.890
where you'll see the greatest difference

00:05:09.890 --> 00:05:11.570
between the population before

00:05:11.570 --> 00:05:13.900
and after the environmental change.

00:05:13.900 --> 00:05:16.140
The white arrow then shows which direction

00:05:16.140 --> 00:05:19.020
the population as a
whole is then shifting.

00:05:19.020 --> 00:05:20.690
Directional selection events

00:05:20.690 --> 00:05:22.820
are those which favor one extreme

00:05:22.820 --> 00:05:24.810
while excluding another.

00:05:24.810 --> 00:05:27.510
However, change itself can be as diverse

00:05:27.510 --> 00:05:29.140
as the organisms it changes.

00:05:29.140 --> 00:05:31.490
And thus, you can also get circumstances

00:05:31.490 --> 00:05:34.440
where selection favors both extremes.

00:05:34.440 --> 00:05:37.000
This is called disruptive selection.

00:05:37.000 --> 00:05:38.070
For this example,

00:05:38.070 --> 00:05:41.450
we look to the pungent
world of dung beetles.

00:05:41.450 --> 00:05:45.220
Some populations of the
beetle Onthophagus acuminatus

00:05:45.220 --> 00:05:47.810
have a bimodaled size distribution

00:05:47.810 --> 00:05:50.460
where you'll see a large
number of large males

00:05:50.460 --> 00:05:53.170
in addition to a large
number of small males

00:05:53.170 --> 00:05:56.273
with very few representatives
of the sizes in between.

00:05:57.140 --> 00:05:59.890
The large males use their superior size,

00:05:59.890 --> 00:06:03.040
strength, and horns to fight other males

00:06:03.040 --> 00:06:05.240
for the opportunity to mate.

00:06:05.240 --> 00:06:08.850
A male having selected a mate
will spend most of his time

00:06:08.850 --> 00:06:11.440
guarding the entrance to the
borough where she dwells,

00:06:11.440 --> 00:06:15.910
grappling with males of
similar size who challenge him

00:06:15.910 --> 00:06:18.630
for the chance to depose
him and mate with her.

00:06:18.630 --> 00:06:21.640
Medium-sized males don't
do well in this context.

00:06:21.640 --> 00:06:24.460
They can't overcome the larger males.

00:06:24.460 --> 00:06:27.250
But why then are there small males

00:06:27.250 --> 00:06:29.860
and why are they so
abundant in the population?

00:06:29.860 --> 00:06:32.620
Well, they've actually got
a trick up their sleeves.

00:06:32.620 --> 00:06:34.410
Instead of trying to fight,

00:06:34.410 --> 00:06:36.550
they forgo the battle entirely

00:06:36.550 --> 00:06:38.840
and simply use their diminutive size

00:06:38.840 --> 00:06:40.900
to sneak past unnoticed.

00:06:40.900 --> 00:06:43.950
These small males typically lack horns,

00:06:43.950 --> 00:06:46.610
which leaves them similar
in appearance to females,

00:06:46.610 --> 00:06:48.110
which makes male dung beetles

00:06:48.110 --> 00:06:50.460
less inclined to engage them in battle.

00:06:50.460 --> 00:06:52.900
And the little guys further avoid trouble

00:06:52.900 --> 00:06:55.220
by deftly digging a backdoor entrance

00:06:55.220 --> 00:06:58.270
into the female beetle's
subterranean nesting chamber,

00:06:58.270 --> 00:06:59.950
such that they rarely have to encounter

00:06:59.950 --> 00:07:02.340
the large male guarding the front door.

00:07:02.340 --> 00:07:04.720
This allows them plenty of time to mate

00:07:04.720 --> 00:07:06.120
before making their exit,

00:07:06.120 --> 00:07:07.560
ensuring the representation

00:07:07.560 --> 00:07:10.250
of their sneaky genetics in the gene pool.

00:07:10.250 --> 00:07:12.040
Disruptive selection selects

00:07:12.040 --> 00:07:14.020
for both extremes on the spectrum

00:07:14.020 --> 00:07:17.230
to the exclusion of the moderate
trait in the distribution.

00:07:17.230 --> 00:07:19.370
But there's also a form of selection

00:07:19.370 --> 00:07:22.870
that is the opposite of
disruptive selection.

00:07:22.870 --> 00:07:24.853
It's called stabilizing selection.

00:07:25.780 --> 00:07:28.670
Stabilizing selection is
observed under circumstances

00:07:28.670 --> 00:07:31.250
where selection is favoring
the moderate set of traits

00:07:31.250 --> 00:07:34.620
in a distribution to the
exclusion of the extremes.

00:07:34.620 --> 00:07:36.780
For this story, we once again turn

00:07:36.780 --> 00:07:39.130
to the wondrous class Insecta,

00:07:39.130 --> 00:07:41.140
where insects have learned to make a home

00:07:41.140 --> 00:07:43.950
in some of the most
ingenious ways possible.

00:07:43.950 --> 00:07:45.640
In the world of gall flies,

00:07:45.640 --> 00:07:47.920
life begins when a female gall fly

00:07:47.920 --> 00:07:51.000
lays an egg in the stem
of a suitable host plant.

00:07:51.000 --> 00:07:53.110
The developing larva induces the plant

00:07:53.110 --> 00:07:56.130
to form a spherical growth
around it called a gall,

00:07:56.130 --> 00:07:58.830
which protects the
developing fly's squishy body

00:07:58.830 --> 00:08:00.790
and provides it with all the food it needs

00:08:00.790 --> 00:08:01.860
to reach adulthood,

00:08:01.860 --> 00:08:04.130
unless a parasitic wasps shows up

00:08:04.130 --> 00:08:07.710
and attempts to lay eggs
on the developing gall fly.

00:08:07.710 --> 00:08:11.220
If this happens, the
developing wasp kills the fly

00:08:11.220 --> 00:08:13.070
and takes over its living space.

00:08:13.070 --> 00:08:15.840
This is easiest to do with small galls

00:08:15.840 --> 00:08:18.940
because the wasp has to stick
his thin egg-laying tube

00:08:18.940 --> 00:08:21.750
through the gall to
access the developing fly.

00:08:21.750 --> 00:08:24.320
A large gall forms enough of a barrier

00:08:24.320 --> 00:08:27.190
between the gall fly and
the wasp's egg-laying tube

00:08:27.190 --> 00:08:29.260
that it can't quite make it.

00:08:29.260 --> 00:08:31.600
So this selects for larger galls

00:08:31.600 --> 00:08:33.670
and would promote directional selection,

00:08:33.670 --> 00:08:36.310
all the other factors in
the environment being equal.

00:08:36.310 --> 00:08:40.360
But what happens when
keen-eyed birds are introduced?

00:08:40.360 --> 00:08:44.250
Well, large galls become
an easy meal for them

00:08:44.250 --> 00:08:46.520
because they're conspicuous
in the environment.

00:08:46.520 --> 00:08:47.800
This creates a situation

00:08:47.800 --> 00:08:51.610
where small galls are selected
against as are large ones,

00:08:51.610 --> 00:08:54.820
ensuring that the medium-sized
galls are favored.

00:08:54.820 --> 00:08:57.000
To remember what this
distribution looks like,

00:08:57.000 --> 00:08:59.130
I tell myself that the trait distribution

00:08:59.130 --> 00:09:03.090
is being stabilized by
pressure on both sides.

00:09:03.090 --> 00:09:06.520
Environmental change
can be pretty diverse.

00:09:06.520 --> 00:09:10.170
Without the incredible adaptive
powers of natural selection,

00:09:10.170 --> 00:09:12.800
our dynamic world would be unsuitable

00:09:12.800 --> 00:09:17.403
for the unpredictability
that is life as we know it.


Factors affecting acid strength

WEBVTT
Kind: captions
Language: en

00:00:00.370 --> 00:00:02.360
- [Instructor] Factors
that affect asset strength

00:00:02.360 --> 00:00:04.640
include bond polarity, bond strength,

00:00:04.640 --> 00:00:06.650
and conjugate base stability.

00:00:06.650 --> 00:00:09.070
Let's think about a generic acid, HA,

00:00:09.070 --> 00:00:11.570
that donates a proton to water

00:00:11.570 --> 00:00:14.470
to form the hydronium ion H3O plus,

00:00:14.470 --> 00:00:18.420
and the conjugate base
to HA, which is A minus.

00:00:18.420 --> 00:00:20.250
First let's consider the polarity

00:00:20.250 --> 00:00:23.000
of the bond between H and A.

00:00:23.000 --> 00:00:25.630
If A is more electronegative than H,

00:00:25.630 --> 00:00:28.090
A withdraws electron density.

00:00:28.090 --> 00:00:32.170
So we could draw an arrow
pointing towards the A, right,

00:00:32.170 --> 00:00:34.130
as the electrons and the bond between them

00:00:34.130 --> 00:00:36.670
are pulled closer to the A.

00:00:36.670 --> 00:00:40.330
As the electronegativity of A increases,

00:00:40.330 --> 00:00:44.913
there's an increase in
the polarity of the bond.

00:00:45.840 --> 00:00:47.800
As the bond polarity increases,

00:00:47.800 --> 00:00:51.180
more electron density is
drawn away from the H,

00:00:51.180 --> 00:00:54.730
which makes it easier for
HA to donate a proton.

00:00:54.730 --> 00:00:56.270
Therefore, in general,

00:00:56.270 --> 00:00:59.450
an increase in the
polarity of the HA bond,

00:00:59.450 --> 00:01:02.823
means an increase in the
strength of the acid.

00:01:03.800 --> 00:01:06.740
Next, let's think about the
factor of bond strength.

00:01:06.740 --> 00:01:10.800
And let's consider the strength
of the bond between H and A.

00:01:10.800 --> 00:01:12.100
The weaker the bond,

00:01:12.100 --> 00:01:15.160
the more easily the proton is donated.

