Pathogens and the environment

WEBVTT
Kind: captions
Language: en

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- [Instructor] In this video,

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we're gonna be talking about pathogens

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and how an environment might help

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or hurt the spread of a pathogen.

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So first of all, let's make
sure we know what a pathogen is.

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Patho comes from Greek pathos,

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which is referring to disease.

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Gen, you might recognize that part of word

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from words like generate or genetics.

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It means to produce.

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So a pathogen is something
that produces disease.

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Now, a good example of a pathogen

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that we're still facing
on our planet is malaria.

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And malaria is a very,
very unpleasant disease.

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It involves the attack on red blood cells.

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It causes fever, chills, sweats,

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severe abdominal pain,
vomiting, headaches.

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It can make you more
susceptible to other diseases.

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And if it's not treated,
it can kill someone.

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Now, the pathogen that creates
or that produces malaria,

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that causes the disease malaria is known

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as plasmodium falciparum.

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And this right over here is a picture

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of it attacking red blood cells.

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We see healthy red blood
cells right over here,

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and then the ones that
are in this deep color,

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this is being attacked
by the malaria pathogen.

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And as it does that, it
destroys those red blood cells,

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and it leads to all of the
symptoms that I talked about.

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So let's think a little
bit about the environment

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in which malaria is likely to be spread.

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Well, there's a few things that we know.

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We know that the malaria
pathogen can only operate,

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can only go through its full life cycle

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in relatively warm conditions.

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It needs to be greater
than 20 degrees Celsius,

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which is the same thing
as 68 degrees Fahrenheit.

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We also know that it is
spread through mosquitoes,

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in particular this
mosquito right over here,

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the anopheles mosquito.

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And mosquitoes are unpleasant even

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when they aren't zoomed in like this,

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but this is actually quite frightening.

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And we all know what
mosquitoes like to do,

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at least to human beings.

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They drink our blood.

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And so you need an environment

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where both mosquitoes can thrive,

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and there's a lot of human hosts

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whose blood they can drink

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and where they can spread
the malaria pathogen

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from one host to another.

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So ideally, you'd like a
high population density.

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And for the mosquito spread,

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you definitely want a hot
and humid environment.

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So you might guess that if
you're looking for things

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that don't dip below this,
because if you dip below this,

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the malaria pathogen's not going

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to be able to go through its life cycle,

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you're likely looking at
regions in the Equator.

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And this map right over
here confirms our intuition.

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What you see in these orange regions are

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where you see the highest
prevalence of malaria.

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The yellow regions are
where you might see malaria,

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but not as high of a prevalence.

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And then the blue areas are
where you don't see malaria.

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And as we guessed, if you look generally

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at where the Equator is,

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that is where you're likely to see malaria

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because it is warmer there.

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You don't see temperatures dip.

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And then you also see
hot, humid environments

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in Sub-Saharan Africa.

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The Sahara Desert is
roughly right over there,

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and that is very dry,
not good for mosquitoes.

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But if you look at
Sub-Sahara right below this,

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you have a high prevalence of malaria.

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Now, one not so comforting realization is

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as the Earth warms, you're
going to have more regions

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that are not just tropical regions,

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not just equatorial regions,

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that are going to be suitable
for the spread of malaria.

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So easily you could have

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what are traditionally
sub-tropical environments

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or even temperate environments

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that if it gets warm and
hot and humid enough,

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you might see the spread
of malaria over time.

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And malaria and the malaria
pathogen aren't the only things

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that might spread.

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You have things like the
Zika virus, which is a virus,

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not a protist in the case of malaria,

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but a virus that's also
spread by mosquitoes.

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So it also thrives

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wherever mosquitoes
might be able to thrive.

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And a warming, more humid
environment isn't just going

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to support the spread of mosquitoes.

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It can support the spread of
other pathogens, like bacteria,

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which can cause diseases like cholera,

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which is caused by infected water.

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So I'll leave you there.

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I like to be a little bit
more upbeat about things.

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But it's just good to realize
that this is out there.

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And as our environment changes,

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it's not just the
environment that it affects,

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but it can also affect
the spread of disease.


Nonrenewable Energy Resources

WEBVTT
Kind: captions
Language: en

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- Today, let's talk
about energy resources.

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You've probably already
done something today

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that used energy resources.

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Even beginning from
the moment you woke up.

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For me, the beginning of my day,

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always starts with making tea.

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I use energy and every
step of this process,

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my car uses oil.

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When I drive to the grocery
store to get the tea,

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my stove uses natural
gas when I boil the water

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and my water heater uses
coal sourced electricity.

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When I wash my mug in
hot water afterwards,

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we use energy constantly
in our daily lives.

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Since the first law of
thermodynamics states that

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"energy cannot be created nor destroyed."

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Where does it all come from?

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And will we have enough?

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When we talk about energy resources,

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we could split it up into two groups,

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renewable energy and non renewable energy.

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I can always remember what
renewable resources are

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because the prefix re means again,

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and the root new refers to the
origin of the energy source.

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So renewable energy sources,

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are the sources that we
can use again and again,

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and are quickly restored
by natural processes.

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Renewable fuel sources include

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sunlight, wind, moving water,

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biomass from fast growing plants

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and geothermal heat from the earth.

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The lifespan of renewable
resources looks like a circle.

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We can use them and then
we can use them again

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without worrying about them running out.

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Non-renewable energy
sources on the other hand,

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are sources that exist in a fixed amount

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and cannot be easily replaced.

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These energy sources must
be extracted from the earth

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and they include things like
nuclear fuel and fossil fuels,

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which are things like
coal oil and natural gas.

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Fossil fuels performed
in the geologic past

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from the remains of ancient organisms,

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plants and animals that
died millions of years ago,

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became buried in the soil,

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partially decomposed,

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and were exposed to a
lot of heat and pressure.

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This heat and pressure
chemically rearranged

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the energy contained within their bodies

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into the fossil fuels we use today.

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Because they take so long to form,

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we have a finite amount
of non-renewable resources

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in the earth right now.

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The lifespan for fossil
fuels is a broken loop,

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a one-way ticket.

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First, the fossil fuel is found,

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then it has to be extracted.

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Extracting fossil fuels can involve

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destructive mining processes

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that can pollute surrounding habitats.

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It then has to be transported
to wherever it will be used.

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And using fossil fuels means burning them,

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which releases greenhouse gases

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and other pollutants into that atmosphere.

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Also, it can take energy to refine

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and process some types of fossil fuels,

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so that they're more helpful to us

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like turning crude oil
into petroleum gas, diesel,

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and jet fuel.

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And burning the fossil
fuels transforms that energy

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into a less organized form of energy,

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like heat and ash.

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Not all fossil fuels are the same.

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And this is because

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different kinds of organic material

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were heated and compressed
in different ways.

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Creating different kinds of fossil fuels.

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One of these kinds of
fossil fuels is petroleum.

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The word petroleum comes from
the Latin word for rock Petra

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and the Latin word for oil Oleum.

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This makes sense because
it's an oily substance

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that's found in reservoirs,
trapped in rock.

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Humans have been using
oil for a very long time.

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Early civilizations found it
bubbling up from the ground

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at natural Wells,

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the oil they saw coming
straight out of the earth

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is called crude oil or crude petroleum,

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which means that it is
unrefined and unprocessed.

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Crude oil was probably one of
the ingredients in Greek fire,

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which was a flame throwing
up in the Byzantines

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with fling at their enemies.

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Today we were fine crude
oil into many products

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like gasoline and diesel for vehicles,

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kerosene for heating and
asphalts for road construction.

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It's also used to make
plastics and synthetic fabrics.

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Another type of fossil fuel is coal,

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which we mostly burned
to make electricity,

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but it can also be used
for heating and cooking.

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It's a solid, shiny black flammable rock,

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which is mostly formed
from the carpenter remains

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of fossilized plants like MOS.

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Over thousands of years,

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plant material transforms

00:04:19.050 --> 00:04:22.060
into a carbon rich compound called peat.

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And then with some pressure,
it becomes soft coal,

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which is called lignite.

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And then with more pressure,
it becomes bituminous coal.

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And then finally it becomes a
hard coal called anthracite.

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The longer the plant remains
are put under pressure,

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the more energy dense it becomes.

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So if you burned anthracite,

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you would get a lot more
energy than if you burned peat,

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burning coal releases carbon
dioxide, sulfur dioxide,

00:04:47.780 --> 00:04:51.330
nitrogen oxides, and
other gases into the air.

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So coal is one of the
dirtiest fossil fuels.

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The cleanest burning
fossil fuel is natural gas,

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meaning that it admits the
least amount of carbon dioxide

00:05:00.810 --> 00:05:02.600
or other air prints.

00:05:02.600 --> 00:05:04.350
This is not the same type of gas

00:05:04.350 --> 00:05:06.130
as we put into our vehicles.

00:05:06.130 --> 00:05:07.940
That type comes from petroleum.

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Instead, natural gas is mostly used

00:05:10.140 --> 00:05:11.800
for generating electricity,

00:05:11.800 --> 00:05:14.600
heating homes and cooking food.

00:05:14.600 --> 00:05:17.230
Your oven, furnace, water heater, dryer,

00:05:17.230 --> 00:05:20.150
or grill all might run on natural gas.

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Natural gas is a naturally
occurring mixture of gases.

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And it's mostly made
up of methane, ethane,

00:05:26.570 --> 00:05:28.760
propane and butane.

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The gas is colorless and odorless.

00:05:31.530 --> 00:05:34.290
So natural gas companies
often add a rotten egg smell

00:05:35.125 --> 00:05:37.520
to the fuel to make
leaks easier to detect.

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But even though we can't
see or smell natural gas,

00:05:40.510 --> 00:05:44.320
humans have been detecting it
and using it for centuries.

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There is evidence that
more than 2000 years ago,

00:05:47.150 --> 00:05:49.530
people in China created bamboo pipelines

00:05:49.530 --> 00:05:51.010
to transport the gas,

00:05:51.010 --> 00:05:53.150
so that they could use
it to boil saltwater

00:05:53.150 --> 00:05:55.210
and separate out the salt.

00:05:55.210 --> 00:05:57.190
And today we still use pipelines

00:05:57.190 --> 00:05:59.010
to transport gas across land.

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Although they're no
longer made out of bamboo.

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Another kind of non-renewable
resource is nuclear fuel.

00:06:06.500 --> 00:06:09.800
Nuclear power plants
don't burn fossil fuels.

00:06:09.800 --> 00:06:12.070
Instead they split uranium

00:06:12.070 --> 00:06:14.750
through something called nuclear fission,

00:06:14.750 --> 00:06:17.070
just one kilogram of uranium,

00:06:17.070 --> 00:06:21.060
can produce 24 million
kilowatt hours of energy.

00:06:21.060 --> 00:06:23.280
Compare that with one kilogram of coal,

00:06:23.280 --> 00:06:26.630
which can produce eight
kilowatt hours of energy.

00:06:26.630 --> 00:06:30.710
In other words, nuclear fusion
produces a lot of energy.

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It also produces a lot of heat,

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which we mostly use to make electricity.

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Also nuclear energy is
known as a clean energy

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because the only greenhouse
gas emission is water vapor,

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which tends to cycle out
of the atmosphere faster

00:06:44.960 --> 00:06:47.170
than other greenhouse gases.

00:06:47.170 --> 00:06:49.320
But the downside is the spent fuel

00:06:49.320 --> 00:06:51.790
is a dangerous radioactive waste

00:06:51.790 --> 00:06:55.673
and recycling the nuclear waste
is dangerous and expensive.

00:06:57.320 --> 00:07:00.810
Petroleum, coal, natural
gas and nuclear fuel

00:07:00.810 --> 00:07:04.300
make up 85% of the world's
energy consumption.

00:07:04.300 --> 00:07:06.890
But as our populations
and our energy needs grow,

00:07:06.890 --> 00:07:08.270
it's becoming more important

00:07:08.270 --> 00:07:10.100
to integrate renewable resources.

