10.4: The Strengths of Acids and Bases

Learning Objectives

• Describe the difference between strong and weak acids and bases.
• Describe how a chemical reaction reaches chemical equilibrium.
• Define the pH scale and use it to describe acids and bases.

Acids and bases do not all demonstrate the same degree of chemical activity in solution. Different acids and bases have different strengths.

Strong and Weak Acids

Let us consider the strengths of acids first. A small number of acids ionize completely in aqueous solution. For example, when HCl dissolves in water, every molecule of HCl separates into a hydronium ion and a chloride ion:

$\ce{HCl(g) + H2O(l) ->[\sim 100\%] H_3O^{+}(aq) + Cl^{-} (aq)} \label{Eq1}$

HCl(aq) is one example of a strong acid, which is a compound that is essentially 100% ionized in aqueous solution. There are very few strong acids. The important ones are listed in Table $$\PageIndex{1}$$.

Table $$\PageIndex{1}$$: Strong Acids and Bases (All in Aqueous Solution)
Acids Bases
HCl LiOH
HBr NaOH
HI KOH
HNO3 Mg(OH)2
H2SO4 Ca(OH)2
HClO4

By analogy, a strong base is a compound that is essentially 100% ionized in aqueous solution. As with acids, there are only a few strong bases, which are also listed in Table $$\PageIndex{1}$$.

If an acid is not listed in Table $$\PageIndex{1}$$, it is likely a weak acid, which is a compound that is not 100% ionized in aqueous solution. Similarly, a weak base is a compound that is not 100% ionized in aqueous solution. For example, acetic acid (HC2H3O2) is a weak acid. The ionization reaction for acetic acid is as follows:

$HC_2H_3O_{2(aq)} + H_2O_{(ℓ)} \rightarrow H_3O^+_{(aq)} + C_2H_3O^−_{2(aq)} \label{Eq2}$

Depending on the concentration of HC2H3O2, the ionization reaction may occur only for 1%–5% of the acetic acid molecules.

Looking Closer: Household Acids and Bases

Many household products are acids or bases. For example, the owner of a swimming pool may use muriatic acid to clean the pool. Muriatic acid is another name for hydrochloric acid [HCl(aq)]. Vinegar has already been mentioned as a dilute solution of acetic acid [HC2H3O2(aq)]. In a medicine chest, one may find a bottle of vitamin C tablets; the chemical name of vitamin C is ascorbic acid (HC6H7O6).

One of the more familiar household bases is ammonia (NH3), which is found in numerous cleaning products. As we mentioned previously, ammonia is a base because it increases the hydroxide ion concentration by reacting with water:

$NH_{3(aq)} + H_2O_{(ℓ)} \rightarrow NH^+_{4(aq)} + OH^−_{(aq)} \label{Eq3}$

Many soaps are also slightly basic because they contain compounds that act as Brønsted-Lowry bases, accepting protons from water and forming excess hydroxide ions. This is one reason that soap solutions are slippery.

Chemical Equilibrium

The behavior of weak acids and bases illustrates a key concept in chemistry. Does the chemical reaction describing the ionization of a weak acid or base just stop when the acid or base is done ionizing? Actually, no. Rather, the reverse process—the reformation of the molecular form of the acid or base—occurs, ultimately at the same rate as the ionization process. For example, the ionization of the weak acid HC2H3O2 (aq) is as follows:

$HC_2H_3O_{2(aq)} + H_2O_{(ℓ)} \rightarrow H_3O^+_{(aq)} + C_2H_3O^−_{2(aq)} \label{Eq4}$

The reverse process also begins to occur:

$H_3O^+_{(aq)} + C_2H_3O^−_{2(aq)} \rightarrow HC_2H_3O_{2(aq)} + H_2O_{(ℓ)} \label{Eq5}$

Eventually, there is a balance between the two opposing processes, and no additional change occurs. The chemical reaction is better represented at this point with a double arrow:

$HC_2H_3O_{2(aq)} + H_2O_{(ℓ)} \rightleftharpoons H_3O^+_{(aq)} + C_2H_3O^−_{2(aq)} \label{Eq6}$

The $$\rightleftharpoons$$ implies that both the forward and reverse reactions are occurring, and their effects cancel each other out. A process at this point is considered to be at chemical equilibrium (or equilibrium). It is important to note that the processes do not stop. They balance out each other so that there is no further net change; that is, chemical equilibrium is a dynamic equilibrium.

Example $$\PageIndex{1}$$: Partial Ionization

Write the equilibrium chemical equation for the partial ionization of each weak acid or base.

1. HNO2(aq)
2. C5H5N(aq)

SOLUTION

1. HNO2(aq) + H2O(ℓ) ⇆ NO2(aq) + H3O+(aq)
2. C5H5N(aq) + H2O(ℓ) ⇆ C5H5NH+(aq) + OH(aq)

Exercise $$\PageIndex{1}$$

Write the equilibrium chemical equation for the partial ionization of each weak acid or base.

1. $$HF_{(aq)}$$
2. $$AgOH_{(aq)}$$

Hydrofluoric acid $$HF_{(aq)}$$ reacts directly with glass (very few chemicals react with glass). Hydrofluoric acid is used in glass etching.

Finally, you may realize that the autoionization of water is actually an equilibrium process, so it is more properly written with the double arrow:

$H_2O_{(ℓ)} + H_2O_{(ℓ)} \rightleftharpoons H_3O^+_{(aq)} + OH^−_{(aq)} \label{Eq7}$

The pH Scale

One qualitative measure of the strength of an acid or a base solution is the pH scale, which is based on the concentration of the hydronium (or hydrogen) ion in aqueous solution.

$pH = -\log[H^+]$

or

$pH = -\log[H_3O^+]$

A neutral (neither acidic nor basic) solution, one that has the same concentration of hydrogen and hydroxide ions, has a pH of 7. A pH below 7 means that a solution is acidic, with lower values of pH corresponding to increasingly acidic solutions. A pH greater than 7 indicates a basic solution, with higher values of pH corresponding to increasingly basic solutions. Thus, given the pH of several solutions, you can state which ones are acidic, which ones are basic, and which are more acidic or basic than others. Table $$\PageIndex{2}$$ lists the pH of several common solutions. Notice that some biological fluids are nowhere near neutral.

Table $$\PageIndex{2}$$: The pH Values of Some Common Solutions
Solution pH
battery acid 0.3
stomach acid 1–2
lemon or lime juice 2.1
vinegar 2.8–3.0
Coca-Cola 3
wine 2.8–3.8
beer 4–5
coffee 5
milk 6
urine 6
pure H2O 7
(human) blood 7.3–7.5
sea water 8
antacid (milk of magnesia) 10.5
NH3 (1 M) 11.6
bleach 12.6
NaOH (1 M) 14.0

Weak acids and bases are relatively common. You may notice from Table $$\PageIndex{2}$$ that many food products are slightly acidic. They are acidic because they contain solutions of weak acids. If the acid components of these foods were strong acids, the food would likely be inedible.

Key Takeaways

• Acids and bases can be strong or weak depending on the extent of ionization in solution.
• Most chemical reactions reach equilibrium at which point there is no net change.
• The pH scale is used to succinctly communicate the acidity or basicity of a solution.