00:01:15.160 --> 00:01:18.320
Therefore, in general, a
decrease in the bond strength,

00:01:18.320 --> 00:01:22.580
means an increase in the
strength of the acid.

00:01:22.580 --> 00:01:24.510
The stability of the conjugate base,

00:01:24.510 --> 00:01:27.060
can also affect the strength of the acid.

00:01:27.060 --> 00:01:28.830
The more stable the conjugate base,

00:01:28.830 --> 00:01:32.320
the more likely the asset
is to donate a proton.

00:01:32.320 --> 00:01:33.740
So if you think about that for HA,

00:01:33.740 --> 00:01:36.860
the conjugate base is A minus.

00:01:36.860 --> 00:01:39.040
And the more stable A minus is,

00:01:39.040 --> 00:01:43.100
the more likely HA will
donate a proton in solution.

00:01:43.100 --> 00:01:44.530
Therefore, in general,

00:01:44.530 --> 00:01:48.460
the more stable the conjugate
base, the stronger the acid.

00:01:48.460 --> 00:01:49.520
So let's go ahead and write here

00:01:49.520 --> 00:01:52.700
an increase in the stability
of the conjugate base,

00:01:52.700 --> 00:01:57.150
means an increase in the
strength of the acid.

00:01:57.150 --> 00:01:58.340
Even though acid strength

00:01:58.340 --> 00:02:00.740
is usually due to all
three of these factors,

00:02:00.740 --> 00:02:02.190
bond polarity, bond strength,

00:02:02.190 --> 00:02:04.010
and conjugate base stability,

00:02:04.010 --> 00:02:05.150
when we look at examples,

00:02:05.150 --> 00:02:07.540
we're only gonna consider
one or two factors

00:02:07.540 --> 00:02:11.590
that are the main
contributors to acid strength.

00:02:11.590 --> 00:02:15.190
Let's look at the binary
acids from group 7A

00:02:15.190 --> 00:02:18.020
on the periodic tables,
that's hydrofluoric acid,

00:02:18.020 --> 00:02:22.340
hydrochloric acid, hydrobromic
acid, and hydro ionic acid.

00:02:22.340 --> 00:02:25.110
As we go down the group
from fluorine to chlorine,

00:02:25.110 --> 00:02:26.890
to bromine, to iodine,

00:02:26.890 --> 00:02:29.600
there's an increase in
the strength of the acid.

00:02:29.600 --> 00:02:33.700
So out of these four, hydro
ionic acid, is the strongest.

00:02:33.700 --> 00:02:35.450
The main factor determining the strength

00:02:35.450 --> 00:02:40.380
of the binary acids in
group 7A, is bond strength.

00:02:40.380 --> 00:02:42.390
Looking at values for bond enthalpy,

00:02:42.390 --> 00:02:45.220
allows us to figure out the
strengths of these bonds.

00:02:45.220 --> 00:02:47.320
For example, the HF bond,

00:02:47.320 --> 00:02:51.480
has a bond enthalpy of
567 kilojoules per mole.

00:02:51.480 --> 00:02:53.040
While the HI bond,

00:02:53.040 --> 00:02:56.570
has a bond enthalpy of
299 kilojoules per mole.

00:02:56.570 --> 00:02:58.690
The lower the value for the bond enthalpy,

00:02:58.690 --> 00:03:01.120
the easier it is to break the bond.

00:03:01.120 --> 00:03:04.760
And because bond enthalpy
decrease as we go down the group,

00:03:04.760 --> 00:03:07.980
that means there's a
decrease in bond strength.

00:03:07.980 --> 00:03:09.330
A decrease in bond strength,

00:03:09.330 --> 00:03:12.210
means it's easier for the
asset to donate a proton.

00:03:12.210 --> 00:03:15.370
Therefore, we see an increase
in the strength of the asset

00:03:15.370 --> 00:03:17.330
as we go down the group.

00:03:17.330 --> 00:03:20.990
Next, let's compare the
strengths of some oxyacids.

00:03:20.990 --> 00:03:23.587
These oxyacids all have
the general formula XOH,

00:03:24.560 --> 00:03:26.660
where X is a halogen.

00:03:26.660 --> 00:03:27.830
The acidic proton,

00:03:27.830 --> 00:03:30.830
is the proton that's directly
bonded to the oxygen.

00:03:30.830 --> 00:03:34.420
And if an oxyacid donates
its proton to water,

00:03:34.420 --> 00:03:37.360
that forms the hydronium ion H3O plus,

00:03:37.360 --> 00:03:40.720
and the conjugate base to the oxyacid.

00:03:40.720 --> 00:03:42.170
For these three oxyacids,

00:03:42.170 --> 00:03:45.600
the halogens are iodine,
bromine, and chlorine.

00:03:45.600 --> 00:03:47.560
And as we go from iodine to bromine,

00:03:47.560 --> 00:03:50.730
to chlorine in group 7A
on the periodic table,

00:03:50.730 --> 00:03:54.450
that's an increase in
the electro negativity.

00:03:54.450 --> 00:03:57.110
So chlorine is the most electronegative

00:03:57.110 --> 00:03:59.560
out of these three halogens.

00:03:59.560 --> 00:04:02.610
And as we go up the group in our halogens,

00:04:02.610 --> 00:04:05.320
there's an increase in
the strength of the acid.

00:04:05.320 --> 00:04:09.060
So hypochlorous acid is
the strongest of the three.

00:04:09.060 --> 00:04:12.360
The main factor determining
the strength of these oxyacids,

00:04:12.360 --> 00:04:13.780
is the bond polarity,

00:04:13.780 --> 00:04:17.420
which is affected by the
electronegativity of the halogen.

00:04:17.420 --> 00:04:21.040
So the polarity of this
oxygen, hydrogen bond,

00:04:21.040 --> 00:04:24.120
is affected by the
presence of the halogen.

00:04:24.120 --> 00:04:27.760
As the electronegativity
of the halogen increases,

00:04:27.760 --> 00:04:30.730
the halogen is able to
withdraw more electron density

00:04:30.730 --> 00:04:33.210
away from the right side of the molecule.

00:04:33.210 --> 00:04:36.330
That increases the
polarity of the OH bond,

00:04:36.330 --> 00:04:39.040
it makes it easier to donate this proton.

00:04:39.040 --> 00:04:40.990
Therefore, as the electronegativity

00:04:40.990 --> 00:04:43.110
of the halogen increases,

00:04:43.110 --> 00:04:46.830
the acidity of the oxyacid increases.

00:04:46.830 --> 00:04:49.280
This effect of the electronegative atom

00:04:49.280 --> 00:04:51.200
increasing the acidity,

00:04:51.200 --> 00:04:54.770
is often referred to as
the inductive effect.

00:04:54.770 --> 00:04:59.110
Let's compare hypochlorous
acid to two other oxyacids,

00:04:59.110 --> 00:05:01.650
and notice how I've
left the formal charges

00:05:01.650 --> 00:05:03.270
off of these acids

00:05:03.270 --> 00:05:06.200
just so we can focus on general structure.

00:05:06.200 --> 00:05:08.410
Notice how the acidic proton

00:05:08.410 --> 00:05:10.630
is directly bonded to the oxygen

00:05:10.630 --> 00:05:13.370
and all three of these oxyacids.

00:05:13.370 --> 00:05:15.270
And in all three of these oxyacids,

00:05:15.270 --> 00:05:18.570
the oxygen is directly
bonded to a chlorine.

00:05:18.570 --> 00:05:20.010
Notice what happens to the structure

00:05:20.010 --> 00:05:21.560
as we move to the right.

00:05:21.560 --> 00:05:24.720
Comparing chlorous acid
to hypochlorous acid,

00:05:24.720 --> 00:05:26.590
chlorous acid has an additional oxygen

00:05:26.590 --> 00:05:28.060
bonded to the chlorine.

00:05:28.060 --> 00:05:30.260
And looking at perchloric acid,

00:05:30.260 --> 00:05:31.620
instead of only one additional oxygen,

00:05:31.620 --> 00:05:34.380
there are three additional oxygens.

00:05:34.380 --> 00:05:35.640
So as we move to the right,

00:05:35.640 --> 00:05:38.070
we're increasing in the number of oxygens

00:05:38.070 --> 00:05:40.180
bonded to the chlorine.

00:05:40.180 --> 00:05:43.100
Oxygen is a very electronegative element.

00:05:43.100 --> 00:05:44.490
So as we move to the right,

00:05:44.490 --> 00:05:48.240
we're increasing in the number
of electronegative atoms

00:05:48.240 --> 00:05:49.900
in the acid.

00:05:49.900 --> 00:05:52.920
And as the number of
electronegative atoms increases,

00:05:52.920 --> 00:05:57.310
more electron density is pulled
away from the acidic proton,

00:05:57.310 --> 00:06:01.590
which increases the polarity
of the oxygen hydrogen bond.

00:06:01.590 --> 00:06:04.980
So bond polarity increases
as we move to the right,

00:06:04.980 --> 00:06:08.260
which predicts an increase
in the strength of the acid

00:06:08.260 --> 00:06:10.210
as we move to the right.

00:06:10.210 --> 00:06:12.160
And that's what we observe experimentally,

00:06:12.160 --> 00:06:15.360
perchloric acid is the
strongest of the three.

00:06:15.360 --> 00:06:17.070
In reality, all three factors

00:06:17.070 --> 00:06:19.040
affect the strength of the acid.

00:06:19.040 --> 00:06:20.820
However, for simplicity sake,

00:06:20.820 --> 00:06:23.560
we could just say that
increasing bond polarity,

00:06:23.560 --> 00:06:26.440
is the main factor for the
increasing acid strength

00:06:26.440 --> 00:06:27.763
in these oxyacids.