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The sources that could be
restored by natural processes

00:07:12.780 --> 00:07:14.550
into our energy mix.

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I'm gonna go make some
tea now, but before I do,

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here's something to think about.

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What's something that you do
in your day that uses energy?

00:07:21.750 --> 00:07:24.120
Does that energy come from fossil fuels?

00:07:24.120 --> 00:07:26.690
And if so, which one?

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Understanding how you get your energy

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is part of understanding
what powers your community,

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your country, and the world.


Integrated pest management

WEBVTT
Kind: captions
Language: en

00:00:00.430 --> 00:00:02.360
- [Instructor] Let's
imagine your corn farmer

00:00:02.360 --> 00:00:05.120
and you would dream of having
nice, healthy corn crop,

00:00:05.120 --> 00:00:06.860
like we see in this picture.

00:00:06.860 --> 00:00:11.380
But being a farmer isn't as
simple as planting the seeds

00:00:11.380 --> 00:00:14.610
and making sure that the
crop gets enough sunshine

00:00:14.610 --> 00:00:19.610
and water and fertilizer, you
also have to deal with pests.

00:00:20.290 --> 00:00:22.840
And pests you can view as anything

00:00:22.840 --> 00:00:25.310
that might eat your crop
or destroy your crop,

00:00:25.310 --> 00:00:28.980
or make it just really hard
for you to have a good crop.

00:00:28.980 --> 00:00:30.120
Now, some of you all are familiar

00:00:30.120 --> 00:00:31.790
with the notion of pesticides,

00:00:31.790 --> 00:00:34.260
where you're essentially
poisoning the pests.

00:00:34.260 --> 00:00:37.250
Now for various reasons, that
might not be a good idea,

00:00:37.250 --> 00:00:38.600
and we'll talk more about it.

00:00:38.600 --> 00:00:42.410
And so there's a notion of
integrated pest management,

00:00:42.410 --> 00:00:45.610
which is how we can do multiple
things which might include

00:00:45.610 --> 00:00:48.810
pesticides or might not include
pesticides, to really keep

00:00:48.810 --> 00:00:50.610
things under control.

00:00:50.610 --> 00:00:54.680
And to get a good sense
of how bad a pest can be,

00:00:54.680 --> 00:00:59.003
I will introduce you to
the Western corn rootworm.

00:00:59.920 --> 00:01:01.430
Now, the reason why it's called a worm,

00:01:01.430 --> 00:01:02.650
even though in its adult phase,

00:01:02.650 --> 00:01:05.790
it does not look like a worm,
is that in its larval phase,

00:01:05.790 --> 00:01:08.220
when it's a baby, it looks like a worm.

00:01:08.220 --> 00:01:12.040
And even though the adult will
also feed on the corn silks,

00:01:12.040 --> 00:01:14.780
the real damage is done
in the larval stage.

00:01:14.780 --> 00:01:16.720
When they're feeding on the roots,

00:01:16.720 --> 00:01:20.080
that's why they're called corn rootworms.

00:01:20.080 --> 00:01:25.040
And the USDA estimates that
just in the United States,

00:01:25.040 --> 00:01:28.927
damage due to Western
corn rootworm is about

00:01:28.927 --> 00:01:33.020
$1 billion per year.

00:01:33.020 --> 00:01:36.790
About 800 million of that,
is just lost corn crop.

00:01:36.790 --> 00:01:40.360
And about 200 million of that
is all the costs in order

00:01:40.360 --> 00:01:44.500
to prevent this pest
from doing their damage.

00:01:44.500 --> 00:01:45.880
Now, as I mentioned, if we think about

00:01:45.880 --> 00:01:47.270
integrated pest management,

00:01:47.270 --> 00:01:49.960
there's many ways of addressing this.

00:01:49.960 --> 00:01:53.070
One way of course, that many
people think of, is pesticides.

00:01:53.070 --> 00:01:56.690
And this is an image of crop dusting.

00:01:56.690 --> 00:01:58.340
I don't think this is a corn field here,

00:01:58.340 --> 00:02:00.750
we can see the pesticides
coming from the biplane.

00:02:00.750 --> 00:02:02.570
And it's a way of
spreading the pesticides,

00:02:02.570 --> 00:02:05.180
not the only way, but for various reasons,

00:02:05.180 --> 00:02:06.610
this is not optimal.

00:02:06.610 --> 00:02:09.080
You could view it as
polluting the environment

00:02:09.080 --> 00:02:10.170
in certain ways.

00:02:10.170 --> 00:02:12.560
A lot of us aren't so keen on eating crops

00:02:12.560 --> 00:02:14.450
that have pesticides on them.

00:02:14.450 --> 00:02:18.030
And we've learned in other
videos on evolution and natural

00:02:18.030 --> 00:02:20.560
selection, there's
variation in any population.

00:02:20.560 --> 00:02:23.050
And the more that you expose
a population to something,

00:02:23.050 --> 00:02:25.040
and it kills off the ones
that are susceptible,

00:02:25.040 --> 00:02:27.570
the more chances that the ones
that aren't as susceptible,

00:02:27.570 --> 00:02:29.230
reproduce and thrive.

00:02:29.230 --> 00:02:32.930
So at some point you might
have an insecticide resistant

00:02:32.930 --> 00:02:35.080
pest on your hands and
then your insecticide,

00:02:35.080 --> 00:02:37.530
isn't going to be all that useful.

00:02:37.530 --> 00:02:39.540
So when we think about
integrated pest management,

00:02:39.540 --> 00:02:40.373
we think about art,

00:02:40.373 --> 00:02:41.830
what are other ways to address this?

00:02:41.830 --> 00:02:43.860
Maybe in conjunction, maybe ways

00:02:43.860 --> 00:02:46.600
that could be perceived
to be more natural?

00:02:46.600 --> 00:02:48.880
Well one obvious thing
is to think about, well,

00:02:48.880 --> 00:02:51.320
what are the predators of that pest?

00:02:51.320 --> 00:02:53.580
And this is the wasp spider,

00:02:53.580 --> 00:02:57.290
and this is what's known as
a ladybug in North America

00:02:57.290 --> 00:02:59.680
and ladybird in much of
the rest of the world.

00:02:59.680 --> 00:03:04.300
And they will actually feed
on the Western corn rootworm.

00:03:04.300 --> 00:03:07.070
So you might want to
introduce this onto your farm

00:03:07.070 --> 00:03:09.350
if you're trying to grow that corn.

00:03:09.350 --> 00:03:11.340
Another thing that farmers have done

00:03:11.340 --> 00:03:14.440
to some degree of success,
is early planting.

00:03:14.440 --> 00:03:16.490
By planting the crop early,

00:03:16.490 --> 00:03:20.400
it's less favorable for the
root worm to lay their eggs.

00:03:20.400 --> 00:03:24.350
And also it gives a chance
for the roots to get stronger,

00:03:24.350 --> 00:03:29.350
so that by the time that they're
attacked by the rootworm,

00:03:29.530 --> 00:03:31.600
they're going to be more resilient.

00:03:31.600 --> 00:03:33.940
You could also have
hybrid versions of corn

00:03:33.940 --> 00:03:36.250
that just have larger roots.

00:03:36.250 --> 00:03:38.020
So once again, they're going to be more

00:03:38.020 --> 00:03:40.200
resilient to the rootworm.

00:03:40.200 --> 00:03:43.400
Another technique is to
genetically modify the corn.

00:03:43.400 --> 00:03:45.760
And that also is a bit
of a controversial topic,

00:03:45.760 --> 00:03:47.370
depending on who you talk to.

00:03:47.370 --> 00:03:49.860
You might've heard things about GMO,

00:03:49.860 --> 00:03:52.160
genetically modified organism.

00:03:52.160 --> 00:03:53.680
Some people think they're fine.

00:03:53.680 --> 00:03:55.320
Some people are wary of them.

00:03:55.320 --> 00:03:57.890
But there is a transgenic version of corn.

00:03:57.890 --> 00:04:02.020
Transgenic means, that we took
DNA from some other organism

00:04:02.020 --> 00:04:03.470
and put it into the corn.

00:04:03.470 --> 00:04:07.970
And that transgenic version
is known as BT corn.

00:04:07.970 --> 00:04:10.690
That actually has DNA from a bacteria,

00:04:10.690 --> 00:04:12.890
which produces a protein,

00:04:12.890 --> 00:04:15.790
which is harmful to certain
types of root worms.

00:04:15.790 --> 00:04:17.880
Now, another really interesting technique,

00:04:17.880 --> 00:04:20.280
which people have used to
some degree of success,

00:04:20.280 --> 00:04:22.380
is known as crop rotation.

00:04:22.380 --> 00:04:24.450
Let's imagine looking down from the sky

00:04:24.450 --> 00:04:26.290
onto a corn crop here.

00:04:26.290 --> 00:04:28.290
So we're looking down from above.

00:04:28.290 --> 00:04:30.310
So what normally happens is,

00:04:30.310 --> 00:04:33.800
is that the eggs are laid near the corn

00:04:33.800 --> 00:04:36.090
and the larvae can't travel so far.

00:04:36.090 --> 00:04:37.930
So they, if they're near the corn,

00:04:37.930 --> 00:04:39.760
they get to the roots
and then they're able to,

00:04:39.760 --> 00:04:42.170
the reproduction cycle goes on.

00:04:42.170 --> 00:04:45.850
But if we rotate the crops so
that the next year we're not

00:04:45.850 --> 00:04:50.850
planting here, but maybe we
are planting the corn over here

00:04:50.980 --> 00:04:53.720
and then in that first place,
we're planting soybean.

00:04:53.720 --> 00:04:55.520
Well, that can solve the problem,

00:04:55.520 --> 00:04:57.080
because the eggs will be laid here.

00:04:57.080 --> 00:04:59.980
But next year the corn
is going to be here.

00:04:59.980 --> 00:05:02.820
And over here, we will have the soybean,

00:05:02.820 --> 00:05:06.890
which is not a suitable
food for the corn rootworm.

00:05:06.890 --> 00:05:08.250
Now what's really interesting,

00:05:08.250 --> 00:05:10.600
and this is another one of
those fascinating stories

00:05:10.600 --> 00:05:12.540
of evolution and natural selection,

00:05:12.540 --> 00:05:15.920
are there cases because there's
variation in populations,

00:05:15.920 --> 00:05:20.920
where some of the adult rootworm
actually doesn't lay their

00:05:21.320 --> 00:05:24.860
eggs in the same corn
crop, where they feed,

00:05:24.860 --> 00:05:27.240
they lay it in an adjacent corn crop.

00:05:27.240 --> 00:05:30.830
So that next year when
the crop rotation happens,

00:05:30.830 --> 00:05:34.170
then their larva are going
to have access to the corn

00:05:34.170 --> 00:05:35.620
in the other field.

00:05:35.620 --> 00:05:37.190
Now you might say, how did they know that?

00:05:37.190 --> 00:05:40.010
How did they know that the
farmers were planning to rotate?

00:05:40.010 --> 00:05:41.930
Well, generally the
rotations are pretty simple.

00:05:41.930 --> 00:05:43.920
You keep alternating between two crops

00:05:43.920 --> 00:05:45.470
like corn and soybean.

00:05:45.470 --> 00:05:48.810
And most of the Western corn rootworm

00:05:48.810 --> 00:05:50.010
would not have done that,

00:05:50.010 --> 00:05:52.880
but just some of them might
have started to randomly travel

00:05:52.880 --> 00:05:53.800
a little bit further.

00:05:53.800 --> 00:05:56.410
And then those would be
more likely to reproduce.

00:05:56.410 --> 00:05:58.920
So the ones are more likely to reproduce

00:05:58.920 --> 00:05:59.753
will then exist more.

00:05:59.753 --> 00:06:01.640
And then that behavior would be passed on,

00:06:01.640 --> 00:06:03.800
if it is a genetic trait.

00:06:03.800 --> 00:06:05.480
There's other situations once again,

00:06:05.480 --> 00:06:07.440
because of the variation in populations,

00:06:07.440 --> 00:06:10.680
where some of the eggs stay
dormant for an extra year.

