12.4 Strong and Weak Acids and Bases and Their Salts

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:

$$\text{HCl }\stackrel{\text{100\%}}{\to }{\text{H}}^{+}{\text{(aq) + Cl}}^{-}\text{(aq)}$$

Any acid that dissociates 100% into ions is called a strong acidAny acid that is 100% dissociated into ions in aqueous solution.. If it does not dissociate 100%, it is a weak acidAny acid that is less than 100% dissociated into ions in aqueous solution.. HC₂H₃O₂ is an example of a weak acid:

$${\text{HC}}_{\text{2}}{\text{H}}_{\text{3}}{\text{O}}_{\text{2}}\stackrel{~5\%}{\to }{\text{H}}^{\text{+}}{\text{(aq) + C}}_{\text{2}}{\text{H}}_{\text{3}}{\text{O}}_{\text{2}}^{-}\text{(aq)}$$

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

$${\text{HC}}_{\text{2}}{\text{H}}_{\text{3}}{\text{O}}_{\text{2}}\rightleftarrows {\text{H}}^{\text{+}}{\text{(aq) + C}}_{\text{2}}{\text{H}}_{\text{3}}{\text{O}}_{\text{2}}^{-}\text{(aq)}$$

As it turns out, there are very few strong acids, which are given in Table 12.2 "Strong Acids and Bases". 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.

The issue is similar with bases: a strong baseAny base that is 100% dissociated into ions in aqueous solution. is a base that is 100% ionized in solution. If it is less than 100% ionized in solution, it is a weak baseAny base that is less than 100% dissociated into ions in aqueous solution.. There are very few strong bases (see Table 12.2 "Strong Acids and Bases"); 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.

Certain salts will also affect the acidity or basicity of aqueous solutions because some of the ions will undergo hydrolysis, just like NH₃ does to make a basic solution. The general rule is that salts with ions that are part of strong acids or bases will not hydrolyze, while salts with ions that are part of weak acids or bases will hydrolyze.

Consider NaCl. When it dissolves in an aqueous solution, it separates into Na⁺ ions and Cl⁻ ions:

NaCl → Na⁺(aq) + Cl⁻(aq)

Will the Na⁺(aq) ion hydrolyze? If it does, it will interact with the OH⁻ ion to make NaOH:

Na⁺(aq) + H₂O → NaOH + H⁺(aq)

However, NaOH is a strong base, which means that it is 100% ionized in solution:

NaOH → Na⁺(aq) + OH⁻(aq)

The free OH⁻(aq) ion reacts with the H⁺(aq) ion to remake a water molecule:

H⁺(aq) + OH⁻(aq) → H₂O

The net result? There is no change, so there is no effect on the acidity or basicity of the solution from the Na⁺(aq) ion. What about the Cl⁻ ion? Will it hydrolyze? If it does, it will take an H⁺ ion from a water molecule:

Cl⁻(aq) + H₂O → HCl + OH⁻

However, HCl is a strong acid, which means that it is 100% ionized in solution:

HCl → H⁺(aq) + Cl⁻(aq)

The free H⁺(aq) ion reacts with the OH⁻(aq) ion to remake a water molecule:

H⁺(aq) + OH⁻(aq) → H₂O

The net result? There is no change, so there is no effect on the acidity or basicity of the solution from the Cl⁻(aq) ion. Because neither ion in NaCl affects the acidity or basicity of the solution, NaCl is an example of a neutral saltAn ionic compound that does not affect the acidity of its aqueous solution..

Things change, however, when we consider a salt like NaC₂H₃O₂. We already know that the Na⁺ ion won’t affect the acidity of the solution. What about the acetate ion? If it hydrolyzes, it will take an H⁺ from a water molecule:

C₂H₃O₂⁻(aq) + H₂O → HC₂H₃O₂ + OH⁻(aq)

Does this happen? Yes, it does. Why? Because HC₂H₃O₂ is a weak acid. Any chance a weak acid has to form, it will (the same with a weak base). As some C₂H₃O₂⁻ ions hydrolyze with H₂O to make the molecular weak acid, OH⁻ ions are produced. OH⁻ ions make solutions basic. Thus NaC₂H₃O₂ solutions are slightly basic, so such a salt is called a basic saltAn ionic compound whose aqueous solution is slightly basic..

There are also salts whose aqueous solutions are slightly acidic. NH₄Cl is an example. When NH₄Cl is dissolved in H₂O, it separates into NH₄⁺ ions and Cl⁻ ions. We have already seen that the Cl⁻ ion does not hydrolyze. However, the NH₄⁺ ion will:

NH₄⁺(aq) + H₂O → NH₃(aq) + H₃O⁺(aq)

Recall from Section 12.1 "Arrhenius Acids and Bases" that H₃O⁺ ion is the hydronium ion, the more chemically proper way to represent the H⁺ ion. This is the classic acid species in solution, so a solution of NH₄⁺(aq) ions is slightly acidic. NH₄Cl is an example of an acid saltAn ionic compound whose aqueous solution is slightly acidic.. The molecule NH₃ is a weak base, and it will form when it can, just like a weak acid will form when it can.

So there are two general rules: (1) If an ion derives from a strong acid or base, it will not affect the acidity of the solution. (2) If an ion derives from a weak acid, it will make the solution basic; if an ion derives from a weak base, it will make the solution acidic.

Some salts are composed of ions that come from both weak acids and weak bases. The overall effect on an aqueous solution depends on which ion exerts more influence on the overall acidity. We will not consider such salts here.