00:06:29.490 --> 00:06:31.630
Carboxylic acids are a group of assets

00:06:31.630 --> 00:06:34.240
that all contain a carboxyl group.

00:06:34.240 --> 00:06:36.670
A carboxylic group consists of carbon,

00:06:36.670 --> 00:06:38.870
oxygen, oxygen and hydrogen.

00:06:38.870 --> 00:06:40.920
So if we look at acetic acid,

00:06:40.920 --> 00:06:43.810
I'll circle the carboxyl
group on acetic acid,

00:06:43.810 --> 00:06:48.040
and I can also circle the
carboxyl group on formic acid.

00:06:48.040 --> 00:06:50.440
The acidic proton in the carboxylic acid,

00:06:50.440 --> 00:06:53.150
is the one that's directly
bonded to the oxygen

00:06:53.150 --> 00:06:54.823
in the carboxyl group.

00:06:55.680 --> 00:06:57.990
One reason why this proton is acidic,

00:06:57.990 --> 00:07:00.097
is because of the presence of this oxygen

00:07:00.097 --> 00:07:02.210
in the carboxyl group.

00:07:02.210 --> 00:07:05.140
This oxygen hydrogen bond
is already polarized,

00:07:05.140 --> 00:07:08.380
but the presence of another
electronegative atom,

00:07:08.380 --> 00:07:12.670
further increases the polarity
of the oxygen hydrogen bond.

00:07:12.670 --> 00:07:14.150
Increasing the bond polarity,

00:07:14.150 --> 00:07:16.840
makes it more likely to donate the proton,

00:07:16.840 --> 00:07:19.460
which increases the acidity.

00:07:19.460 --> 00:07:20.730
For carboxylic acids,

00:07:20.730 --> 00:07:22.460
it's also important to consider

00:07:22.460 --> 00:07:25.400
the stability of the conjugate base.

00:07:25.400 --> 00:07:28.080
When acetic acid donates its proton,

00:07:28.080 --> 00:07:29.820
it turns into its conjugate base,

00:07:29.820 --> 00:07:31.860
which is the acetate anion.

00:07:31.860 --> 00:07:33.050
Notice that the oxygen

00:07:33.050 --> 00:07:34.670
that used to be bonded to the proton,

00:07:34.670 --> 00:07:37.320
now has a negative formal charge on it.

00:07:37.320 --> 00:07:39.160
There are two possible
resonance structures

00:07:39.160 --> 00:07:41.400
that you can draw for the acetate anion.

00:07:41.400 --> 00:07:42.820
The first is with the negative charge

00:07:42.820 --> 00:07:44.000
on this oxygen.

00:07:44.000 --> 00:07:46.330
And then we could draw
another resonance structure

00:07:46.330 --> 00:07:49.230
with a negative charge
on the other oxygen.

00:07:49.230 --> 00:07:51.280
In reality, neither resonant structure

00:07:51.280 --> 00:07:54.330
is a perfect representation
of the acetate anion.

00:07:54.330 --> 00:07:56.330
And we need to think about a hybrid

00:07:56.330 --> 00:07:59.020
of these two resonant structures.

00:07:59.020 --> 00:07:59.853
In the hybrid,

00:07:59.853 --> 00:08:02.670
the negative charge isn't
on one of the oxygens,

00:08:02.670 --> 00:08:03.790
that one negative charge,

00:08:03.790 --> 00:08:06.950
is spread out or de-localized over two,

00:08:06.950 --> 00:08:08.640
over the two oxygens.

00:08:08.640 --> 00:08:11.140
So it's like one oxygen
has negative one half,

00:08:11.140 --> 00:08:14.310
and the other oxygen
has negative one half.

00:08:14.310 --> 00:08:16.960
This de-localization
of the negative charge,

00:08:16.960 --> 00:08:19.500
stabilizes the conjugate base.

00:08:19.500 --> 00:08:21.700
And the more stable the conjugate base,

00:08:21.700 --> 00:08:23.720
the stronger the acid.

00:08:23.720 --> 00:08:25.810
Therefore, the stability
of the conjugate base

00:08:25.810 --> 00:08:29.490
also affects the acidity
of the carboxylic acid.

00:08:29.490 --> 00:08:31.160
So because carboxylic acids

00:08:31.160 --> 00:08:34.400
have conjugate bases that
are resonant stabilized,

00:08:34.400 --> 00:08:37.250
carboxylic acids like
acetic acid and formic acid,

00:08:37.250 --> 00:08:38.353
are more acidic.


Acid–base properties of salts

WEBVTT
Kind: captions
Language: en

00:00:00.260 --> 00:00:02.290
- [Instructor] Salts can
form acidic solutions,

00:00:02.290 --> 00:00:03.390
neutral solutions,

00:00:03.390 --> 00:00:06.760
or basic solutions when
dissolved in water.

00:00:06.760 --> 00:00:10.200
For example, if we dissolve
sodium chloride in water,

00:00:10.200 --> 00:00:13.110
solid sodium chloride
turns into sodium cations

00:00:13.110 --> 00:00:16.100
and chloride anions in solution.

00:00:16.100 --> 00:00:17.910
At 25 degrees Celsius,

00:00:17.910 --> 00:00:20.960
the aqueous solution of
sodium chloride is neutral

00:00:20.960 --> 00:00:23.240
and has a pH of seven.

00:00:23.240 --> 00:00:25.480
The reason why the pH is equal to seven is

00:00:25.480 --> 00:00:29.520
because neither the cation nor
the anion reacts with water,

00:00:29.520 --> 00:00:33.730
and water has a pH of seven
at 25 degrees Celsius.

00:00:33.730 --> 00:00:37.380
And since neither the cation
nor the anion react with water,

00:00:37.380 --> 00:00:39.630
the pH remains seven.

00:00:39.630 --> 00:00:42.030
In a different solution,
it's possible for the cation

00:00:42.030 --> 00:00:44.230
or the anion to react with water

00:00:44.230 --> 00:00:48.100
and turn the solution
either acidic or basic.

00:00:48.100 --> 00:00:50.580
Therefore, to predict if
a salt solution is going

00:00:50.580 --> 00:00:52.830
to be acidic, neutral, or basic,

00:00:52.830 --> 00:00:55.280
we have to analyze
whether or not the cation

00:00:55.280 --> 00:00:58.160
and the anion will react with water.

00:00:58.160 --> 00:01:00.580
And there are four possible combinations

00:01:00.580 --> 00:01:01.937
of cation and anion.

00:01:02.850 --> 00:01:06.140
The first combination
is neither the cation

00:01:06.140 --> 00:01:09.240
nor the anion will react with water.

00:01:09.240 --> 00:01:10.660
And if that's the case,

00:01:10.660 --> 00:01:13.740
the resulting solution will be neutral.

00:01:13.740 --> 00:01:15.430
We've already talked
about an aqueous solution

00:01:15.430 --> 00:01:18.550
of sodium chloride being
a neutral solution.

00:01:18.550 --> 00:01:21.710
The way to approach this is to
look at the chemical formula

00:01:21.710 --> 00:01:26.080
and say sodium chloride
consists of a sodium cation

00:01:26.080 --> 00:01:28.460
and a chloride anion.

00:01:28.460 --> 00:01:31.300
The next step is to analyze
the cation and the anion

00:01:31.300 --> 00:01:34.240
and think about if they
react with water or not.

00:01:34.240 --> 00:01:37.020
To determine whether or not a
cation will react with water,

00:01:37.020 --> 00:01:40.890
it's helpful to think about
a list of common strong bases

00:01:40.890 --> 00:01:44.660
that consist of Group 1A metal hydroxides

00:01:44.660 --> 00:01:49.150
and the heavier Group 2A metal hydroxides.

00:01:49.150 --> 00:01:52.530
I the cation is from Group
1A or the heavier Group 2A,

00:01:52.530 --> 00:01:55.440
the cation will not react with water.

00:01:55.440 --> 00:01:58.480
For example, in our case,
we have the sodium cation.

00:01:58.480 --> 00:02:01.730
And since the sodium
cation is in Group 1A,

00:02:01.730 --> 00:02:04.590
the sodium cation will
not react with water.

00:02:04.590 --> 00:02:06.550
Next, we think about the anion.

00:02:06.550 --> 00:02:09.480
To determine whether or not the
anion will react with water,

00:02:09.480 --> 00:02:13.570
it's helpful to think about a
list of common strong acids.

00:02:13.570 --> 00:02:15.880
If the anion is the conjugate base

00:02:15.880 --> 00:02:17.640
to one of the strong acids,

00:02:17.640 --> 00:02:20.600
the anion will not react with water.

00:02:20.600 --> 00:02:23.850
For example, in our case,
we have the chloride anion,

00:02:23.850 --> 00:02:27.560
which is the conjugate base to HCl.

00:02:27.560 --> 00:02:30.530
Since Cl- is the conjugate base to HCl,

00:02:30.530 --> 00:02:33.730
Cl- will not react with water.

00:02:33.730 --> 00:02:35.380
A good way to think about this is

00:02:35.380 --> 00:02:38.430
to think about hydrochloric
acid being a strong acid.

00:02:38.430 --> 00:02:42.210
And the stronger the acid,
the weaker the conjugate base.

00:02:42.210 --> 00:02:45.650
Therefore, the chloride
anion is such a weak base,

00:02:45.650 --> 00:02:48.010
it will not react with water.

00:02:48.010 --> 00:02:52.100
So we say the chloride anion
is of negligible basicity.

00:02:52.100 --> 00:02:56.020
And since neither the cation
nor the anion react with water,

00:02:56.020 --> 00:02:59.840
an aqueous solution of sodium
chloride will be neutral.

00:02:59.840 --> 00:03:02.880
As another example, let's
think about barium nitrate.