00:06:10.680 --> 00:06:13.380
So even though you might
rotate to corn this year,

00:06:13.380 --> 00:06:15.440
the year after that,
you rotate back to corn

00:06:15.440 --> 00:06:19.070
and then the eggs hatch
and the larva can feed.

00:06:19.070 --> 00:06:21.800
So it's a fascinating, almost arms race,

00:06:21.800 --> 00:06:24.470
between the farmer and these pests.

00:06:24.470 --> 00:06:26.760
But I hope you appreciate,
it's far more interesting

00:06:26.760 --> 00:06:28.220
than you might have realized before.

00:06:28.220 --> 00:06:30.440
And I haven't even given
you a comprehensive list

00:06:30.440 --> 00:06:32.900
of all the types of
pests and all of the ways

00:06:32.900 --> 00:06:33.870
that you might want to manage it.

00:06:33.870 --> 00:06:36.330
You can do physical removal,
you could do barriers.

00:06:36.330 --> 00:06:38.560
Maybe you'll think of another creative way

00:06:38.560 --> 00:06:41.873
to have integrated pest management.


Demographic transition model

WEBVTT
Kind: captions
Language: en

00:00:00.710 --> 00:00:02.650
- [Narrator] In this video,
we're going to study something

00:00:02.650 --> 00:00:05.530
called the demographic transition model,

00:00:05.530 --> 00:00:07.510
which is something demographers use.

00:00:07.510 --> 00:00:11.140
Demographers are people who
study the makeup of populations

00:00:11.140 --> 00:00:13.410
and how those transition over time

00:00:13.410 --> 00:00:15.140
and why that might happen.

00:00:15.140 --> 00:00:18.240
And this model in particular
is one to think about

00:00:18.240 --> 00:00:21.450
how a country goes from
being pre-industrial

00:00:21.450 --> 00:00:25.150
to post-industrial, and what
happens to the birth rate,

00:00:25.150 --> 00:00:27.480
the death rate, and the total population

00:00:27.480 --> 00:00:29.850
over time as they make that transition.

00:00:29.850 --> 00:00:32.480
And as we'll see, it
happens over four stages.

00:00:32.480 --> 00:00:34.060
So let's just first understand

00:00:34.060 --> 00:00:36.860
what's going on in this graphic year.

00:00:36.860 --> 00:00:41.410
So our horizontal axis
we see is our time axis.

00:00:41.410 --> 00:00:43.690
And depending on the
country we're studying,

00:00:43.690 --> 00:00:44.870
this might occur.

00:00:44.870 --> 00:00:45.840
Oh, in fact, it's definitely

00:00:45.840 --> 00:00:47.330
going to occur over many decades,

00:00:47.330 --> 00:00:49.040
but some countries still
haven't even gotten

00:00:49.040 --> 00:00:50.640
through all of these phases.

00:00:50.640 --> 00:00:52.700
So it might've been a century already.

00:00:52.700 --> 00:00:55.230
And they might still be
in say this second phase

00:00:55.230 --> 00:00:57.500
which we'll describe in
a little bit more detail.

00:00:57.500 --> 00:01:01.130
The vertical axis, you can see
it says rates per thousand.

00:01:01.130 --> 00:01:04.900
And this is useful for
looking at the birth rate

00:01:04.900 --> 00:01:07.460
and the death rate, the
total population here

00:01:07.460 --> 00:01:09.170
which is in this light blue color,

00:01:09.170 --> 00:01:10.900
that really shouldn't be read as a rate,

00:01:10.900 --> 00:01:12.650
that is the total population.

00:01:12.650 --> 00:01:14.550
You could probably add another axis here

00:01:14.550 --> 00:01:16.450
to measure total population,

00:01:16.450 --> 00:01:17.640
but it's telling us indicatively

00:01:17.640 --> 00:01:20.040
where the total population is.

00:01:20.040 --> 00:01:23.170
Now we see the birth
rate in this blue color

00:01:23.170 --> 00:01:24.280
and we see the death rate

00:01:24.280 --> 00:01:27.760
in this salmon or this
pink, orange, red color.

00:01:27.760 --> 00:01:30.640
And we can see that as a country is,

00:01:30.640 --> 00:01:31.940
I guess you could say, starting out,

00:01:31.940 --> 00:01:33.650
or really according to this model,

00:01:33.650 --> 00:01:36.603
we would call it pre-industrial phase,

00:01:37.870 --> 00:01:39.900
which is pre-industrial,

00:01:39.900 --> 00:01:42.880
which is this phase right over here.

00:01:42.880 --> 00:01:45.040
You have relatively high birth rates

00:01:45.040 --> 00:01:47.140
and relatively high death rates.

00:01:47.140 --> 00:01:48.520
Why is that happening?

00:01:48.520 --> 00:01:50.330
Well, in most pre-industrial societies,

00:01:50.330 --> 00:01:53.660
you have a high death
rate because healthcare

00:01:53.660 --> 00:01:56.960
is either non-existent
or it isn't that good.

00:01:56.960 --> 00:01:59.040
So you could have high child mortality.

00:01:59.040 --> 00:02:01.920
A lot of people are dying from diseases

00:02:01.920 --> 00:02:04.540
that are easy to cure
in an industrial society

00:02:04.540 --> 00:02:07.000
or in a post-industrial
society, which we'll talk about.

00:02:07.000 --> 00:02:09.360
And why is there a high birth rate?

00:02:09.360 --> 00:02:11.640
Oftentimes, culturally women still haven't

00:02:11.640 --> 00:02:13.820
entered the workforce in a major way.

00:02:13.820 --> 00:02:16.410
Education generally is at a low level

00:02:16.410 --> 00:02:19.030
especially for women at this
stage because fundamentally

00:02:19.030 --> 00:02:21.420
of a lack of a healthcare
system for most people,

00:02:21.420 --> 00:02:23.990
you might not have things
like family planning.

00:02:23.990 --> 00:02:25.930
And so you could imagine that might lead

00:02:25.930 --> 00:02:28.070
to an overall higher birth rate.

00:02:28.070 --> 00:02:29.490
Now, because both the birth rate

00:02:29.490 --> 00:02:32.490
and the death rate are high,
but they're about the same,

00:02:32.490 --> 00:02:35.190
you can see that you
have a relatively stable

00:02:35.190 --> 00:02:38.600
but we'll say in absolute
terms, low population.

00:02:38.600 --> 00:02:42.550
And then countries might
start to industrialize.

00:02:42.550 --> 00:02:46.673
And so we call this state right
here the transitional state,

00:02:48.970 --> 00:02:51.690
and we can see that at
least on this graphic,

00:02:51.690 --> 00:02:54.660
it is elongated and different
countries go through this

00:02:54.660 --> 00:02:56.120
in different amounts of time.

00:02:56.120 --> 00:02:58.300
You might have a country like Japan

00:02:58.300 --> 00:03:01.410
that went through this in
less than half a century,

00:03:01.410 --> 00:03:02.670
but you might have other countries

00:03:02.670 --> 00:03:03.930
that are still going through this,

00:03:03.930 --> 00:03:07.920
even after a half a century
or even a century or more.

00:03:07.920 --> 00:03:09.560
But in the transitional state,

00:03:09.560 --> 00:03:13.390
this country will start to industrialize.

00:03:13.390 --> 00:03:15.390
And this whole model is based on

00:03:15.390 --> 00:03:18.950
looking at what happened in
Europe, in the 19th century,

00:03:18.950 --> 00:03:21.100
as much of Europe
industrialized and saying, okay,

00:03:21.100 --> 00:03:23.700
maybe other countries will
go through the same pattern

00:03:23.700 --> 00:03:26.600
of demographics as they industrialize.

00:03:26.600 --> 00:03:28.350
So let's think about
what is happening here.

00:03:28.350 --> 00:03:30.320
We see that the death rate

00:03:30.320 --> 00:03:32.880
is starting to come
down pretty reasonably.

00:03:32.880 --> 00:03:34.500
Now, the argument why that would happen

00:03:34.500 --> 00:03:37.290
as this country is industrializing

00:03:37.290 --> 00:03:40.270
is because they would now
have better healthcare,

00:03:40.270 --> 00:03:42.670
they might have better nutrition now.

00:03:42.670 --> 00:03:43.670
In fact, that's another reason

00:03:43.670 --> 00:03:46.260
why the pre-industrial society
might have a high death rate.

00:03:46.260 --> 00:03:49.410
People are even having illnesses
or dying from malnutrition.

00:03:49.410 --> 00:03:52.010
But now, because say farm
productivity is higher

00:03:52.010 --> 00:03:54.630
because they're able to
use more modern methods,

00:03:54.630 --> 00:03:58.000
the nutrition is better,
death rates start coming down.

00:03:58.000 --> 00:04:01.320
And birth rates might start
coming down a little bit,

00:04:01.320 --> 00:04:03.300
but not as quickly as the death rates.

00:04:03.300 --> 00:04:06.190
Now, why the birth rates
might start coming down?

00:04:06.190 --> 00:04:08.410
Well, once again, access to healthcare.

00:04:08.410 --> 00:04:10.660
More women might be able
to enter the workforce.

00:04:10.660 --> 00:04:13.040
You're having a higher level of education

00:04:13.040 --> 00:04:17.220
as the society starts to
develop and becomes wealthier.

00:04:17.220 --> 00:04:20.000
Now, with the death rate
going down at a faster rate

00:04:20.000 --> 00:04:21.770
than the birth rate,

00:04:21.770 --> 00:04:24.380
all of a sudden this will
add to the population.

00:04:24.380 --> 00:04:26.710
If fewer people are dying than being born,

00:04:26.710 --> 00:04:30.130
the population is increasing over time.

00:04:30.130 --> 00:04:33.150
And in fact, the rate of
population increase is increasing

00:04:33.150 --> 00:04:37.810
as this gap between birth
rate and death rate increases.

00:04:37.810 --> 00:04:42.470
Now, the next stage of our
demographic transition model,

00:04:42.470 --> 00:04:45.060
we would consider industrial.

00:04:45.060 --> 00:04:48.290
And the real marker of that

00:04:48.290 --> 00:04:51.200
is we see that in the industrial phase,

00:04:51.200 --> 00:04:55.980
our birth rate is starting to
catch up with our death rate.

00:04:55.980 --> 00:04:58.690
And so at this phase,
women might be entering

00:04:58.690 --> 00:05:00.170
in the workforce in a major way.

00:05:00.170 --> 00:05:02.480
Culturally, it is far more acceptable

00:05:02.480 --> 00:05:04.060
for things like family planning,

00:05:04.060 --> 00:05:05.800
culturally far more acceptable

00:05:05.800 --> 00:05:08.460
for people to delay giving birth,

00:05:08.460 --> 00:05:11.720
especially for women so that
they could get their education.

00:05:11.720 --> 00:05:12.980
And there's just more awareness

00:05:12.980 --> 00:05:14.670
that by having fewer children,

00:05:14.670 --> 00:05:17.280
the children you do have
will be more prosperous,

00:05:17.280 --> 00:05:18.730
will get better nutrition.

00:05:18.730 --> 00:05:21.640
The family is overall
likely to be wealthier.

00:05:21.640 --> 00:05:23.860
So as the birth rate
and the death rate curve

00:05:23.860 --> 00:05:26.010
get closer and closer together,

00:05:26.010 --> 00:05:29.280
the rate of population
growth starts to slow.

00:05:29.280 --> 00:05:31.850
And then this last phase
right here over here,

00:05:31.850 --> 00:05:35.060
the fourth phase in our
demographic transition model

00:05:35.060 --> 00:05:38.363
which we could call post industrial,

00:05:39.660 --> 00:05:42.540
we see that not only has the birth rate

00:05:42.540 --> 00:05:44.300
caught up to the death rate,

00:05:44.300 --> 00:05:47.940
that it actually can go
below the death rate.

