14.7: Strong and Weak Acids and Bases

LEARNING OBJECTIVES

• Define a strong and a weak acid and base.
• Recognize an acid or a base as strong or weak.
• Determine if a salt produces an acidic or a basic solution.

Strong and Weak Acids

Except for their names and formulas, so far we have treated all acids as equals, especially in a chemical reaction. However, acids can be very different in a very important way. Consider HCl(aq). When HCl is dissolved in H2O, it completely dissociates into H+(aq) and Cl(aq) ions; all the HCl molecules become ions:

$HCl\overset{100\%}{\rightarrow}H^{+}(aq)+Cl^{-}(aq)$

Any acid that dissociates 100% into ions is called a strong acid. If it does not dissociate 100%, it is a weak acid. HC2H3O2 is an example of a weak acid:

$HC_{2}H_{3}O_{2}\overset{\sim 5\%}{\longrightarrow}H^{+}(aq)+C_{2}H_{3}O_{2}^{-}(aq)$

Because this reaction does not go 100% to completion, it is more appropriate to write it as a reversible reaction:

$HC_{2}H_{3}O_{2}\rightleftharpoons H^{+}(aq)+C_{2}H_{3}O_{2}^{-}(aq)$

As it turns out, there are very few strong acids, which are given in Table $$\PageIndex{1}$$. If an acid is not listed here, it is a weak acid. It may be 1% ionized or 99% ionized, but it is still classified as a weak acid.

Any acid that dissociates 100% into ions is called a strong acid. If it does not dissociate 100%, it is a weak acid.

Table $$\PageIndex{1}$$: Strong Acids and Bases
Acids Bases
HCl LiOH
HBr NaOH
HI KOH
HNO3 RbOH
H2SO4 CsOH
HClO3 Mg(OH)2
HClO4 Ca(OH)2
Sr(OH)2
Ba(OH)2

Strong and Weak Bases

The issue is similar with bases: a strong base is a base that is 100% ionized in solution. If it is less than 100% ionized in solution, it is a weak base. There are very few strong bases (Table $$\PageIndex{1}$$); any base not listed is a weak base. All strong bases are OH compounds. So a base based on some other mechanism, such as NH3 (which does not contain OH ions as part of its formula), will be a weak base.

Example $$\PageIndex{1}$$: Identifying Strong and Weak Acids and Bases

Identify each acid or base as strong or weak.

1. HCl
2. Mg(OH)2
3. C5H5N

Solution

1. Because HCl is listed in Table $$\PageIndex{1}$$, it is a strong acid.
2. Because Mg(OH)2 is listed in Table $$\PageIndex{1}$$, it is a strong base.
3. The nitrogen in C5H5N would act as a proton acceptor and therefore can be considered a base, but because it does not contain an OH compound, it cannot be considered a strong base; it is a weak base.

Exercise $$\PageIndex{1}$$

Identify each acid or base as strong or weak.

1. $$\ce{RbOH}$$
2. $$\ce{HNO_2}$$
strong base
weak acid

Example $$\PageIndex{2}$$: Characterizing Base IOnization

Write the balanced chemical equation for the dissociation of Ca(OH)2 and indicate whether it proceeds 100% to products or not.

Solution

This is an ionic compound of Ca2+ ions and OH ions. When an ionic compound dissolves, it separates into its constituent ions:

$\ce{Ca(OH)2 → Ca^{2+}(aq) + 2OH^{−}(aq)} \nonumber$

Because Ca(OH)2 is listed in Table $$\PageIndex{1}$$, this reaction proceeds 100% to products.

Exercise $$\PageIndex{2}$$

Write the balanced chemical equation for the dissociation of hydrazoic acid (HN3) and indicate whether it proceeds 100% to products or not.

$\ce{HN3 → H^{+}(aq) + N3^{−}(aq)} \nonumber$