00:03:02.880 --> 00:03:06.770
Barium nitrate consists
of the barium 2+ cation

00:03:06.770 --> 00:03:09.380
and the nitrate anion.

00:03:09.380 --> 00:03:14.160
Because the barium 2+ cation
is from the heavier Group 2A,

00:03:14.160 --> 00:03:17.430
the barium 2+ cation will
not react with water.

00:03:17.430 --> 00:03:21.800
And because the nitrate
anion is the conjugate base

00:03:21.800 --> 00:03:24.080
to a strong acid, which is nitric acid,

00:03:24.080 --> 00:03:26.920
the nitrate anion will
not react with water.

00:03:26.920 --> 00:03:28.360
Since neither the cation

00:03:28.360 --> 00:03:30.610
nor the anion will react with water,

00:03:30.610 --> 00:03:34.630
an aqueous solution of barium
nitrate will be neutral.

00:03:34.630 --> 00:03:37.860
The second possible combination
of cation and anion is

00:03:37.860 --> 00:03:40.110
where the cation does
not react with water,

00:03:40.110 --> 00:03:41.850
but the anion does.

00:03:41.850 --> 00:03:43.300
When this combination occurs,

00:03:43.300 --> 00:03:46.530
the resulting solution will be basic.

00:03:46.530 --> 00:03:48.490
An example of the second combination

00:03:48.490 --> 00:03:50.710
would be barium acetate.

00:03:50.710 --> 00:03:55.040
So barium acetate consists
of the barium 2+ cation

00:03:55.040 --> 00:03:57.643
and the acetate anion.

00:03:58.800 --> 00:04:02.777
Since barium 2+ is on our
list for heavier Group 2A,

00:04:02.777 --> 00:04:06.220
barium 2+ will not react with water.

00:04:06.220 --> 00:04:09.760
However, the acetate anion
is the conjugate base

00:04:09.760 --> 00:04:11.310
to acetic acid.

00:04:11.310 --> 00:04:13.840
And acetic acid is a weak acid

00:04:13.840 --> 00:04:16.830
and is not on our list of strong acids.

00:04:16.830 --> 00:04:19.270
Since acetic acid is a weak acid,

00:04:19.270 --> 00:04:23.690
its conjugate base is strong
enough to react with water.

00:04:23.690 --> 00:04:26.410
So the acetate anion
is a strong enough base

00:04:26.410 --> 00:04:27.690
to react with water.

00:04:27.690 --> 00:04:30.400
And when acetate anion reacts with water,

00:04:30.400 --> 00:04:33.540
it forms acetic acid and hydroxide ions.

00:04:33.540 --> 00:04:36.340
Since the concentration of hydroxide ions

00:04:36.340 --> 00:04:38.220
in solution have increased,

00:04:38.220 --> 00:04:41.000
that's what makes the solution basic.

00:04:41.000 --> 00:04:43.440
This is called anion hydrolysis

00:04:43.440 --> 00:04:45.580
when an anion reacts with water

00:04:45.580 --> 00:04:50.070
to increase the concentration
of hydroxide ions in solution.

00:04:50.070 --> 00:04:51.000
As another example,

00:04:51.000 --> 00:04:53.460
let's think about sodium hypochlorite,

00:04:53.460 --> 00:04:56.210
which consists of the sodium cation

00:04:56.210 --> 00:04:58.093
and the hypochlorite anion.

00:04:59.260 --> 00:05:02.540
Since the sodium cation is from Group 1A,

00:05:02.540 --> 00:05:05.023
the sodium cation will
not react with water.

00:05:05.023 --> 00:05:08.240
The hypochlorite anion
is the conjugate base

00:05:08.240 --> 00:05:10.140
to hypochlorous acid.

00:05:10.140 --> 00:05:12.110
Since hypochlorous acid is not

00:05:12.110 --> 00:05:16.400
on our list of common strong
acids, it must be a weak acid.

00:05:16.400 --> 00:05:18.840
And if it's a weak acid,
its conjugate base,

00:05:18.840 --> 00:05:20.673
the hypochlorite anion,

00:05:20.673 --> 00:05:23.220
is a strong enough base
to react with water,

00:05:23.220 --> 00:05:25.260
therefore increasing the concentration

00:05:25.260 --> 00:05:27.920
of hydroxide ions in solution.

00:05:27.920 --> 00:05:30.720
So since we have a cation
that does not react with water

00:05:30.720 --> 00:05:33.250
and an anion that does react with water,

00:05:33.250 --> 00:05:38.250
an aqueous solution of sodium
hypochlorite will be basic.

00:05:38.330 --> 00:05:41.520
The third possible combination
of cation and anion is

00:05:41.520 --> 00:05:43.750
where the cation will react with water,

00:05:43.750 --> 00:05:45.570
but the anion will not.

00:05:45.570 --> 00:05:48.420
In this case, the resulting
solution will be acidic.

00:05:48.420 --> 00:05:50.270
As an example of this third combination,

00:05:50.270 --> 00:05:53.680
let's consider an aqueous
solution of ammonium nitrate.

00:05:53.680 --> 00:05:56.320
Looking at the chemical
formula for ammonium nitrate,

00:05:56.320 --> 00:05:58.820
it consists of the ammonium cation NH4+

00:06:00.390 --> 00:06:04.450
and the nitrate anion NO3-.

00:06:04.450 --> 00:06:06.200
First, let's talk about the anion.

00:06:06.200 --> 00:06:10.740
The nitrate anion is the
conjugate base to nitric acid,

00:06:10.740 --> 00:06:12.230
which is HNO3.

00:06:12.230 --> 00:06:14.010
Since nitric acid is a strong acid,

00:06:14.010 --> 00:06:17.180
its conjugate base is
of negligible basicity,

00:06:17.180 --> 00:06:20.210
so the nitrate anion does not
affect the pH of the solution.

00:06:20.210 --> 00:06:22.040
It does not react with water.

00:06:22.040 --> 00:06:23.943
When thinking about the cation NH4+,

00:06:25.030 --> 00:06:30.030
NH4+ is the conjugate acid
to NH3, which is ammonia.

00:06:30.130 --> 00:06:33.750
And since NH3 or ammonia is a weak base,

00:06:33.750 --> 00:06:37.520
its conjugate acid NH4+ is strong enough

00:06:37.520 --> 00:06:39.390
to react with water.

00:06:39.390 --> 00:06:43.410
And when the ammonium cation
NH4+ reacts with water,

00:06:43.410 --> 00:06:47.000
it forms the hydronium
ion H3O+ and ammonia.

00:06:47.000 --> 00:06:50.810
So the NH4+ is increasing
the concentration

00:06:50.810 --> 00:06:53.390
of hydronium ion in solution,

00:06:53.390 --> 00:06:56.110
which is what's making the solution,

00:06:56.110 --> 00:06:59.100
this aqueous solution of
ammonium nitrate, acidic.

00:06:59.100 --> 00:07:01.680
As another example, let's
consider aluminum chloride,

00:07:01.680 --> 00:07:03.740
an aqueous solution of aluminum chloride.

00:07:03.740 --> 00:07:05.300
Looking at the chemical formula,

00:07:05.300 --> 00:07:09.200
aluminum chloride consists
of the aluminum 3+ cation

00:07:09.200 --> 00:07:11.250
and the chloride anion.

00:07:11.250 --> 00:07:13.430
The chloride anion is the conjugate base

00:07:13.430 --> 00:07:15.760
to hydrochloric acid,
which is a strong acid.

00:07:15.760 --> 00:07:18.950
Therefore, the chloride anion
does not react with water.

00:07:18.950 --> 00:07:23.240
The aluminum 3+ cation
is not from Group 1A

00:07:23.240 --> 00:07:25.520
or heavier Group 2A.

00:07:25.520 --> 00:07:27.560
Therefore, we can conclude
that this is going

00:07:27.560 --> 00:07:29.440
to react with water.

00:07:29.440 --> 00:07:31.700
So small cations with charges

00:07:31.700 --> 00:07:36.660
of 2+ or greater can react with water.

00:07:36.660 --> 00:07:37.760
Let's look in more detail

00:07:37.760 --> 00:07:42.070
how the aluminum 3+ cation
can function as an acid.

00:07:42.070 --> 00:07:45.220
In aqueous solution, the
aluminum 3+ cation interacts

00:07:45.220 --> 00:07:48.983
with water molecules to
form a hydrated metal ion.

00:07:50.020 --> 00:07:52.520
Water is a polar molecule,

00:07:52.520 --> 00:07:57.310
and the positively-charged
aluminum 3+ cation interacts

00:07:57.310 --> 00:08:00.150
with the negative end
of the water molecule.

00:08:00.150 --> 00:08:02.230
So there's an electrostatic attraction

00:08:02.230 --> 00:08:04.870
between the negative end
of the water molecule

00:08:04.870 --> 00:08:07.770
and the 3+ charge on the aluminum cation.

00:08:07.770 --> 00:08:09.900
I only drew in one water molecule.

00:08:09.900 --> 00:08:13.190
However, keep in mind there's
actually six water molecules

00:08:13.190 --> 00:08:16.020
interacting with one Al3+ cation

00:08:16.020 --> 00:08:18.320
in the hydrated metal ion.

00:08:18.320 --> 00:08:20.240
The strong electrostatic attraction

00:08:20.240 --> 00:08:24.920
withdraws electron density
from this oxygen-hydrogen bond,

00:08:24.920 --> 00:08:28.000
which makes this proton easier to remove.

00:08:28.000 --> 00:08:30.820
Therefore, when another
molecule comes along,

00:08:30.820 --> 00:08:33.090
so right here in the equation,

00:08:33.090 --> 00:08:35.860
this other water molecule
can take this proton,

00:08:35.860 --> 00:08:38.150
leaving these electrons
behind in the oxygen.

00:08:38.150 --> 00:08:43.150
And if you're adding an H+
onto H2O, you form H3O+.