00:05:47.940 --> 00:05:51.230
And so in that situation,
you have a stable population,

00:05:51.230 --> 00:05:52.440
but if the birth rate

00:05:52.440 --> 00:05:54.496
actually even goes below the death rate,

00:05:54.496 --> 00:05:58.200
you can even see the
population began to decline.

00:05:58.200 --> 00:06:01.030
And there are societies
that are already there.

00:06:01.030 --> 00:06:04.270
For example, Japan has
a declining population,

00:06:04.270 --> 00:06:06.110
even places like the United States.

00:06:06.110 --> 00:06:07.920
If we did not have immigration,

00:06:07.920 --> 00:06:10.090
you would have a declining population

00:06:10.090 --> 00:06:13.720
because we are in this
last phase right over here.

00:06:13.720 --> 00:06:16.610
And this post-industrial
declining population

00:06:16.610 --> 00:06:18.640
because birth rates have gotten so low,

00:06:18.640 --> 00:06:20.660
have its own set of issues.

00:06:20.660 --> 00:06:22.680
You now have an aging population.

00:06:22.680 --> 00:06:24.000
If birth rates are much lower

00:06:24.000 --> 00:06:25.750
and people are living much longer,

00:06:25.750 --> 00:06:28.750
a higher and higher percentage
of the population is older.

00:06:28.750 --> 00:06:31.670
Older people, especially once
they are out of the workforce,

00:06:31.670 --> 00:06:32.930
who's going to support them?

00:06:32.930 --> 00:06:34.870
Who's going to provide
the healthcare for them?

00:06:34.870 --> 00:06:37.430
Where are those resources
going to come from?

00:06:37.430 --> 00:06:40.550
If your population is shrinking,

00:06:40.550 --> 00:06:43.030
then your economy might
not be growing as fast,

00:06:43.030 --> 00:06:45.830
which would make it even
harder to support the costs

00:06:45.830 --> 00:06:47.320
of an aging population.

00:06:47.320 --> 00:06:48.570
So I'll leave you there.

00:06:48.570 --> 00:06:51.150
Once again, this is a
model, it's a hypothesis,

00:06:51.150 --> 00:06:53.690
but it's a useful one to
at least have a framework

00:06:53.690 --> 00:06:56.620
of how things might transition
in the society over time.

00:06:56.620 --> 00:06:59.260
And people use this type of
model to advise countries,

00:06:59.260 --> 00:07:01.770
especially ones that might be
in this transitional state.

00:07:01.770 --> 00:07:03.280
And the transitional state,

00:07:03.280 --> 00:07:05.830
sometimes is the place where
countries might get stuck

00:07:05.830 --> 00:07:08.650
because they're getting
delayed economic development.

00:07:08.650 --> 00:07:10.920
So they're not quite able
to get too industrial.

00:07:10.920 --> 00:07:13.800
And so demographers, economists,
others will think about,

00:07:13.800 --> 00:07:17.863
well, how can you make this
transition as well as possible?


Weak acid–weak base reactions

WEBVTT
Kind: captions
Language: en

00:00:00.380 --> 00:00:01.560
- [Instructor] Let's say the HA

00:00:01.560 --> 00:00:04.670
represents a generic weak acid

00:00:04.670 --> 00:00:08.680
and B represents a generic weak base.

00:00:08.680 --> 00:00:11.860
If our weak acid donates
a proton to our weak base,

00:00:11.860 --> 00:00:15.480
that would form A minus and HB plus.

00:00:15.480 --> 00:00:17.860
To identify conjugate acid base pairs

00:00:17.860 --> 00:00:21.260
remember there's only one
proton or one H plus difference

00:00:21.260 --> 00:00:24.590
between an acid and it's conjugate base.

00:00:24.590 --> 00:00:28.300
So when HA donates its proton
and turns into A minus,

00:00:28.300 --> 00:00:33.103
A minus must be the conjugate base to HA.

00:00:34.570 --> 00:00:38.760
So there's one H plus difference
between HA and A minus.

00:00:38.760 --> 00:00:41.650
They are a conjugate acid base pair.

00:00:41.650 --> 00:00:43.210
When B accepts a proton,

00:00:43.210 --> 00:00:48.130
it turns into HB plus
therefore HB plus must be

00:00:48.130 --> 00:00:52.490
the conjugate acid to B.

00:00:52.490 --> 00:00:56.210
Since there's one H plus
difference between B and HB plus,

00:00:56.210 --> 00:00:59.410
they are a conjugate acid base pair.

00:00:59.410 --> 00:01:02.240
Let's look at the situation
where the equilibrium constant

00:01:02.240 --> 00:01:04.600
for this reaction is greater than one.

00:01:04.600 --> 00:01:07.390
If K is greater than one, that
means that at equilibrium,

00:01:07.390 --> 00:01:09.840
there are more products than reactants

00:01:09.840 --> 00:01:13.170
therefore, the equilibrium
favors the formation

00:01:13.170 --> 00:01:15.100
of the products.

00:01:15.100 --> 00:01:17.270
For acid-base reactions the equilibrium

00:01:17.270 --> 00:01:19.770
always favors the side
with the weaker acid

00:01:19.770 --> 00:01:21.630
and the weaker base.

00:01:21.630 --> 00:01:24.730
Therefore, since the
equilibrium favors the products,

00:01:24.730 --> 00:01:27.090
the weaker acid and the weaker base,

00:01:27.090 --> 00:01:28.710
are on the product side

00:01:28.710 --> 00:01:31.170
and the stronger acid
and the stronger base

00:01:31.170 --> 00:01:33.000
are on the reactant side.

00:01:33.000 --> 00:01:34.750
So when comparing our two acids,

00:01:34.750 --> 00:01:39.750
HA and HB plus, HA is the
stronger acid of the two.

00:01:40.580 --> 00:01:44.660
And when comparing our
two bases B and A minus,

00:01:44.660 --> 00:01:47.273
B is the stronger base of the two.

00:01:48.110 --> 00:01:50.420
When the equilibrium
constant is less than one

00:01:50.420 --> 00:01:51.990
that means at equilibrium,

00:01:51.990 --> 00:01:54.520
there are more reactants
than there are products.

00:01:54.520 --> 00:01:55.720
Therefore, this time,

00:01:55.720 --> 00:01:59.470
the equilibrium favors the
formation of the reactants.

00:01:59.470 --> 00:02:01.820
And the equilibrium always favors the side

00:02:01.820 --> 00:02:04.640
with the weaker acid and the weaker base.

00:02:04.640 --> 00:02:07.760
Since the equilibrium
favors the reactants,

00:02:07.760 --> 00:02:09.840
the weaker acid and the weaker base,

00:02:09.840 --> 00:02:11.180
are on the reactant side

00:02:11.180 --> 00:02:13.530
and the stronger acid
and the stronger base

00:02:13.530 --> 00:02:15.520
are on the product side.

00:02:15.520 --> 00:02:19.340
So comparing our two acids, HA and HB plus

00:02:19.340 --> 00:02:22.570
this time, HB plus is the
stronger acid of the two

00:02:22.570 --> 00:02:25.490
and for our bases B and A minus

00:02:25.490 --> 00:02:29.720
this time, A minus is the
stronger base of the two.

00:02:29.720 --> 00:02:33.870
Let's look at an example of a
weak acid, weak base reaction.

00:02:33.870 --> 00:02:35.500
The Hydrogen Sulfate anion

00:02:35.500 --> 00:02:37.400
we'll react with the Carbonate anion

00:02:37.400 --> 00:02:39.470
to form the Sulfate anion

00:02:39.470 --> 00:02:42.150
and the Hydrogen Carbonate anion.

00:02:42.150 --> 00:02:44.760
Looking at the reaction,
the Hydrogen Sulfate anion

00:02:44.760 --> 00:02:48.000
is donating its proton
so that must be the acid

00:02:48.000 --> 00:02:51.610
and the Carbonate anion
is accepting a proton

00:02:51.610 --> 00:02:54.450
so that must be the base.

00:02:54.450 --> 00:02:57.210
After the Hydrogen Sulfate
anion donates it's proton

00:02:57.210 --> 00:03:00.870
it turns into the Sulfate
anion, SO four, two minus.

00:03:00.870 --> 00:03:05.320
Therefore the Sulfate anion
must be the conjugate base

00:03:05.320 --> 00:03:07.910
to HSO four minus.

00:03:07.910 --> 00:03:11.940
So one conjugate acid base
pair is HSO four minus

00:03:11.940 --> 00:03:14.390
and SO four, two minus.

00:03:14.390 --> 00:03:16.770
And when the Carbonate anion
and accepts the proton,

00:03:16.770 --> 00:03:19.760
it turns into the
Hydrogen Carbonate anion.

00:03:19.760 --> 00:03:21.980
Therefore the Hydrogen Carbonate anion,

00:03:21.980 --> 00:03:26.980
must be the conjugate acid
to the Carbonate anion.

00:03:29.170 --> 00:03:31.240
So our other conjugate acid-base pair

00:03:31.240 --> 00:03:33.490
consists of the Carbonate anion,

00:03:33.490 --> 00:03:36.000
and the Hydrogen Carbonate anion.

00:03:36.000 --> 00:03:38.380
The equilibrium constant for this reaction

00:03:38.380 --> 00:03:41.210
at 25 degrees Celsius is greater than one,

00:03:41.210 --> 00:03:43.350
therefore the equilibrium favors

00:03:43.350 --> 00:03:46.350
the formation of the products.

00:03:46.350 --> 00:03:48.050
And because the equilibrium favors

00:03:48.050 --> 00:03:50.920
the formation of the weaker
acid and the weaker base,

00:03:50.920 --> 00:03:53.110
we know that the weaker
acid and the weaker base

00:03:53.110 --> 00:03:54.560
must be on the product side

00:03:54.560 --> 00:03:56.860
and the stronger acid
and the stronger base

00:03:56.860 --> 00:03:58.630
are on the reactant side.

00:03:58.630 --> 00:04:02.339
Therefore the Hydrogen Sulfate
anion is a stronger acid

00:04:02.339 --> 00:04:04.503
than the Hydrogen Carbonate anion,

00:04:04.503 --> 00:04:07.330
and the Carbonate anion
in as a stronger base

00:04:07.330 --> 00:04:09.730
than the Sulfate anion.

00:04:09.730 --> 00:04:11.010
Let's look at another example

00:04:11.010 --> 00:04:13.630
of a weak acid, weak base reaction.

00:04:13.630 --> 00:04:14.463
In this case,

00:04:14.463 --> 00:04:16.740
Hydrofluoric acid reacts with Ammonia

00:04:16.740 --> 00:04:19.290
to form the Ammonium ion an NH four plus,

00:04:19.290 --> 00:04:22.090
and the Fluoride anion, F minus.

00:04:22.090 --> 00:04:26.100
Because Hydrofluoric acid
donates a proton, it's an acid

00:04:26.100 --> 00:04:29.650
and because Ammonia accepts a proton

00:04:29.650 --> 00:04:32.960
Ammonia functions as a base.

00:04:32.960 --> 00:04:35.020
And when HF donates a proton,

00:04:35.020 --> 00:04:37.760
it turns into F minus the Fluoride anion,

00:04:37.760 --> 00:04:42.760
therefore the Fluoride anion
is the conjugate base to HF.

00:04:43.560 --> 00:04:45.360
And when Ammonia accepts a proton,

00:04:45.360 --> 00:04:48.670
it turns into NH four
Plus the Ammonium ion,

00:04:48.670 --> 00:04:52.540
therefore the Ammonium
ion is the conjugate acid

00:04:52.540 --> 00:04:54.223
to NH three.

00:04:55.130 --> 00:04:58.860
Let's say that we mix equal
moles of our weak acid HF

00:04:58.860 --> 00:05:01.200
with our weak base NH three

00:05:01.200 --> 00:05:04.280
and our goal is to figure
out if the resulting solution

00:05:04.280 --> 00:05:07.310
is acidic, basic or neutral.

