Learning Objectives
- Write the acid dissociation constant (\(K_a\)) expression.
- Determine the relative strength of an acid using the (\(K_a\)) value.
The ionization for a general weak acid, \(\ce{HA}\), can be written as follows:
\[\ce{HA} \left( aq \right) \rightleftharpoons \ce{H^+} \left( aq \right) + \ce{A^-} \left( aq \right) \nonumber \]
Because the acid is weak, an equilibrium expression can be written. An acid ionization constant \(\left( K_\text{a} \right)\) is the equilibrium constant for the ionization of an acid.
\[K_\text{a} = \frac{\left[ \ce{H^+} \right] \left[ \ce{A^-} \right]}{\left[ \ce{HA} \right]} \nonumber \]
The acid ionization represents the fraction of the original acid that has been ionized in solution. Therefore, the numerical value of \(K_\text{a}\) is a reflection of the strength of the acid. Weak acids with relatively higher \(K_\text{a}\) values are stronger than acids with relatively lower \(K_\text{a}\) values. Because strong acids are essentially \(100\%\) ionized, the concentration of the acid in the denominator is nearly zero and the \(K_\text{a}\) value approaches infinity. For this reason, \(K_\text{a}\) values are generally reported for weak acids only.
The table below is a listing of acid ionization constants for several acids. Note that polyprotic acids have a distinct ionization constant for each ionization step, with each successive ionization constant being smaller than the previous one.
Name of Acid |
Ionization Equation |
\(K_\text{a}\) |
Table \(\PageIndex{1}\): Acid Ionization Constants at \(25^\text{o} \text{C}\)
Sulfuric acid |
\(\ce{H_2SO_4} \rightleftharpoons \ce{H^+} + \ce{HSO_4^-}\)
\(\ce{HSO_4} \rightleftharpoons \ce{H^+} + \ce{SO_4^{2-}}\)
|
very large
\(1.3 \times 10^{-2}\)
|
Hydrofluoric acid |
\(\ce{HF} \rightleftharpoons \ce{H^+} + \ce{F^-}\) |
\(7.1 \times 10^{-4}\) |
Nitrous acid |
\(\ce{HNO_2} \rightleftharpoons \ce{H^+} + \ce{NO_2^-}\) |
\(4.5 \times 10^{-4}\) |
Benzoic acid |
\(\ce{C_6H_5COOH} \rightleftharpoons \ce{H^+} + \ce{C_6H_5COO^-}\) |
\(6.5 \times 10^{-5}\) |
Acetic acid |
\(\ce{CH_3COOH} \rightleftharpoons \ce{H^+} + \ce{CH_3COO^-}\) |
\(1.8 \times 10^{-5}\) |
Carbonic acid |
\(\ce{H_2CO_3} \rightleftharpoons \ce{H^+} + \ce{HCO_3^-}\)
\(\ce{HCO_3^-} \rightleftharpoons \ce{H^+} + \ce{CO_3^{2-}}\)
|
\(4.2 \times 10^{-7}\)
\(4.8 \times 10^{-11}\)
|
Hydrofluoric acid \(HF_{(aq)}\) reacts directly with glass (very few chemicals react with glass). Hydrofluoric acid is used in glass etching.