00:08:43.450 --> 00:08:46.990
Losing an H+ means the charge
in the hydrated ion went

00:08:46.990 --> 00:08:50.802
from a 3+ down to only a 2+.

00:08:50.802 --> 00:08:53.300
An increase in the
concentration of hydronium ions

00:08:53.300 --> 00:08:57.770
in solution means the pH of
the solution will decrease.

00:08:57.770 --> 00:09:02.440
Therefore, a hydrated metal
ion can function as an acid.

00:09:02.440 --> 00:09:04.830
However, it's the strength
of this interaction

00:09:04.830 --> 00:09:08.420
between the cation and the
water molecules that determines

00:09:08.420 --> 00:09:12.070
if the hydrated metal ion will
function as an acid or not.

00:09:12.070 --> 00:09:15.610
For a small cation with
charge of 2+ or greater,

00:09:15.610 --> 00:09:17.720
that gives a large
electrostatic attraction

00:09:17.720 --> 00:09:18.750
to the water molecule,

00:09:18.750 --> 00:09:21.370
which makes this proton easier to donate.

00:09:21.370 --> 00:09:23.190
However, if the cation is too large

00:09:23.190 --> 00:09:24.650
or the charge is too small,

00:09:24.650 --> 00:09:27.170
the electrostatic attraction
is not strong enough

00:09:27.170 --> 00:09:29.460
to make the proton easy to donate,

00:09:29.460 --> 00:09:31.690
and that's the reason
why cations from Group 1A

00:09:31.690 --> 00:09:35.100
or the heavier Group 2A don't
interact strongly enough

00:09:35.100 --> 00:09:38.500
with water to affect
the pH of the solution.

00:09:38.500 --> 00:09:40.330
Therefore, if we have an aqueous solution

00:09:40.330 --> 00:09:42.070
of aluminum chloride,

00:09:42.070 --> 00:09:45.520
the cation aluminum 3+
will react with water,

00:09:45.520 --> 00:09:48.310
but the anion, the
chloride anion, will not.

00:09:48.310 --> 00:09:49.890
Since the cation reacts with water,

00:09:49.890 --> 00:09:52.670
the resulting solution will be acidic.

00:09:52.670 --> 00:09:54.450
The fourth possible combination is

00:09:54.450 --> 00:09:56.550
where a cation will react with water,

00:09:56.550 --> 00:09:59.300
and the anion will also react with water.

00:09:59.300 --> 00:10:02.350
In this case, the resulting
solution can be acidic,

00:10:02.350 --> 00:10:04.860
neutral, or basic.

00:10:04.860 --> 00:10:07.130
As an example, let's think
about an aqueous solution

00:10:07.130 --> 00:10:08.940
of ammonium carbonate.

00:10:08.940 --> 00:10:12.040
Looking at the chemical
formula for ammonium carbonate,

00:10:12.040 --> 00:10:16.380
the cation would be the
ammonium cation, so that's NH4+.

00:10:16.380 --> 00:10:19.900
And the anion would be the
carbonate anion, CO3 2-.

00:10:21.770 --> 00:10:23.470
We've already seen that
the ammonium cation

00:10:23.470 --> 00:10:25.430
will react with water.

00:10:25.430 --> 00:10:28.370
And the carbonate anion, if
you add a proton onto it,

00:10:28.370 --> 00:10:31.960
that doesn't give us an acid
on our list of strong acids.

00:10:31.960 --> 00:10:34.980
Therefore, the carbonate
anion is a strong enough base

00:10:34.980 --> 00:10:36.690
to react with water.

00:10:36.690 --> 00:10:38.010
First, let's look at the equation

00:10:38.010 --> 00:10:41.180
showing the ammonium
ion reacting with water.

00:10:41.180 --> 00:10:43.430
So this is cation hydrolysis

00:10:43.430 --> 00:10:45.640
since the cation is reacting with water.

00:10:45.640 --> 00:10:49.390
In this case, we would form
the hydronium ion and ammonia.

00:10:49.390 --> 00:10:52.180
Next, we think about the
carbonate anion reacting

00:10:52.180 --> 00:10:55.110
with water, so this is anion hydrolysis.

00:10:55.110 --> 00:10:57.780
When the carbonate
anion reacts with water,

00:10:57.780 --> 00:11:00.210
it forms hydroxide ions in solution

00:11:00.210 --> 00:11:02.890
and also hydrogen carbonate.

00:11:02.890 --> 00:11:07.430
Since the cation hydrolysis
forms hydronium ions,

00:11:07.430 --> 00:11:09.600
if we were to write an
equilibrium constant

00:11:09.600 --> 00:11:13.140
for this acid-base
reaction, it would be Ka.

00:11:13.140 --> 00:11:15.010
And since the anion hydrolysis

00:11:15.010 --> 00:11:17.610
forms hydroxide ions in solution,

00:11:17.610 --> 00:11:19.740
if we were to write an
equilibrium constant

00:11:19.740 --> 00:11:23.420
for this acid-base
reaction, it would be Kb.

00:11:23.420 --> 00:11:25.680
To figure out if the aqueous solution

00:11:25.680 --> 00:11:29.160
of ammonium carbonate is
acidic, neutral, or basic,

00:11:29.160 --> 00:11:32.550
we need to compare the
Ka value for ammonium

00:11:32.550 --> 00:11:35.880
to the Kb value for carbonate.

00:11:35.880 --> 00:11:37.673
If the Ka value is
greater than the Kb value,

00:11:39.220 --> 00:11:40.940
the solution is acidic.

00:11:40.940 --> 00:11:44.470
If the Ka and the Kb values
are approximately the same,

00:11:44.470 --> 00:11:46.190
the solution is about neutral.

00:11:46.190 --> 00:11:48.300
And if Ka is less than Kb,

00:11:48.300 --> 00:11:50.690
or you could say Kb is greater than Ka,

00:11:50.690 --> 00:11:53.090
the solution will be basic.

00:11:53.090 --> 00:11:55.800
To find the Ka for the ammonium cation,

00:11:55.800 --> 00:11:57.470
we're gonna use the following equation:

00:11:57.470 --> 00:12:00.697
Ka times Kb is equal to Kw.

00:12:00.697 --> 00:12:04.850
And this equation is true for
a conjugate acid-base pair.

00:12:04.850 --> 00:12:09.070
So what this is saying is the
Ka for the ammonium cation

00:12:09.070 --> 00:12:14.070
times the Kb for ammonia is equal to Kw,

00:12:14.070 --> 00:12:16.310
which at 25 degrees Celsius is equal

00:12:16.310 --> 00:12:19.460
to 1.0 times 10 to the -14th.

00:12:19.460 --> 00:12:21.730
Textbooks often have the Kb values

00:12:21.730 --> 00:12:24.340
for common weak bases like ammonia,

00:12:24.340 --> 00:12:29.310
and the Kb value for ammonia
is 1.8 times 10 to the -5th.

00:12:29.310 --> 00:12:31.620
So we solve for Ka,

00:12:31.620 --> 00:12:34.500
and we find that Ka for the
ammonium cation is equal

00:12:34.500 --> 00:12:36.833
to 5.6 times 10 to the -10th.

00:12:37.720 --> 00:12:40.500
To calculate the Kb value
for the carbonate anion,

00:12:40.500 --> 00:12:41.760
we use the same equation.

00:12:41.760 --> 00:12:44.650
Ka times Kb is equal to Kw.

00:12:44.650 --> 00:12:47.200
However, remember, this
is the Ka and the Kb

00:12:47.200 --> 00:12:49.260
for a conjugate acid-base pair,

00:12:49.260 --> 00:12:53.710
so we're talking about the
carbonate anion as the base

00:12:53.710 --> 00:12:54.840
and its conjugate acid,

00:12:54.840 --> 00:12:57.557
which would be a hydrogen
carbonate, HCO3-.

00:12:58.460 --> 00:13:01.860
So we plug in the Ka value for HCO3-

00:13:01.860 --> 00:13:04.430
and solve for Kb.

00:13:04.430 --> 00:13:07.820
So Kb for the carbonate anion is equal

00:13:07.820 --> 00:13:12.490
to 1.8 times 10 to the
-4th at 25 degrees Celsius.

00:13:12.490 --> 00:13:17.490
And this Ka value is also
at 25 degrees Celsius.

00:13:17.640 --> 00:13:20.640
Looking at the values for Ka and Kb,

00:13:20.640 --> 00:13:24.880
Ka is less than Kb.

00:13:24.880 --> 00:13:27.210
Since Ka is less than Kb,

00:13:27.210 --> 00:13:29.790
or you could say Kb is greater than Ka,

00:13:29.790 --> 00:13:31.653
this solution is basic.


Weak base equilibria

WEBVTT
Kind: captions
Language: en

00:00:00.670 --> 00:00:04.510
- [Instructor] An example of
a weak base is ammonia, NH3,

00:00:04.510 --> 00:00:06.550
and ammonia will react with water.

00:00:06.550 --> 00:00:07.410
And in this reaction,

00:00:07.410 --> 00:00:09.760
water functions as a Bronsted-Lowry acid

00:00:09.760 --> 00:00:12.410
and donates a proton to ammonia,

00:00:12.410 --> 00:00:14.340
which functions as a Bronsted-Lowry base

00:00:14.340 --> 00:00:16.000
and accepts a proton.

00:00:16.000 --> 00:00:17.400
A proton is H plus.

00:00:17.400 --> 00:00:20.410
So if you add on H plus onto NH3,

00:00:20.410 --> 00:00:23.720
you form NH4 plus, which
is the ammonium ion.

00:00:23.720 --> 00:00:26.770
And if you take away NH plus from H2O,

00:00:26.770 --> 00:00:30.640
you form the hydroxide
anion, which is OH minus.