00:05:07.310 --> 00:05:09.860
First, we need to determine
if the reactants or products

00:05:09.860 --> 00:05:11.830
are favored at equilibrium.

00:05:11.830 --> 00:05:13.210
And to help us determine that

00:05:13.210 --> 00:05:15.660
here we have the Ka and Kb values

00:05:15.660 --> 00:05:17.630
for our acids and bases.

00:05:17.630 --> 00:05:20.380
So here's the Ka value
for Hydrofluoric acid

00:05:20.380 --> 00:05:22.400
at 25 degrees Celsius,

00:05:22.400 --> 00:05:24.520
so all of these are at 25 degrees Celsius.

00:05:24.520 --> 00:05:27.217
Here's the Kb value for Ammonia,

00:05:27.217 --> 00:05:29.709
the Ka value for the Ammonium ion

00:05:29.709 --> 00:05:33.070
and the Kb value for the Fluoride anion.

00:05:33.070 --> 00:05:36.160
So looking at our two acids
that was Hydrofluoric acid

00:05:36.160 --> 00:05:38.410
and the Ammonium cation,

00:05:38.410 --> 00:05:42.070
the Ka for Hydrofluoric
acid is a higher value

00:05:42.070 --> 00:05:44.640
than the Ka for the Ammonium cation.

00:05:44.640 --> 00:05:49.440
Therefore Hydrofluoric acid
is the stronger of the two,

00:05:49.440 --> 00:05:51.690
the stronger acid of the two.

00:05:51.690 --> 00:05:53.100
And for the two bases,

00:05:53.100 --> 00:05:56.180
we have Ammonia and we also
have the Fluoride anion,

00:05:56.180 --> 00:05:58.080
looking at the Kb values,

00:05:58.080 --> 00:06:01.240
for Ammonia the Kb value is higher

00:06:01.240 --> 00:06:03.410
than the Kb value for the Fluoride anion,

00:06:03.410 --> 00:06:07.283
therefore Ammonia is the
stronger base of the two.

00:06:08.200 --> 00:06:10.510
So we have the stronger
acid and the stronger base

00:06:10.510 --> 00:06:11.660
on the reactant side

00:06:11.660 --> 00:06:13.840
and the weaker acid and the weaker base

00:06:13.840 --> 00:06:15.400
on the product side.

00:06:15.400 --> 00:06:17.560
We know the equilibrium favors the side

00:06:17.560 --> 00:06:19.930
with the weaker acid and the weaker base.

00:06:19.930 --> 00:06:21.610
Therefore the equilibrium

00:06:21.610 --> 00:06:25.160
favors the formation of the products.

00:06:25.160 --> 00:06:27.800
So we could go ahead and
write the equilibrium constant

00:06:27.800 --> 00:06:31.110
for this reaction is greater than one.

00:06:31.110 --> 00:06:32.310
And at equilibrium

00:06:32.310 --> 00:06:33.640
we're going to have more of our products

00:06:33.640 --> 00:06:35.410
than we do of our reactants.

00:06:35.410 --> 00:06:38.240
So to determine at the solution
as acidic, basic or neutral,

00:06:38.240 --> 00:06:40.650
we have to think about the Ammonium cation

00:06:40.650 --> 00:06:44.160
and the Fluoride anion
in aqueous solution.

00:06:44.160 --> 00:06:46.666
Since the Ka value for the Ammonium cation

00:06:46.666 --> 00:06:47.573
is greater than the Kb value
for the Fluoride anion,

00:06:50.201 --> 00:06:53.005
the Ammonium cation is better
at producing Hydronium ions

00:06:53.005 --> 00:06:54.357
in aqueous solution

00:06:54.357 --> 00:06:57.773
than the Fluoride anion is
at producing Hydroxide anions

00:06:57.773 --> 00:06:59.710
in aqueous solution.

00:06:59.710 --> 00:07:02.040
So the concentration of Hydronium ions

00:07:02.040 --> 00:07:06.270
will be greater than the
concentration of Hydroxide anions

00:07:06.270 --> 00:07:10.253
and therefore the resulting
solution will be acidic.


Weak base–strong acid reactions

WEBVTT
Kind: captions
Language: en

00:00:00.380 --> 00:00:03.350
- [Instructor] Ammonia is
an example of a weak base.

00:00:03.350 --> 00:00:07.280
and hydrochloric acid is an
example of a strong acid.

00:00:07.280 --> 00:00:09.170
Ammonia reacts with hydrochloric acid

00:00:09.170 --> 00:00:12.280
to form an aqueous solution
of ammonium chloride.

00:00:12.280 --> 00:00:15.240
And because this is an acid-base
neutralization reaction,

00:00:15.240 --> 00:00:17.420
there's only a single
arrow going to the right,

00:00:17.420 --> 00:00:20.380
indicating the reaction
goes to completion.

00:00:20.380 --> 00:00:24.120
Next, let's write the overall
or complete ionic equation.

00:00:24.120 --> 00:00:25.760
Let's start with ammonia.

00:00:25.760 --> 00:00:27.230
Ammonia is a weak base,

00:00:27.230 --> 00:00:31.010
and weak bases only partly
ionize in aqueous solution.

00:00:31.010 --> 00:00:34.020
Therefore, since weak
bases only partly ionize,

00:00:34.020 --> 00:00:35.480
we're not gonna show this as an ion.

00:00:35.480 --> 00:00:39.550
We're simply gonna write
NH3 in our equation.

00:00:39.550 --> 00:00:41.350
However, for hydrochloric acid,

00:00:41.350 --> 00:00:43.320
hydrochloric acid is a strong acid,

00:00:43.320 --> 00:00:46.340
and strong acids ionize 100%.

00:00:46.340 --> 00:00:47.760
Therefore, an aqueous solution,

00:00:47.760 --> 00:00:49.760
we need to show this as the ions,

00:00:49.760 --> 00:00:53.593
so H plus and Cl minus.

00:00:54.860 --> 00:00:57.660
Ammonium chloride is a soluble salt,

00:00:57.660 --> 00:00:59.150
therefore, an aqueous solution,

00:00:59.150 --> 00:01:01.100
we show it as the ions.

00:01:01.100 --> 00:01:03.930
So ammonium chloride
consists of the ammonium ion,

00:01:03.930 --> 00:01:08.930
NH4 plus, and the
chloride anion, Cl minus.

00:01:09.410 --> 00:01:10.340
To save some time,

00:01:10.340 --> 00:01:12.320
I've drawn in the aqueous subscripts,

00:01:12.320 --> 00:01:15.530
and also put in the reaction
arrow and a plus sign.

00:01:15.530 --> 00:01:17.860
So this represents the overall,

00:01:17.860 --> 00:01:20.660
or the complete ionic equation.

00:01:20.660 --> 00:01:22.900
And we can use the complete ionic equation

00:01:22.900 --> 00:01:24.970
to find the net ionic equation

00:01:24.970 --> 00:01:27.740
for this weak base, strong acid reaction.

00:01:27.740 --> 00:01:31.150
To do that, we first need to
identify these spectator ions.

00:01:31.150 --> 00:01:33.440
And remember, these are the
ions that do not take part

00:01:33.440 --> 00:01:35.120
in the chemical reaction.

00:01:35.120 --> 00:01:37.460
Since there's a chloride
anion on the left side

00:01:37.460 --> 00:01:38.510
and on the right side,

00:01:38.510 --> 00:01:42.780
the chloride anion is the
spectator ion for this reaction.

00:01:42.780 --> 00:01:44.590
And once we take out our spectator ion,

00:01:44.590 --> 00:01:47.330
we're left with our net ionic equation,

00:01:47.330 --> 00:01:52.330
which is aqueous ammonia
plus H plus yields NH4 plus.

00:01:52.610 --> 00:01:55.920
So this is one way to write
our net ionic equation.

00:01:55.920 --> 00:01:59.480
However, remember that H plus and H3O plus

00:01:59.480 --> 00:02:02.070
are used interchangeably in chemistry.

00:02:02.070 --> 00:02:05.200
Therefore, another way to
write the net ionic equation

00:02:05.200 --> 00:02:08.080
is to show aqueous ammonia
plus the hydronium ion,

00:02:08.080 --> 00:02:12.683
H3O plus, yields the ammonium
ion, NH4 plus, plus water.

00:02:13.550 --> 00:02:15.420
Now that we have our net ionic equation,

00:02:15.420 --> 00:02:18.010
we're gonna consider three
different situations.

00:02:18.010 --> 00:02:19.470
In the first situation,

00:02:19.470 --> 00:02:23.350
we have equal moles of our
weak base and strong acid.

00:02:23.350 --> 00:02:24.990
Looking at our net ionic equation,

00:02:24.990 --> 00:02:28.750
the mole ratio of ammonia to
hydronium ion is one to one.

00:02:28.750 --> 00:02:31.800
Therefore, if we have equal
moles of our weak base

00:02:31.800 --> 00:02:33.500
and strong acid,

00:02:33.500 --> 00:02:34.870
the weak base and strong acid

00:02:34.870 --> 00:02:36.650
will completely neutralize each other

00:02:36.650 --> 00:02:40.450
and produce the ammonium ion NH4 plus.

00:02:40.450 --> 00:02:42.210
So if our goal is to figure out the pH

00:02:42.210 --> 00:02:43.480
of the resulting solution,

00:02:43.480 --> 00:02:45.440
we don't need to consider the weak base,

00:02:45.440 --> 00:02:47.010
or this strong acid.

00:02:47.010 --> 00:02:49.600
We need to think about the ammonium cation

00:02:49.600 --> 00:02:51.820
in aqueous solution.

00:02:51.820 --> 00:02:52.720
And in solution,

00:02:52.720 --> 00:02:55.170
the ammonium cation acts as a weak acid

00:02:55.170 --> 00:02:56.830
and donates a proton to water

00:02:56.830 --> 00:03:01.740
to form the hydronium ion,
H3O plus, and aqueous ammonia.

00:03:01.740 --> 00:03:05.240
The ammonium cation, NH4
plus, is a weak acid.

00:03:05.240 --> 00:03:08.410
Therefore, the Ka value is less than one.

00:03:08.410 --> 00:03:11.930
And since Ka is less
than one at equilibrium,

00:03:11.930 --> 00:03:15.050
there are mostly reactants
and not very many products.

00:03:15.050 --> 00:03:18.960
However, the concentration
of hydronium ions in solution

00:03:18.960 --> 00:03:21.270
is increased, and therefore,

00:03:21.270 --> 00:03:24.620
the resulting solution will be acidic.

00:03:24.620 --> 00:03:27.220
So the resulting solution
will be slightly acidic.

00:03:27.220 --> 00:03:29.240
And at 25 degrees Celsius,

00:03:29.240 --> 00:03:32.650
the pH of the solution
will be less than seven.

00:03:32.650 --> 00:03:34.860
If we wanted to calculate the actual pH,

00:03:34.860 --> 00:03:38.330
we would treat this like a
weak acid equilibrium problem.

00:03:38.330 --> 00:03:41.040
Also, it's important to
emphasize that the hydronium ions

00:03:41.040 --> 00:03:43.950
that gave the resulting
solution a pH less than seven

00:03:43.950 --> 00:03:47.740
came from the reaction of the
ammonium cation with water.

00:03:47.740 --> 00:03:50.630
The hydronium ions did not
come from the strong acid.

00:03:50.630 --> 00:03:52.900
All of those hydronium ions were used up

00:03:52.900 --> 00:03:56.050
in the acid-base neutralization reaction.

00:03:56.050 --> 00:03:57.290
For the second situation,

00:03:57.290 --> 00:04:00.180
we have more of the weak
base than the strong acid,

00:04:00.180 --> 00:04:03.240
therefore, we have the
weak base in excess.

00:04:03.240 --> 00:04:04.930
And because the mole
ratio of the weak base

00:04:04.930 --> 00:04:07.360
to the strong acid is one to one,

00:04:07.360 --> 00:04:10.400
if we have more of the weak
base than the strong acid,

00:04:10.400 --> 00:04:13.790
all of the strong acid will be used up.

