11.4: 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.

Acids and bases do not all demonstrate the same degree of chemical activity in solution. Different acids and bases have different strengths. In other words, some are strong electrolytes and some are weak electrolytes. 

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	Na OH
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 (\(\ce{HC2H3O2}\)) is a weak acid. The ionization reaction for acetic acid is as follows:

\[\ce{HC2H3O2(aq) + H2O(ℓ) \rightarrow H3O^{+}(aq) + C2H3O^{−}2(aq)} \label{Eq2}\]

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 [\(\ce{HCl(aq)}\)]. Vinegar has already been mentioned as a dilute solution of acetic acid [\(\ce{HC2H3O2(aq)}\)]. In a medicine chest, one may find a bottle of vitamin C tablets; the chemical name of vitamin C is ascorbic acid (\(\ce{HC6H7O6}\)).

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:

\[\ce{NH3(aq) + H2O(ℓ) \rightarrow NH^{+}4(aq) + OH^{−}(aq)} \label{Eq3}\]

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.

Figure \(\PageIndex{1}\): (left) Bottles of alkaline drain cleaners containing sodium hydroxide can dissolve greases and hair. (right) Chemical burns caused by sodium hydroxide solution photographed 44 hours after exposure. Figures used with permission from Wikipedia.

Perhaps the most dangerous household chemical is the lye-based drain cleaner. Lye is a common name for sodium hydroxide, although it is also used as a synonym for potassium hydroxide. Lye is an extremely caustic chemical that can react with grease, hair, food particles, and other substances that may build up and form a clog in a pipe. Unfortunately, lye can also attack tissues and other substances in our bodies. Thus, when we use lye-based drain cleaners, we must be very careful not to touch any of the solid drain cleaner or spill the water it was poured into. Safer, nonlye drain cleaners use peroxide compounds to react on the materials in the clog and clear the drain.

Chemical Equilibrium in Weak Acids and Bases

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 \(\ce{HC2H3O2 (aq)}\) is as follows:

\[\ce{HC2H3O2(aq) + H2O(ℓ) \rightarrow H3O^{+}(aq) + C2H3O^{−}2(aq)} \label{Eq4}\]

The reverse process also begins to occur:

\[\ce{H3O^{+}(aq) + C2H3O^{−}2(aq) \rightarrow HC2H3O2(aq) + H2O(ℓ)} \label{Eq5}\]

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:

\[\ce{HC2H3O2(aq) + H2O(ℓ) \leftrightharpoons H3O^{+}(aq) + C2H3O^{-}2(aq)} \label{Eq6}\]

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. \(AgOH_{(aq)}\)
3. CH₃NH₂(aq)

Answer

a. HF(aq) + H₂O(ℓ) ⇆ F⁻(aq) + H₃O⁺(aq)

b. AgOH(aq) ⇆ Ag⁺(aq) + OH⁻(aq)

c. CH₃NH₂(aq) + H₂O(ℓ) ⇆ CH₃NH₃⁺(aq) + OH⁻(aq)

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.

Concept Review Exercises

1. Explain the difference between a strong acid or base and a weak acid or base.
2. Explain what is occurring when a chemical reaction reaches equilibrium.

Answers

1. A strong acid or base is 100% ionized in aqueous solution; a weak acid or base is less than 100% ionized.
2. The overall reaction progress stops because the reverse process balances out the forward process.

Exercises

1. Name a strong acid and a weak acid. (Hint: use Table 10.4.1.)
2. Name a strong base and a weak base. (Hint: use Table 10.4.1.)
3. Is each compound a strong acid or a weak acid? Assume all are in aqueous solution. 
   1. HF
   2. HC₂H₃O₂
   3. HCl
   4. HClO₄
4. Is each compound a strong acid or a weak acid? Assume all are in aqueous solution. 
   1. H₂SO₄
   2. HSO₄⁻
   3. HPO₄²⁻
   4. HNO₃
5. Is each compound a strong base or a weak base? Assume all are in aqueous solution. 
   1. NH₃
   2. NaOH
   3. Mg(OH)₂
   4. Cu(OH)₂
6. Is each compound a strong base or a weak base? Assume all are in aqueous solution. 
   1. KOH
   2. H₂O
   3. Fe(OH)₂
   4. Fe(OH)₃
7. Write the chemical equation for the equilibrium process for each weak acid in Exercise 3.
8. Write the chemical equation for the equilibrium process for each weak acid in Exercise 4.
9. Write the chemical equation for the equilibrium process for each weak base in Exercise 5.
10. Write the chemical equation for the equilibrium process for each weak base in Exercise 6.

Answers

1. strong acid: HCl; weak acid: HC₂H₃O₂ (answers will vary)
2. strong base: NaOH; weak base: NH₃ (answers will vary)

3. 1. weak
   2. weak
   3. strong
   4. strong
4. 1. strong
   2. weak
   3. weak
   4. strong

5. 1. weak
   2. strong
   3. strong
   4. weak
6. 1. strong
   2. weak
   3. weak
   4. weak

7. HF(aq) ⇆ H⁺(aq) + F⁻(aq); 3b: HC₂H₃O₂(aq) ⇆ H⁺(aq) + C₂H₃O₂⁻(aq)

8. HSO₄⁻(aq) ⇆ H⁺(aq) + SO₄²⁻(aq); 4c: HPO₄²⁻(aq) ⇆ H⁺(aq) + PO₄³⁻(aq)

9. NH₃(aq) + H₂O ⇆ NH₄⁺(aq) + OH⁻(aq); 5d: Cu(OH)₂(aq) ⇆ Cu²⁺(aq) + 2OH⁻(aq)

10. H₂O + H₂O ⇆ H₃O⁺(aq) + OH⁻(aq); 6c: Fe(OH)₂(aq) ⇆ Fe²⁺(aq) + 2OH⁻(aq); 6d: Fe(OH)3(aq) ⇆ Fe³⁺(aq) + 3OH⁻(aq)