00:00:30.640 --> 00:00:32.620
This reaction comes to an equilibrium,

00:00:32.620 --> 00:00:34.540
so we have an equilibrium arrow in here,

00:00:34.540 --> 00:00:38.370
and we can also write an
equilibrium constant expression.

00:00:38.370 --> 00:00:39.960
So first, we look at our products.

00:00:39.960 --> 00:00:41.480
We have NH4 plus,

00:00:41.480 --> 00:00:43.290
and there's a coefficient
of one in front of it,

00:00:43.290 --> 00:00:45.680
so it'd be the concentration
of NH4 plus raised

00:00:45.680 --> 00:00:48.160
to the first power times the concentration

00:00:48.160 --> 00:00:50.130
of the other product, hydroxide,

00:00:50.130 --> 00:00:51.810
which also has a coefficient of one,

00:00:51.810 --> 00:00:53.850
so it's the concentration
of hydroxide raised

00:00:53.850 --> 00:00:55.030
to the first power,

00:00:55.030 --> 00:00:58.720
divided by the concentration of ammonia,

00:00:58.720 --> 00:01:00.270
and the coefficient is also one,

00:01:00.270 --> 00:01:02.280
so that's also raised to the first power.

00:01:02.280 --> 00:01:03.480
And water is left out

00:01:03.480 --> 00:01:06.170
of our equilibrium constant expression.

00:01:06.170 --> 00:01:09.640
The equilibrium constant
K has a subscript of b,

00:01:09.640 --> 00:01:11.170
which stands for base.

00:01:11.170 --> 00:01:14.900
And this Kb is called the
base ionization constant,

00:01:14.900 --> 00:01:18.020
or the base dissociation constant.

00:01:18.020 --> 00:01:19.390
Ammonia is a weak base,

00:01:19.390 --> 00:01:22.530
which means it only
partially ionizes in water.

00:01:22.530 --> 00:01:25.760
And if it only partially
ionizes, at equilibrium,

00:01:25.760 --> 00:01:28.280
there's going to be a
small amount of products

00:01:28.280 --> 00:01:30.890
and a large amount of reactants.

00:01:30.890 --> 00:01:33.870
Therefore, looking at the
equilibrium constant expression,

00:01:33.870 --> 00:01:36.530
the numerator is gonna
be very small compared

00:01:36.530 --> 00:01:38.470
to the denominator.

00:01:38.470 --> 00:01:40.950
So with a small numerator
and a large denominator,

00:01:40.950 --> 00:01:44.260
that means that the Kb
value for a weak base

00:01:44.260 --> 00:01:46.330
will be less than one.

00:01:46.330 --> 00:01:49.220
Let's look at a few
examples of weak bases.

00:01:49.220 --> 00:01:50.940
Here's the dot structure for ammonia,

00:01:50.940 --> 00:01:53.010
and notice how ammonia has a nitrogen

00:01:53.010 --> 00:01:54.830
with a lone pair of electrons

00:01:54.830 --> 00:01:57.330
that's capable of accepting a proton.

00:01:57.330 --> 00:01:59.030
A proton is H plus.

00:01:59.030 --> 00:02:04.000
So if we add an H plus
onto NH3, we form NH4 plus,

00:02:04.000 --> 00:02:06.180
which is the ammonium ion,

00:02:06.180 --> 00:02:09.510
and it's the conjugate acid to ammonia.

00:02:09.510 --> 00:02:11.290
So think about this lone pair of electrons

00:02:11.290 --> 00:02:15.570
that I circled here picking
up a proton to form this bond.

00:02:15.570 --> 00:02:16.930
If we replace the hydrogen

00:02:16.930 --> 00:02:19.650
with what's called a methyl group, CH3,

00:02:19.650 --> 00:02:23.270
we form a new weak base
called methylamine.

00:02:23.270 --> 00:02:25.410
Like ammonia, methylamine has a nitrogen

00:02:25.410 --> 00:02:26.720
with a lone pair of electrons

00:02:26.720 --> 00:02:29.880
that's capable of accepting a proton.

00:02:29.880 --> 00:02:32.410
So if methylamine picks up a proton,

00:02:32.410 --> 00:02:34.733
it forms the methyl ammonium ion.

00:02:35.660 --> 00:02:37.410
Both ammonia and methylamine

00:02:37.410 --> 00:02:39.940
are examples of neutral substances.

00:02:39.940 --> 00:02:42.730
However, a weak base
doesn't have to be neutral.

00:02:42.730 --> 00:02:47.410
If we look at the
hypochlorite anion, ClO minus,

00:02:47.410 --> 00:02:49.650
if the hypochlorite
anion picks up a proton,

00:02:49.650 --> 00:02:51.180
let's say this lone pair of electrons

00:02:51.180 --> 00:02:52.963
in the oxygen picks up a proton,

00:02:54.832 --> 00:02:57.140
it would form hypochlorous acid,

00:02:57.140 --> 00:03:01.670
and hypochlorous is the
conjugate acid to hypochlorite.

00:03:01.670 --> 00:03:04.760
So these are three examples of weak bases,

00:03:04.760 --> 00:03:07.300
which we can tell by
looking at their Kb values

00:03:07.300 --> 00:03:08.920
at 25 degrees Celsius.

00:03:08.920 --> 00:03:12.450
Notice they all have Kb
values less than one.

00:03:12.450 --> 00:03:14.360
However, the higher the Kb value,

00:03:14.360 --> 00:03:17.760
the higher the equilibrium
concentration of hydroxide ion,

00:03:17.760 --> 00:03:20.070
and therefore the stronger the base.

00:03:20.070 --> 00:03:22.420
So out of these three week bases,

00:03:22.420 --> 00:03:26.060
the one with the higher Kb
value is the most basic.

00:03:26.060 --> 00:03:28.800
So looking at these three values, again,

00:03:28.800 --> 00:03:33.400
the Kb value for methylamine
is the highest of the three,

00:03:33.400 --> 00:03:35.550
so it's the strongest base of the three,

00:03:35.550 --> 00:03:38.770
although all three are
considered to be weak.

00:03:38.770 --> 00:03:41.470
Let's do a calculation with a weak base.

00:03:41.470 --> 00:03:44.770
Let's say we have a .14 molar solution,

00:03:44.770 --> 00:03:47.940
aqueous solution of ammonia
at 25 degrees Celsius,

00:03:47.940 --> 00:03:50.720
and our goal is to
calculate the concentration

00:03:50.720 --> 00:03:52.930
of hydroxide ions in solution,

00:03:52.930 --> 00:03:56.930
and also the pH of the
solution at 25 degrees Celsius.

00:03:56.930 --> 00:03:58.240
So first, we're gonna calculate

00:03:58.240 --> 00:04:01.070
the equilibrium concentration
of hydroxide ions,

00:04:01.070 --> 00:04:02.080
and to help us do that,

00:04:02.080 --> 00:04:03.800
we're gonna use an ICE table,

00:04:03.800 --> 00:04:05.910
where I stands for the
initial concentration,

00:04:05.910 --> 00:04:07.770
C is the change in concentration,

00:04:07.770 --> 00:04:10.690
and E is the equilibrium concentration.

00:04:10.690 --> 00:04:14.430
The initial concentration
of ammonia is .14 molar.

00:04:14.430 --> 00:04:16.850
So we can put that in on our ICE table.

00:04:16.850 --> 00:04:20.450
And if we assume that ammonia
hasn't ionized at all yet,

00:04:20.450 --> 00:04:23.050
the initial concentrations of ammonium

00:04:23.050 --> 00:04:26.720
and hydroxide ions would both be zero.

00:04:26.720 --> 00:04:29.500
Next, we think about some
of the ammonia ionizing,

00:04:29.500 --> 00:04:31.090
and we don't know how much,

00:04:31.090 --> 00:04:35.260
so we write minus x under
ammonia for the change.

00:04:35.260 --> 00:04:39.180
When NH3 gains a proton,
it turns into NH4 plus,

00:04:39.180 --> 00:04:43.600
and the mole ratio of ammonia
to ammonium is one to one.

00:04:43.600 --> 00:04:46.720
Therefore, if we write
minus x for ammonia,

00:04:46.720 --> 00:04:50.180
we must write plus x for ammonium.

00:04:50.180 --> 00:04:53.170
And the hydroxide ion also
has a one as a coefficient

00:04:53.170 --> 00:04:54.570
in a balanced equation.

00:04:54.570 --> 00:04:58.340
Therefore, it would also
be plus x in our ICE table.

00:04:58.340 --> 00:05:00.780
So the equilibrium
concentration of ammonia

00:05:00.780 --> 00:05:03.780
would be .14 four minus x.

00:05:03.780 --> 00:05:06.630
So let's write that in there, .14 minus x.

00:05:06.630 --> 00:05:09.130
For ammonium, it would be zero plus x,

00:05:09.130 --> 00:05:11.470
which is just x, and for hydroxide,

00:05:11.470 --> 00:05:15.220
it would also be zero
plus x, which is just x.

00:05:15.220 --> 00:05:16.340
The next step is to write out

00:05:16.340 --> 00:05:18.260
the equilibrium constant expression

00:05:18.260 --> 00:05:20.050
for the ionization of ammonia,

00:05:20.050 --> 00:05:21.410
which we've already talked about.

00:05:21.410 --> 00:05:22.970
And since the concentrations

00:05:22.970 --> 00:05:25.090
in our equilibrium constant expression

00:05:25.090 --> 00:05:27.310
are equilibrium concentrations,

00:05:27.310 --> 00:05:28.610
we can go ahead and plug in

00:05:28.610 --> 00:05:31.650
the equilibrium concentrations
of the ammonium ion,

00:05:31.650 --> 00:05:33.210
the hydroxide ion,

00:05:33.210 --> 00:05:36.840
and ammonia into our
equilibrium constant expression.