00:04:13.790 --> 00:04:15.610
So when the reaction goes to completion,

00:04:15.610 --> 00:04:18.240
we'll have ammonium cations in solution,

00:04:18.240 --> 00:04:21.290
and we'll also have some leftover ammonia.

00:04:21.290 --> 00:04:24.540
So after the neutralization
reaction is complete

00:04:24.540 --> 00:04:26.800
and all the H3O plus is used up,

00:04:26.800 --> 00:04:28.880
we'll have some leftover ammonia.

00:04:28.880 --> 00:04:31.090
That ammonia will react with water

00:04:31.090 --> 00:04:35.730
to form hydroxide anions and NH4 plus.

00:04:35.730 --> 00:04:39.180
Because the concentration of
hydroxide ions in solution

00:04:39.180 --> 00:04:43.000
has increased at 25 degrees Celsius,

00:04:43.000 --> 00:04:45.230
the resulting solution will be basic

00:04:45.230 --> 00:04:48.690
and the pH will be greater than seven.

00:04:48.690 --> 00:04:50.920
If we wanted to calculate the actual pH,

00:04:50.920 --> 00:04:54.190
we would treat this like a
weak base equilibria problem.

00:04:54.190 --> 00:04:57.340
However, we have two sources
for the ammonium cation.

00:04:57.340 --> 00:05:00.190
One source is from ammonia
reacting with water

00:05:00.190 --> 00:05:01.740
to form NH4 plus,

00:05:01.740 --> 00:05:06.180
and the other source came from
the neutralization reaction.

00:05:06.180 --> 00:05:09.970
So actually, this would be
a common-ion effect problem.

00:05:09.970 --> 00:05:12.560
The other way to calculate
the pH of this solution

00:05:12.560 --> 00:05:16.270
is to realize that ammonium
NH4 plus is a weak acid,

00:05:16.270 --> 00:05:19.480
and ammonia NH3 is its conjugate base,

00:05:19.480 --> 00:05:22.350
therefore, if we have similar
amounts of a weak acid

00:05:22.350 --> 00:05:23.380
and its conjugate base,

00:05:23.380 --> 00:05:25.660
we have a buffer solution
and we could calculate

00:05:25.660 --> 00:05:29.100
the pH using the
Henderson-Hasselbalch equation.

00:05:29.100 --> 00:05:30.430
For our third situation,

00:05:30.430 --> 00:05:33.980
let's say we have the
strong acid in excess.

00:05:33.980 --> 00:05:36.500
Since the mole ratio of
weak base to strong acid

00:05:36.500 --> 00:05:37.740
is one to one,

00:05:37.740 --> 00:05:40.590
if we have more of the strong
acid than the weak base,

00:05:40.590 --> 00:05:43.650
all of the weak base will be used up

00:05:43.650 --> 00:05:46.740
and we'll have some strong acid in excess.

00:05:46.740 --> 00:05:50.660
Therefore, there'll be a
concentration of hydronium ions

00:05:50.660 --> 00:05:55.300
in solution, which would make
the resulting solution acidic.

00:05:55.300 --> 00:06:00.300
So at 25 degrees Celsius, the
pH would be less than seven.

00:06:00.480 --> 00:06:03.540
We could calculate the actual
pH of the resulting solution

00:06:03.540 --> 00:06:07.390
by doing a strong acid
pH calculation problem.

00:06:07.390 --> 00:06:09.260
And while it's true
that the ammonium cation

00:06:09.260 --> 00:06:10.660
can function as a weak acid

00:06:10.660 --> 00:06:13.910
and also increase the
concentration of hydronium ions,

00:06:13.910 --> 00:06:15.990
it's such a small increase compared to

00:06:15.990 --> 00:06:19.370
the hydronium ions we have in
solution from our strong acid

00:06:19.370 --> 00:06:21.790
that we don't need to worry
about the contribution

00:06:21.790 --> 00:06:23.310
of the ammonium cations.

00:06:23.310 --> 00:06:24.390
We can just treat this

00:06:24.390 --> 00:06:27.123
like a strong acid pH calculation problem.


Worked example: Calculating the pH after a weak acid–strong base reaction (excess acid)

WEBVTT
Kind: captions
Language: en

00:00:00.280 --> 00:00:01.280
- [Instructor] Let's look at a reaction

00:00:01.280 --> 00:00:03.850
between a weak acid, acetic acid,

00:00:03.850 --> 00:00:07.660
and a strong base, sodium hydroxide.

00:00:07.660 --> 00:00:11.800
Let's say we have 100 milliliters
of a 2.0 molar solution

00:00:11.800 --> 00:00:13.330
of aqueous acetic acid,

00:00:13.330 --> 00:00:15.300
and that's mixed with 100 milliliters

00:00:15.300 --> 00:00:19.190
of a 1.0 molar solution of
aqueous sodium hydroxide.

00:00:19.190 --> 00:00:22.180
Our goal is to find the pH
of the resulting solution

00:00:22.180 --> 00:00:24.830
at 25 degrees Celsius.

00:00:24.830 --> 00:00:26.950
When the weak acid reacts
with the strong base,

00:00:26.950 --> 00:00:29.320
a neutralization reaction occurs.

00:00:29.320 --> 00:00:30.990
So our first step is to figure out

00:00:30.990 --> 00:00:33.930
how many moles of our
weak acid are present

00:00:33.930 --> 00:00:37.330
and also how many moles of
our strong base are present.

00:00:37.330 --> 00:00:38.710
Let's start with the weak acid.

00:00:38.710 --> 00:00:40.350
We're gonna use the molarity equation.

00:00:40.350 --> 00:00:42.950
Molarity is equal to moles over liters.

00:00:42.950 --> 00:00:46.550
So for our weak acid, the
concentration is 2.0 molar,

00:00:46.550 --> 00:00:47.740
so we plug that in,

00:00:47.740 --> 00:00:49.930
and the volume is 100 milliliters

00:00:49.930 --> 00:00:53.140
which is equal to 0.100 liters.

00:00:53.140 --> 00:00:54.410
Solving for x,

00:00:54.410 --> 00:00:59.410
we find that there are
0.20 moles of acetic acid.

00:00:59.710 --> 00:01:03.230
For our strong base, the
concentration is 1.0 molar,

00:01:03.230 --> 00:01:07.420
and the volume is 100 milliliters
which is 0.100 liters.

00:01:07.420 --> 00:01:11.720
So solving for x, x is
equal to 0.10 moles of NaOH.

00:01:12.600 --> 00:01:16.700
And because NaOH is a strong
base, it dissociates 100%.

00:01:16.700 --> 00:01:19.200
So if there's 0.10 moles of NaOH,

00:01:19.200 --> 00:01:24.200
there's also 0.10 moles
of hydroxide ions, OH-.

00:01:26.130 --> 00:01:26.963
In solution,

00:01:26.963 --> 00:01:30.460
the hydroxide anions will
react with acetic acid.

00:01:30.460 --> 00:01:33.820
So our next step is to look
at the net ionic equation

00:01:33.820 --> 00:01:37.290
for this weak acid-strong base reaction.

00:01:37.290 --> 00:01:38.880
In the net ionic equation,

00:01:38.880 --> 00:01:41.640
acetic acid reacts with hydroxide anions

00:01:41.640 --> 00:01:44.640
to form the acetate anion and water.

00:01:44.640 --> 00:01:47.430
Because this neutralization
reaction goes to completion,

00:01:47.430 --> 00:01:49.230
we draw an arrow going to the right

00:01:49.230 --> 00:01:51.640
instead of an equilibrium arrow.

00:01:51.640 --> 00:01:52.970
And instead of an ICE table

00:01:52.970 --> 00:01:54.620
where the E stands for equilibrium,

00:01:54.620 --> 00:01:56.490
we use an ICF table,

00:01:56.490 --> 00:01:58.620
where I is the initial amount of moles,

00:01:58.620 --> 00:01:59.990
C is the change in moles,

00:01:59.990 --> 00:02:02.760
and F is the final amount of moles.

00:02:02.760 --> 00:02:04.980
We've already calculated
the initial amount of moles

00:02:04.980 --> 00:02:07.980
of acetic acid to be 0.20,

00:02:07.980 --> 00:02:09.250
and the initial amount of moles

00:02:09.250 --> 00:02:12.440
of hydroxide anions to be 0.10.

00:02:12.440 --> 00:02:14.550
If we assume the reaction
hasn't happened yet,

00:02:14.550 --> 00:02:18.570
the initial amount of moles
of acetate would be zero.

00:02:18.570 --> 00:02:20.030
Next, we look at the coefficients

00:02:20.030 --> 00:02:22.210
in our balanced net ionic equation.

00:02:22.210 --> 00:02:25.680
The mole ratio of hydroxide
anions to acetic acid

00:02:25.680 --> 00:02:27.130
is one-to-one.

00:02:27.130 --> 00:02:31.650
Therefore, if we have 0.10
moles of hydroxide anions,

00:02:31.650 --> 00:02:35.920
that's gonna react with
0.10 moles of acetic acid.

00:02:35.920 --> 00:02:36.900
So for the change,

00:02:36.900 --> 00:02:41.610
we can write minus 0.10
under hydroxide anions,

00:02:41.610 --> 00:02:46.010
and also minus 0.10 under acetic acid.

00:02:46.010 --> 00:02:48.700
For the acetate anion,
there's a coefficient of one

00:02:48.700 --> 00:02:51.010
in the balanced net ionic equation.

00:02:51.010 --> 00:02:55.190
Therefore, if we're losing 0.10
moles for our two reactants,

00:02:55.190 --> 00:03:00.190
we're going to gain 0.10
moles of the acetate anion.

00:03:00.560 --> 00:03:03.810
So when the reaction comes to
completion for acetic acid,

00:03:03.810 --> 00:03:07.800
we started with 0.20
moles and we lost 0.10,

00:03:07.800 --> 00:03:11.920
so we're gonna have 0.10 moles
of acetic acid left over.

00:03:11.920 --> 00:03:13.420
For hydroxide anions,

00:03:13.420 --> 00:03:16.960
we start with 0.10 moles
and we lost 0.10 moles.

00:03:16.960 --> 00:03:19.350
Therefore, when the reaction
comes to completion,

00:03:19.350 --> 00:03:22.390
there'll be zero moles
of hydroxide anions.

00:03:22.390 --> 00:03:23.730
And for the acetate anion,

00:03:23.730 --> 00:03:27.090
we started off with
zero and we gained 0.10.

00:03:27.090 --> 00:03:31.160
Therefore, we will have 0.10
moles of the acetate anion.

00:03:31.160 --> 00:03:33.620
So in this case, we started
with more of our weak acid

00:03:33.620 --> 00:03:35.620
than we did of our strong base.

00:03:35.620 --> 00:03:40.220
And therefore, we ended up
with acid being in excess.

00:03:40.220 --> 00:03:42.740
The next step is to
calculate the concentration

00:03:42.740 --> 00:03:45.300
of acetic acid in solution.

00:03:45.300 --> 00:03:48.870
So we ended up with 0.10
moles of acetic acid.

00:03:48.870 --> 00:03:51.540
So we plug that into our
equation for molarity.

00:03:51.540 --> 00:03:54.540
And when we mixed our
two solutions together,

00:03:54.540 --> 00:03:56.440
the 100 milliliters of our weak acid

00:03:56.440 --> 00:03:58.350
with the 100 milliliters
of our strong base,

00:03:58.350 --> 00:04:01.670
the total volume of the
solution is 200 milliliters

00:04:01.670 --> 00:04:04.620
or 0.200 liters.

00:04:04.620 --> 00:04:07.920
Therefore, 0.1 divided by 0.200,

00:04:07.920 --> 00:04:12.330
gives the concentration
of acetic of 0.50 molar.

00:04:12.330 --> 00:04:16.240
And for the acetate anion,
we also had 0.10 moles.

00:04:16.240 --> 00:04:17.820
The total volume is the same,

00:04:17.820 --> 00:04:19.980
so it's the same calculation as before.

