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 \(K_\text{a}\) 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.