00:05:36.840 --> 00:05:39.810
We also need to plug in for Kb for ammonia

00:05:39.810 --> 00:05:42.550
at 25 degrees Celsius.

00:05:42.550 --> 00:05:44.490
Here's our equilibrium constant expression

00:05:44.490 --> 00:05:46.220
with everything plugged in.

00:05:46.220 --> 00:05:48.070
And our goal is to solve for x,

00:05:48.070 --> 00:05:50.970
because x represented the
equilibrium concentration

00:05:50.970 --> 00:05:52.760
of hydroxide ions.

00:05:52.760 --> 00:05:54.820
However, if we solve for x now,

00:05:54.820 --> 00:05:57.830
we would get a quadratic equation.

00:05:57.830 --> 00:05:59.330
So to simplify the math,

00:05:59.330 --> 00:06:01.420
we're gonna use an approximation.

00:06:01.420 --> 00:06:04.450
We're gonna say that .14 minus x

00:06:04.450 --> 00:06:06.947
is approximately equal to .14.

00:06:08.440 --> 00:06:10.300
The reason why we can
make this approximation

00:06:10.300 --> 00:06:14.140
is because the Kb value
for ammonia is quite small,

00:06:14.140 --> 00:06:17.300
which means that ammonia
doesn't ionize very much at all,

00:06:17.300 --> 00:06:19.670
which tells us the
equilibrium concentration

00:06:19.670 --> 00:06:22.870
of hydroxide ions will
be a very small number.

00:06:22.870 --> 00:06:25.857
And if x is a really small
number compared to .14,

00:06:26.897 --> 00:06:29.907
.14 minus x is just
approximately equal to .14.

00:06:31.190 --> 00:06:33.530
So instead of writing .14 minus x,

00:06:33.530 --> 00:06:34.907
we can just write in .14.

00:06:36.000 --> 00:06:37.960
So now we solve for x and we find

00:06:37.960 --> 00:06:41.420
that x is equal to 1.6 times
10 to the negative third molar,

00:06:41.420 --> 00:06:45.900
which is the equilibrium
concentration of hydroxide ions.

00:06:45.900 --> 00:06:48.480
Our goal is to calculate
the pH of our solution.

00:06:48.480 --> 00:06:50.790
And first, we could find the pOH,

00:06:50.790 --> 00:06:53.270
because pOH is equal to the negative log

00:06:53.270 --> 00:06:55.870
of the concentration of hydroxide ions.

00:06:55.870 --> 00:07:00.870
So the negative log of 1.6
times 10 to the negative third

00:07:00.900 --> 00:07:03.663
is equal to 2.80.

00:07:04.920 --> 00:07:07.420
And at 25 degrees Celsius,

00:07:07.420 --> 00:07:12.343
the pH plus the pOH is equal to 14.00,

00:07:14.280 --> 00:07:17.440
so we could plug in the
pOH into our equation

00:07:17.440 --> 00:07:19.220
and solve for the pH,

00:07:19.220 --> 00:07:23.523
and we would find the pH of
our solution is equal to 11.20.

00:07:25.570 --> 00:07:28.870
And this makes sense, because
we have a basic solution,

00:07:28.870 --> 00:07:31.860
so the pH should be greater than seven.

00:07:31.860 --> 00:07:35.200
Finally, to make sure that
we can use our approximation,

00:07:35.200 --> 00:07:39.800
let's find the percent
ionization for our weak base.

00:07:39.800 --> 00:07:43.050
To calculate the percent
ionization of ammonia,

00:07:43.050 --> 00:07:45.170
we take x, which you could think about

00:07:45.170 --> 00:07:47.930
as the concentration of
ammonia that ionized,

00:07:47.930 --> 00:07:48.800
or you could think about it

00:07:48.800 --> 00:07:51.910
as the equilibrium
concentration of hydroxide ions.

00:07:51.910 --> 00:07:55.710
And we divide x by the initial
concentration of ammonia

00:07:55.710 --> 00:07:59.650
and multiply it by 100 to
get a percent ionization.

00:07:59.650 --> 00:08:00.580
So for this problem,

00:08:00.580 --> 00:08:04.400
x is equal to 1.6 times 10
to the negative third molar.

00:08:04.400 --> 00:08:05.840
So we go ahead and plug that in here.

00:08:05.840 --> 00:08:08.740
So 1.6 times 10 to the
negative third molar,

00:08:08.740 --> 00:08:10.750
and the initial concentration of ammonia

00:08:10.750 --> 00:08:13.030
was equal to .14 molar.

00:08:13.030 --> 00:08:15.560
So molar cancels out and we end up

00:08:15.560 --> 00:08:20.560
with a percent ionization of 1.1%.

00:08:21.640 --> 00:08:23.880
Once you calculate the percent ionization,

00:08:23.880 --> 00:08:25.680
you need to think about the 5% rule.

00:08:27.232 --> 00:08:28.450
And the 5% rule says,

00:08:28.450 --> 00:08:31.874
if your percent ionization
is less than 5%,

00:08:31.874 --> 00:08:34.300
it's okay to use the approximation.

00:08:34.300 --> 00:08:38.130
So in our case, our percent
ionization was 1.1%,

00:08:38.130 --> 00:08:40.060
which is less than 5%.

00:08:40.060 --> 00:08:42.890
So it's okay to say that .14 minus x

00:08:42.890 --> 00:08:45.460
is approximately equal to .14,

00:08:45.460 --> 00:08:47.590
because with such a
low percent ionization,

00:08:47.590 --> 00:08:50.217
x is a really small value compared to .14.

00:08:51.130 --> 00:08:55.500
However, if you get a percent
ionization greater than 5%,

00:08:55.500 --> 00:08:57.540
you can't use the approximation

00:08:57.540 --> 00:09:01.890
and you need to use the quadratic
equation to calculate x.

00:09:01.890 --> 00:09:04.630
Finally, when we calculated
our equilibrium concentration

00:09:04.630 --> 00:09:06.050
of hydroxide ions,

00:09:06.050 --> 00:09:09.730
we only considered the
ionization of ammonia.

00:09:09.730 --> 00:09:12.050
It's true that the autoionization of water

00:09:12.050 --> 00:09:15.180
would produce a small amount
of hydroxide ions in solution.

00:09:15.180 --> 00:09:18.400
However, that concentration
is extremely small compared

00:09:18.400 --> 00:09:20.940
to the concentration of hydroxide ions

00:09:20.940 --> 00:09:22.460
from the ionization of ammonia.

00:09:22.460 --> 00:09:25.800
Therefore, we ignore the
contribution of hydroxide ions

00:09:25.800 --> 00:09:29.203
from the auto ionization of
water in our calculations.


Worked example: Finding the percent ionization of a weak acid

WEBVTT
Kind: captions
Language: en

00:00:00.420 --> 00:00:01.253
- [Instructor] Let's say we have

00:00:01.253 --> 00:00:04.780
a 0.20 Molar aqueous
solution of acidic acid.

00:00:04.780 --> 00:00:07.120
And our goal is to calculate the pH

00:00:07.120 --> 00:00:09.840
and the percent ionization.

00:00:09.840 --> 00:00:11.970
The Ka value for acidic acid

00:00:11.970 --> 00:00:14.630
is equal to 1.8 times
10 to the negative fifth

00:00:14.630 --> 00:00:16.970
at 25 degrees Celsius.

00:00:16.970 --> 00:00:19.210
First, we need to write out
the balanced equation showing

00:00:19.210 --> 00:00:21.810
the ionization of acidic acid.

00:00:21.810 --> 00:00:25.277
So acidic acid reacts with
water to form the hydronium ion,

00:00:25.277 --> 00:00:26.570
H3O+,

00:00:26.570 --> 00:00:30.350
and acetate, which is the
conjugate base to acidic acid.

00:00:30.350 --> 00:00:32.150
Because acidic acid is a weak acid,

00:00:32.150 --> 00:00:34.500
it only partially ionizes.

00:00:34.500 --> 00:00:36.780
Therefore, we need to set up an ICE table

00:00:36.780 --> 00:00:37.730
so we can figure out

00:00:37.730 --> 00:00:41.000
the equilibrium concentration
of hydronium ion,

00:00:41.000 --> 00:00:43.010
which will allow us to calculate the pH

00:00:43.010 --> 00:00:45.060
and the percent ionization.

00:00:45.060 --> 00:00:48.250
In an ICE table, the I stands
for initial concentration,

00:00:48.250 --> 00:00:50.230
C is for change in concentration,

00:00:50.230 --> 00:00:53.140
and E is equilibrium concentration.

00:00:53.140 --> 00:00:57.240
The initial concentration of
acidic acid is 0.20 Molar.

00:00:57.240 --> 00:01:00.720
So we can put that in our
ICE table under acidic acid.

00:01:00.720 --> 00:01:03.850
And if we assume that the
reaction hasn't happened yet,

00:01:03.850 --> 00:01:06.160
the initial concentrations
of hydronium ion

00:01:06.160 --> 00:01:09.560
and acetate anion would both be zero.

00:01:09.560 --> 00:01:11.920
Some of the acidic acid will ionize,

00:01:11.920 --> 00:01:13.270
but since we don't know how much,

00:01:13.270 --> 00:01:14.600
we're gonna call that x.

00:01:14.600 --> 00:01:17.590
So we write -x under acidic acid

00:01:17.590 --> 00:01:20.010
for the change part of our ICE table.

00:01:20.010 --> 00:01:21.910
And when acidic acid reacts with water,

00:01:21.910 --> 00:01:24.830
we form hydronium and acetate.

00:01:24.830 --> 00:01:27.940
So we're going to gain in
the amount of our products.

00:01:27.940 --> 00:01:30.040
To figure out how much
we look at mole ratios

00:01:30.040 --> 00:01:31.380
from the balanced equation.