00:04:19.980 --> 00:04:23.680
0.10 moles divided by 0.200 liters,

00:04:23.680 --> 00:04:28.500
gives the concentration of
acetate anions of 0.50 molar.

00:04:28.500 --> 00:04:30.790
Remember, our goal was to calculate the pH

00:04:30.790 --> 00:04:32.780
of the resulting solution.

00:04:32.780 --> 00:04:34.100
So to calculate the pH,

00:04:34.100 --> 00:04:36.730
we're gonna need the
concentration of acetic acid

00:04:36.730 --> 00:04:39.980
and the concentration of acetate anions.

00:04:39.980 --> 00:04:42.830
So we know we have some
acetic acid in solution,

00:04:42.830 --> 00:04:44.760
and acetic acid reacts with water

00:04:44.760 --> 00:04:47.580
to form the hydronium ion, H3O+,

00:04:47.580 --> 00:04:51.340
and the acetate anion, CH3COO-.

00:04:51.340 --> 00:04:54.450
This reaction is a weak
acid equilibrium problem,

00:04:54.450 --> 00:04:56.200
so we have an equilibrium arrow,

00:04:56.200 --> 00:04:58.240
and we're gonna set up an ICE table

00:04:58.240 --> 00:05:00.710
for initial concentration,
change in concentration,

00:05:00.710 --> 00:05:03.370
and equilibrium concentration.

00:05:03.370 --> 00:05:05.520
We just calculated the
initial concentration

00:05:05.520 --> 00:05:08.390
of acetic acid to be 0.50 molar.

00:05:08.390 --> 00:05:12.500
And if we pretend like the
acetic acid hasn't ionized yet,

00:05:12.500 --> 00:05:15.730
the initial concentration of
hydronium ions would be zero

00:05:15.730 --> 00:05:18.420
and the initial concentration
of acetate anions

00:05:18.420 --> 00:05:21.030
from the ionization would be zero.

00:05:21.030 --> 00:05:22.780
However, we just calculated

00:05:22.780 --> 00:05:25.730
that there is a concentration
of acetate anions

00:05:25.730 --> 00:05:27.580
already in solution,

00:05:27.580 --> 00:05:30.670
and that concentration was 0.50 molar.

00:05:30.670 --> 00:05:35.230
So on the ICE table, I'm
gonna write in here 0.50

00:05:35.230 --> 00:05:37.440
for the acetate anion.

00:05:37.440 --> 00:05:40.800
Next, we think about some
of the acetic acid ionizing,

00:05:40.800 --> 00:05:43.510
and we don't know how much,
so we're gonna call that x.

00:05:43.510 --> 00:05:46.520
So under the change,
we're gonna write minus x.

00:05:46.520 --> 00:05:49.450
The mole ratio of acetic
acid to hydronium ion

00:05:49.450 --> 00:05:50.820
is one-to-one.

00:05:50.820 --> 00:05:53.280
So if we write minus x under acetic acid,

00:05:53.280 --> 00:05:56.530
we need to write plus x
under the hydronium ion.

00:05:56.530 --> 00:05:57.850
And for the acetate anion,

00:05:57.850 --> 00:06:00.190
there's also a coefficient of one,

00:06:00.190 --> 00:06:03.980
so we're gonna write plus
x under the acetate anion.

00:06:03.980 --> 00:06:05.900
Therefore, the equilibrium concentration

00:06:05.900 --> 00:06:09.310
of acetic acid is 0.50 minus x.

00:06:09.310 --> 00:06:11.240
And for hydronium ion,

00:06:11.240 --> 00:06:13.950
it would be zero plus x or just x.

00:06:13.950 --> 00:06:15.510
And for the acetate anion,

00:06:15.510 --> 00:06:18.800
it would be zero plus 0.50 plus x

00:06:18.800 --> 00:06:22.930
or just 0.50 plus x.

00:06:22.930 --> 00:06:26.705
Notice that there are two
sources of the acetate anion.

00:06:26.705 --> 00:06:28.950
The 0.50 molar

00:06:28.950 --> 00:06:33.070
came from the weak acid-strong
base neutralization reaction

00:06:33.070 --> 00:06:35.120
that we previously discussed.

00:06:35.120 --> 00:06:37.400
The other source of the acetate anion

00:06:37.400 --> 00:06:40.970
comes from the ionization of acetic acid,

00:06:40.970 --> 00:06:42.430
and that's this x here.

00:06:42.430 --> 00:06:46.610
So 0.50 plus x indicates
there are two sources

00:06:46.610 --> 00:06:48.590
for this common ion.

00:06:48.590 --> 00:06:51.803
Therefore, this is a
common-ion effect problem.

00:06:52.680 --> 00:06:54.960
The next step is to
write the Ka expression

00:06:54.960 --> 00:06:56.840
for acetic acid.

00:06:56.840 --> 00:06:58.840
So we can get that from
the balanced equation.

00:06:58.840 --> 00:07:02.490
The Ka value for acetic
acid at 25 degrees Celsius

00:07:02.490 --> 00:07:05.440
is equal to the concentration
of hydronium ions

00:07:05.440 --> 00:07:06.880
raised to the first power

00:07:06.880 --> 00:07:09.480
times the concentration of acetate anions

00:07:09.480 --> 00:07:10.830
raised to the first power

00:07:10.830 --> 00:07:13.640
divided by the
concentration of acetic acid

00:07:13.640 --> 00:07:14.850
raised to the first power,

00:07:14.850 --> 00:07:15.980
with water being left

00:07:15.980 --> 00:07:18.750
out of our equilibrium
constant expression.

00:07:18.750 --> 00:07:21.460
Next, we plug in our
equilibrium concentrations

00:07:21.460 --> 00:07:22.660
from our ICE table.

00:07:22.660 --> 00:07:24.970
For the hydronium ion, it's x.

00:07:24.970 --> 00:07:28.550
For the acetate anion, it's 0.50 plus x.

00:07:28.550 --> 00:07:33.000
And for acetic acid it's 0.50 minus x.

00:07:33.000 --> 00:07:36.000
Here, we have our equilibrium
concentrations plugged in

00:07:36.000 --> 00:07:40.620
and also the Ka value for acetic
acid at 25 degrees Celsius.

00:07:40.620 --> 00:07:42.380
Next, we need to solve for x.

00:07:42.380 --> 00:07:46.040
And to make the math easier,
we can make an approximation.

00:07:46.040 --> 00:07:50.260
With a relatively low value
for Ka for acetic acid,

00:07:50.260 --> 00:07:52.570
acetic acid doesn't ionize very much.

00:07:52.570 --> 00:07:55.520
Therefore, we know that x is
gonna be a very small number.

00:07:55.520 --> 00:07:58.892
And if x is a very small
number compared to 0.50,

00:07:58.892 --> 00:08:03.420
0.50 plus x is
approximately equal to 0.50.

00:08:03.420 --> 00:08:06.010
The same idea applies to 0.50 minus x.

00:08:06.010 --> 00:08:08.949
If x is a very small
number compared to 0.50,

00:08:08.949 --> 00:08:13.440
0.50 minus x is
approximately equal to 0.50.

00:08:13.440 --> 00:08:14.730
And with our approximations,

00:08:14.730 --> 00:08:17.500
the 0.50s would cancel out and give us x

00:08:17.500 --> 00:08:20.520
is equal to 1.8 times 10
to the negative fifth.

00:08:20.520 --> 00:08:21.530
From our ICE table,

00:08:21.530 --> 00:08:25.750
we know that x is equal to
the equilibrium concentration

00:08:25.750 --> 00:08:27.820
of hydronium ions.

00:08:27.820 --> 00:08:29.930
Therefore, at equilibrium,

00:08:29.930 --> 00:08:31.420
the concentration of hydronium ions

00:08:31.420 --> 00:08:35.520
is equal to 1.8 times 10 to
the negative fifth molar.

00:08:35.520 --> 00:08:38.170
And because our goal was to
find the pH of the solution,

00:08:38.170 --> 00:08:40.700
pH is equal to the negative
log of the concentration

00:08:40.700 --> 00:08:42.160
of hydronium ions.

00:08:42.160 --> 00:08:44.970
So when we plug in that concentration,

00:08:44.970 --> 00:08:46.090
we get the pH is equal

00:08:46.090 --> 00:08:48.890
to negative log of 1.8 times
10 to the negative fifth

00:08:48.890 --> 00:08:52.100
which is equal to 4.74.

00:08:52.100 --> 00:08:55.570
So if you react a weak
acid with a strong base

00:08:55.570 --> 00:08:58.590
and the weak acid is in excess,

00:08:58.590 --> 00:09:02.620
the pH of the solution
will be less than seven.

00:09:02.620 --> 00:09:03.553
It'll be acidic.


Weak acid–strong base reactions

WEBVTT
Kind: captions
Language: en

00:00:00.680 --> 00:00:04.540
- [Instructor] Acetic acid
is an example of a weak acid

00:00:04.540 --> 00:00:08.783
and sodium hydroxide is an
example of a strong base.

00:00:08.783 --> 00:00:12.040
When acetic acid reacts
with sodium hydroxide,

00:00:12.040 --> 00:00:14.480
an aqueous solution of
sodium acetate is formed

00:00:14.480 --> 00:00:16.070
along with water.

00:00:16.070 --> 00:00:19.830
Since this reaction is an
acid-base neutralization reaction,

00:00:19.830 --> 00:00:22.060
and these reactions go to completion,

00:00:22.060 --> 00:00:23.890
instead of using an equilibrium arrow,

00:00:23.890 --> 00:00:26.630
we simply draw an arrow
going to the right.

00:00:26.630 --> 00:00:29.610
Next, let's write the
overall ionic equation.

00:00:29.610 --> 00:00:33.270
This is also called the
complete ionic equation.

00:00:33.270 --> 00:00:34.890
Let's start with acetic acid.

00:00:34.890 --> 00:00:37.080
Acetic acid is a weak acid

00:00:37.080 --> 00:00:40.200
that only partly ionizes in solution.

00:00:40.200 --> 00:00:43.010
Therefore, an aqueous
solution of acetic acid,

00:00:43.010 --> 00:00:46.210
most of the acetic acid
molecules stay protonated

00:00:46.210 --> 00:00:48.500
and don't turn into acetate.

00:00:48.500 --> 00:00:53.230
Therefore, we're just gonna
write CH3COOH down here.

00:00:53.230 --> 00:00:56.420
We're not gonna show acetic acid ionizing

00:00:56.420 --> 00:00:59.400
into H+ and the conjugate base.

00:00:59.400 --> 00:01:01.770
However, things are different
for sodium hydroxide.

00:01:01.770 --> 00:01:03.730
Sodium hydroxide is a strong base,

00:01:03.730 --> 00:01:07.600
and strong bases dissociate
100% in solution.

00:01:07.600 --> 00:01:10.310
So an aqueous solution of sodium hydroxide

00:01:10.310 --> 00:01:15.310
consists of sodium cations
and hydroxide anions.

00:01:16.340 --> 00:01:17.910
Next, let's think about our products.

00:01:17.910 --> 00:01:21.040
We have an aqueous
solution of sodium acetate.

00:01:21.040 --> 00:01:23.840
Sodium acetate is a soluble salt.

00:01:23.840 --> 00:01:26.160
Therefore, in aqueous solution,

00:01:26.160 --> 00:01:29.290
we would have sodium cations, Na+,

00:01:29.290 --> 00:01:31.673
and the acetate anion, CH3COO-.

00:01:35.140 --> 00:01:38.080
Because water ionizes only
to an extremely small extent,

00:01:38.080 --> 00:01:39.690
we don't write it as the ions.

00:01:39.690 --> 00:01:42.950
We simply write H2O.

00:01:42.950 --> 00:01:45.770
To save some time, I've added
in some aqueous subscripts

00:01:45.770 --> 00:01:47.670
and a liquid one for water,

00:01:47.670 --> 00:01:50.050
some plus signs, and the reaction arrow.

