# 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.

The reaction is as follows:
$\ce{HN3 → H^{+}(aq) + N3^{−}(aq)} \nonumber$
It does not proceed 100% to products because hydrazoic acid is not a strong acid.

## Key Takeaways

• Strong acids and bases are 100% ionized in aqueous solution.
• Weak acids and bases are less than 100% ionized in aqueous solution.
• Salts of weak acids or bases can affect the acidity or basicity of their aqueous solutions.