00:01:31.380 --> 00:01:33.650
There's a one to one mole ratio

00:01:33.650 --> 00:01:36.280
of acidic acid to hydronium ion.

00:01:36.280 --> 00:01:39.510
Therefore, if we write -x for acidic acid,

00:01:39.510 --> 00:01:42.360
we're gonna write +x under hydronium.

00:01:42.360 --> 00:01:45.010
And for the acetate
anion, there's also a one

00:01:45.010 --> 00:01:47.460
as a coefficient in the balanced equation.

00:01:47.460 --> 00:01:51.470
Therefore, we can write
+x under acetate as well.

00:01:51.470 --> 00:01:54.460
So the equilibrium
concentration of acidic acid

00:01:54.460 --> 00:01:56.820
would be 0.20 minus x.

00:01:56.820 --> 00:01:58.230
Let's go ahead and write that in here,

00:01:58.230 --> 00:02:00.580
0.20 minus x.

00:02:00.580 --> 00:02:02.560
The equilibrium concentration of hydronium

00:02:02.560 --> 00:02:05.550
would be zero plus x, which is just x.

00:02:05.550 --> 00:02:08.450
And for acetate, it would
also be zero plus x,

00:02:08.450 --> 00:02:10.720
so we can just write x here.

00:02:10.720 --> 00:02:13.310
Next, we brought out the
equilibrium constant expression,

00:02:13.310 --> 00:02:15.680
which we can get from
the balanced equation.

00:02:15.680 --> 00:02:18.530
So the Ka is equal to the concentration

00:02:18.530 --> 00:02:20.320
of the hydronium ion.

00:02:20.320 --> 00:02:21.840
And since there's a coefficient of one,

00:02:21.840 --> 00:02:22.680
that's the concentration

00:02:22.680 --> 00:02:24.870
of hydronium ion raised
to the first power,

00:02:24.870 --> 00:02:27.600
times the concentration
of the acetate anion

00:02:27.600 --> 00:02:29.600
also raised to the first power,

00:02:29.600 --> 00:02:31.370
divided by the concentration

00:02:31.370 --> 00:02:34.270
of acidic acid raised to the first power.

00:02:34.270 --> 00:02:35.630
And water is left out of

00:02:35.630 --> 00:02:38.070
our equilibrium constant expression.

00:02:38.070 --> 00:02:39.260
Because the concentrations

00:02:39.260 --> 00:02:41.130
in our equilibrium constant expression

00:02:41.130 --> 00:02:43.290
or equilibrium concentrations,

00:02:43.290 --> 00:02:45.750
we can plug in what we
have from our ICE table.

00:02:45.750 --> 00:02:48.470
So we can plug in x for the
equilibrium concentration

00:02:48.470 --> 00:02:50.030
of hydronium ion,

00:02:50.030 --> 00:02:52.330
x is also the equilibrium concentration

00:02:52.330 --> 00:02:53.690
of the acetate anion,

00:02:53.690 --> 00:02:57.570
and 0.20 minus x is the
equilibrium concentration

00:02:57.570 --> 00:02:58.950
of acidic acid.

00:02:58.950 --> 00:03:02.170
We also need to plug in the
Ka value for acidic acid

00:03:02.170 --> 00:03:04.940
at 25 degrees Celsius.

00:03:04.940 --> 00:03:07.580
Here we have our equilibrium
concentrations plugged in

00:03:07.580 --> 00:03:09.440
and also the Ka value.

00:03:09.440 --> 00:03:10.970
Our goal is to solve for x,

00:03:10.970 --> 00:03:13.190
which would give us the
equilibrium concentration

00:03:13.190 --> 00:03:14.760
of hydronium ions.

00:03:14.760 --> 00:03:17.030
However, if we solve for x here,

00:03:17.030 --> 00:03:19.940
we would need to use a quadratic equation.

00:03:19.940 --> 00:03:21.730
So to make the math a little bit easier,

00:03:21.730 --> 00:03:23.670
we're gonna use an approximation.

00:03:23.670 --> 00:03:27.520
We're gonna say that 0.20 minus x

00:03:27.520 --> 00:03:31.990
is approximately equal to 0.20.

00:03:31.990 --> 00:03:34.080
The reason why we can
make this approximation

00:03:34.080 --> 00:03:36.350
is because acidic acid is a weak acid,

00:03:36.350 --> 00:03:38.550
which we know from its Ka value.

00:03:38.550 --> 00:03:40.080
Ka is less than one.

00:03:40.080 --> 00:03:42.870
And that means it's only
going to partially ionize.

00:03:42.870 --> 00:03:45.400
It's going to ionize
to a very small extent,

00:03:45.400 --> 00:03:48.350
which means that x must
be a very small number.

00:03:48.350 --> 00:03:51.700
And if x is a really small
number compared to 0.20,

00:03:52.539 --> 00:03:56.290
0.20 minus x is approximately
just equal to 0.20.

00:03:57.300 --> 00:03:59.280
So we can go ahead and rewrite this.

00:03:59.280 --> 00:04:02.910
So we would have 1.8 times
10 to the negative fifth

00:04:02.910 --> 00:04:05.510
is equal to x squared over,

00:04:05.510 --> 00:04:07.640
and instead of 0.20 minus x,

00:04:07.640 --> 00:04:09.253
we're just gonna write 0.20.

00:04:10.970 --> 00:04:15.970
Solving for x, we would
find that x is equal to 1.9,

00:04:15.970 --> 00:04:18.790
times 10 to the negative third.

00:04:18.790 --> 00:04:23.170
And remember, this is equal to
the equilibrium concentration

00:04:23.170 --> 00:04:25.000
of hydronium ions.

00:04:25.000 --> 00:04:25.833
So let's write in here,

00:04:25.833 --> 00:04:28.590
the equilibrium concentration
of hydronium ions.

00:04:28.590 --> 00:04:32.100
So this is 1.9 times 10 to
the negative third Molar.

00:04:32.100 --> 00:04:34.820
If we would have used the
quadratic equation to solve for x,

00:04:34.820 --> 00:04:38.070
we would have also gotten 1.9
times 10 to the negative third

00:04:38.070 --> 00:04:40.160
to two significant figures.

00:04:40.160 --> 00:04:43.390
Therefore, using the approximation
got us the same answer

00:04:43.390 --> 00:04:45.320
and saved us some time.

00:04:45.320 --> 00:04:47.130
Also, now that we have a value for x,

00:04:47.130 --> 00:04:48.840
we can go back to our approximation

00:04:48.840 --> 00:04:52.930
and see that x is very
small compared to 0.20.

00:04:52.930 --> 00:04:57.570
So 0.20 minus x is
approximately equal to 0.20.

00:04:57.570 --> 00:05:00.580
Also, this concentration of hydronium ion

00:05:00.580 --> 00:05:03.360
is only from the
ionization of acidic acid.

00:05:03.360 --> 00:05:05.650
And it's true that
there's some contribution

00:05:05.650 --> 00:05:08.690
of hydronium ion from the
autoionization of water.

00:05:08.690 --> 00:05:11.680
However, that concentration
is much smaller than this.

00:05:11.680 --> 00:05:14.330
So for this problem, we
can ignore the contribution

00:05:14.330 --> 00:05:17.900
of hydronium ions from the
autoionization of water.

00:05:17.900 --> 00:05:20.030
Next, we can find the pH of our solution

00:05:20.030 --> 00:05:21.900
at 25 degrees Celsius.

00:05:21.900 --> 00:05:25.140
So pH is equal to the negative
log of the concentration

00:05:25.140 --> 00:05:26.510
of hydronium ions.

00:05:26.510 --> 00:05:30.530
So that's the negative log of 1.9 times 10

00:05:30.530 --> 00:05:32.600
to the negative third,

00:05:32.600 --> 00:05:35.603
which is equal to 2.72.

00:05:37.730 --> 00:05:40.940
We also need to calculate
the percent ionization.

00:05:40.940 --> 00:05:44.520
So the equation 4% ionization is equal to

00:05:44.520 --> 00:05:47.770
the equilibrium concentration
of hydronium ions,

00:05:47.770 --> 00:05:51.070
divided by the initial
concentration of the acid,

00:05:51.070 --> 00:05:52.900
times 100%.

00:05:52.900 --> 00:05:55.180
The equilibrium concentration
of hydronium ions

00:05:55.180 --> 00:05:58.540
is equal to 1.9 times 10
to negative third Molar.

00:05:58.540 --> 00:05:59.980
So we plug that in.

00:05:59.980 --> 00:06:03.520
And the initial concentration
of our weak acid,

00:06:03.520 --> 00:06:06.590
which was acidic acid is 0.20 Molar.

00:06:06.590 --> 00:06:07.780
So the Molars cancel,

00:06:07.780 --> 00:06:12.780
and we get a percent ionization of 0.95%.

00:06:12.950 --> 00:06:15.540
A low value for the percent
ionization makes sense

00:06:15.540 --> 00:06:18.750
because acidic acid is a weak acid.

00:06:18.750 --> 00:06:22.420
We can also use the percent
ionization to justify

00:06:22.420 --> 00:06:25.110
the approximation that
we made earlier using

00:06:25.110 --> 00:06:26.853
what's called the 5% rule.

00:06:28.100 --> 00:06:31.470
So let me write that
down here, the 5% rule.

00:06:31.470 --> 00:06:35.641
If the percent ionization is less than 5%

00:06:35.641 --> 00:06:38.690
as it was in our case, it
was less than 1% actually,

00:06:38.690 --> 00:06:41.110
then the approximation is valid.

00:06:41.110 --> 00:06:44.592
If the percent ionization
is greater than 5%,

00:06:44.592 --> 00:06:46.470
then the approximation is not valid

00:06:46.470 --> 00:06:49.033
and you have to use
the quadratic equation.