00:01:50.050 --> 00:01:51.440
So this balanced equation

00:01:51.440 --> 00:01:54.960
represents the complete ionic equation.

00:01:54.960 --> 00:01:57.220
Next, we're gonna use the
complete ionic equation

00:01:57.220 --> 00:01:59.440
to write the net ionic equation

00:01:59.440 --> 00:02:01.900
for this weak acid-strong base reaction.

00:02:01.900 --> 00:02:02.733
And to do that,

00:02:02.733 --> 00:02:05.660
we first need to identify spectator ions.

00:02:05.660 --> 00:02:09.830
Remember, spectator ions don't
participate in the reaction.

00:02:09.830 --> 00:02:12.030
So looking at the complete ionic equation,

00:02:12.030 --> 00:02:13.920
there are sodium cations on the left

00:02:13.920 --> 00:02:15.720
and sodium cations on the right,

00:02:15.720 --> 00:02:17.720
so we can cross out the sodium cation,

00:02:17.720 --> 00:02:19.730
that's our spectator ion.

00:02:19.730 --> 00:02:21.580
After we take out the sodium cation,

00:02:21.580 --> 00:02:24.480
what's left over is
our net ionic equation.

00:02:24.480 --> 00:02:26.070
So for our net equation,

00:02:26.070 --> 00:02:29.410
we have acetic acid plus hydroxide anion,

00:02:29.410 --> 00:02:32.450
forms the acetate anion and water.

00:02:32.450 --> 00:02:34.280
Now that we have our net ionic equation,

00:02:34.280 --> 00:02:36.890
we're gonna think about
three different situations.

00:02:36.890 --> 00:02:38.450
And the first situation,

00:02:38.450 --> 00:02:42.710
we have equal moles of our
weak acid and our strong base.

00:02:42.710 --> 00:02:45.330
Looking at our balanced
net ionic equation,

00:02:45.330 --> 00:02:48.540
the mole ratio of our weak
acid to our strong base

00:02:48.540 --> 00:02:50.040
is one-to-one.

00:02:50.040 --> 00:02:52.700
Therefore, if we have equal
moles of our weak acid

00:02:52.700 --> 00:02:53.800
and our strong base

00:02:53.800 --> 00:02:56.100
and the mole ratio is one-to-one,

00:02:56.100 --> 00:02:57.410
the weak and the strong base

00:02:57.410 --> 00:02:59.730
are going to completely
neutralize each other

00:02:59.730 --> 00:03:02.250
and form the acetate anion.

00:03:02.250 --> 00:03:03.840
So if our goal is to figure out the pH

00:03:03.840 --> 00:03:05.380
of the resulting solution,

00:03:05.380 --> 00:03:07.370
we don't need to consider acetic acid

00:03:07.370 --> 00:03:09.730
and we don't need to
consider the hydroxide ions

00:03:09.730 --> 00:03:12.040
'cause these have been completely used up.

00:03:12.040 --> 00:03:14.560
We need to think about the acetate anion

00:03:14.560 --> 00:03:16.250
in aqueous solution.

00:03:16.250 --> 00:03:19.730
And in aqueous solution,
acetate anions react with water

00:03:19.730 --> 00:03:23.460
to form acetic acid and hydroxide ions.

00:03:23.460 --> 00:03:25.550
This reaction does come to an equilibrium

00:03:25.550 --> 00:03:27.880
and since Kb is less than one,

00:03:27.880 --> 00:03:31.140
at equilibrium, there will be
a large amount of reactants

00:03:31.140 --> 00:03:33.470
and only a small amount of products.

00:03:33.470 --> 00:03:36.230
However, since we are
increasing the concentration

00:03:36.230 --> 00:03:39.310
of hydroxide ions in solution,

00:03:39.310 --> 00:03:42.310
the resulting solution will be basic.

00:03:42.310 --> 00:03:44.690
Therefore, at 25 degrees Celsius,

00:03:44.690 --> 00:03:48.530
the pH of the resulting solution
will be greater than seven.

00:03:48.530 --> 00:03:50.500
If we wanted to calculate the actual pH,

00:03:50.500 --> 00:03:52.730
we would need some more information,

00:03:52.730 --> 00:03:53.563
but we would treat it

00:03:53.563 --> 00:03:56.720
just like a weak base equilibria problem.

00:03:56.720 --> 00:04:00.110
Also note that the hydroxide
ions that we wrote down here

00:04:00.110 --> 00:04:02.830
are not the same thing
as the hydroxide ions

00:04:02.830 --> 00:04:05.110
from our net ionic equation.

00:04:05.110 --> 00:04:08.490
The hydroxide ions from
our net ionic equation

00:04:08.490 --> 00:04:09.650
went away completely.

00:04:09.650 --> 00:04:13.820
They were neutralized by
reacting with the acetic acid.

00:04:13.820 --> 00:04:16.110
So these hydroxide ions were generated

00:04:16.110 --> 00:04:19.460
from the anion hydrolysis
reaction of acetate plus water,

00:04:19.460 --> 00:04:21.610
and even though it's only a small amount

00:04:21.610 --> 00:04:24.080
or a small concentration
of hydroxide ions,

00:04:24.080 --> 00:04:25.590
those are the hydroxide ions

00:04:25.590 --> 00:04:27.970
that make the pH greater than seven.

00:04:27.970 --> 00:04:29.310
For the second situation,

00:04:29.310 --> 00:04:33.100
let's think about the
weak acid being in excess.

00:04:33.100 --> 00:04:36.240
Since the mole ratio of our
weak acid to our strong base

00:04:36.240 --> 00:04:37.550
is one-to-one,

00:04:37.550 --> 00:04:39.240
if we have more of the weak acid

00:04:39.240 --> 00:04:40.630
than we do have the strong base,

00:04:40.630 --> 00:04:43.160
all of the strong base will be used up,

00:04:43.160 --> 00:04:46.070
and we will form the acetate anion.

00:04:46.070 --> 00:04:47.840
And when the reaction comes to completion,

00:04:47.840 --> 00:04:51.800
there'll be no more
hydroxide ion in solution.

00:04:51.800 --> 00:04:54.700
So if our goal is to find the
pH of the resulting solution,

00:04:54.700 --> 00:04:57.690
we don't need to consider
the hydroxide ion at all

00:04:57.690 --> 00:04:59.440
since it's all been used up.

00:04:59.440 --> 00:05:01.840
We need to think about the weak acid

00:05:01.840 --> 00:05:03.717
that was in excess, acetic acid,

00:05:03.717 --> 00:05:06.200
and we also have to think
about the acetate anion

00:05:06.200 --> 00:05:09.390
that was formed in the
neutralization reaction.

00:05:09.390 --> 00:05:12.560
First, let's think about the
weak acid that was in excess.

00:05:12.560 --> 00:05:14.730
Acetic acid will react with water

00:05:14.730 --> 00:05:17.200
to form the hydronium ion, H3O+,

00:05:17.200 --> 00:05:21.330
and the acetate anion, CH3COO-.

00:05:21.330 --> 00:05:24.050
Because the concentration
of hydronium ions

00:05:24.050 --> 00:05:26.410
in solution increases,

00:05:26.410 --> 00:05:29.210
that makes the solution more acidic.

00:05:29.210 --> 00:05:32.300
Therefore, when the
weak acid is in excess,

00:05:32.300 --> 00:05:35.530
the pH of the resulting
solution will be less than seven

00:05:35.530 --> 00:05:37.750
at 25 degrees Celsius.

00:05:37.750 --> 00:05:40.080
If our goal is to calculate the actual pH

00:05:40.080 --> 00:05:41.680
of the resulting solution,

00:05:41.680 --> 00:05:42.730
our first thought would be,

00:05:42.730 --> 00:05:45.250
this is a weak acid equilibria problem.

00:05:45.250 --> 00:05:46.740
And it is, however,

00:05:46.740 --> 00:05:49.620
there are two sources of acetate anion.

00:05:49.620 --> 00:05:51.750
One source of acetate anion

00:05:51.750 --> 00:05:54.630
is from the reaction of
acetic acid with water,

00:05:54.630 --> 00:05:57.280
and then the other source
of the acetate anion

00:05:57.280 --> 00:06:00.890
came from our acid-base
neutralization reaction.

00:06:00.890 --> 00:06:03.530
Therefore, one way to think
about this problem is,

00:06:03.530 --> 00:06:06.170
it's actually a common-ion effect problem

00:06:06.170 --> 00:06:09.710
where the common ion is the acetate anion.

00:06:09.710 --> 00:06:11.730
The presence of a common ion

00:06:11.730 --> 00:06:15.580
decreases the ionization
of acetic acid in water.

00:06:15.580 --> 00:06:18.380
However, since the
concentration of hydronium ions

00:06:18.380 --> 00:06:19.870
will still increase,

00:06:19.870 --> 00:06:23.220
the pH of the solution
will be less than seven.

00:06:23.220 --> 00:06:25.520
Another way to think
about this problem is,

00:06:25.520 --> 00:06:28.290
when we formed the acetate anion

00:06:28.290 --> 00:06:29.690
from the neutralization reaction,

00:06:29.690 --> 00:06:32.130
we had some of the weak acid in excess.

00:06:32.130 --> 00:06:35.900
And when we have a weak acid
and its conjugate base present,

00:06:35.900 --> 00:06:37.540
a buffer solution forms.

00:06:37.540 --> 00:06:40.670
And so we could also use the
Henderson-Hasselbalch equation

00:06:40.670 --> 00:06:43.920
to calculate the pH of
the resulting solution.

00:06:43.920 --> 00:06:45.130
For the third situation,

00:06:45.130 --> 00:06:48.510
let's think about the
strong base being in excess.

00:06:48.510 --> 00:06:50.330
Since the mole ratio of our weak acid

00:06:50.330 --> 00:06:52.520
to our strong base is one-to-one,

00:06:52.520 --> 00:06:55.100
if we have the strong base in excess,

00:06:55.100 --> 00:06:59.080
the strong base is going to
neutralize all of the weak acid.

00:06:59.080 --> 00:07:02.280
Therefore, when the reaction
comes to completion,

00:07:02.280 --> 00:07:05.950
there will be no more weak acid present.

00:07:05.950 --> 00:07:08.910
So if our goal is to find the
pH of the resulting solution,

00:07:08.910 --> 00:07:12.510
we would just have to think
about the extra hydroxide ions

00:07:12.510 --> 00:07:13.660
in solution.

00:07:13.660 --> 00:07:16.070
And since there are extra hydroxide ions

00:07:16.070 --> 00:07:17.660
from our strong base,

00:07:17.660 --> 00:07:19.960
we know at 25 degrees Celsius,

00:07:19.960 --> 00:07:23.480
the pH of the solution
will be greater than seven,

00:07:23.480 --> 00:07:26.300
so the resulting solution will be basic.

00:07:26.300 --> 00:07:29.240
If we wanted to calculate the
actual pH of the solution,

00:07:29.240 --> 00:07:33.410
this would become a strong
base pH calculation problem.

00:07:33.410 --> 00:07:35.900
And while it is true
that the acetate anions

00:07:35.900 --> 00:07:38.440
that were produced from
the neutralization reaction

00:07:38.440 --> 00:07:39.710
will react with water

00:07:39.710 --> 00:07:43.000
to also increase the
concentration of hydroxide ions,

00:07:43.000 --> 00:07:45.330
the concentration of hydroxide
ions that we would get

00:07:45.330 --> 00:07:48.350
from the anion hydrolysis
of the acetate anion

00:07:48.350 --> 00:07:50.860
would be negligible to the hydroxide ions

00:07:50.860 --> 00:07:52.340
that were left over.

00:07:52.340 --> 00:07:55.010
Therefore, we could just
consider the strong base

00:07:55.010 --> 00:07:56.550
to calculate the pH,

00:07:56.550 --> 00:07:59.190
and we don't need to worry
about anion hydrolysis

00:07:59.190 --> 00:08:01.073
for this particular situation.