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21.12: Strong and Weak Acids and Acid Ionization Constant \(\left( K_\text{a} \right)\)

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  • Etching of glass is a slow process that can produce beautiful artwork. Traditionally, the glass has been treated with dilute hydrofluoric acid which gradually dissolves the glass under it. Parts of the piece that should not be etched are covered with wax or some other non-reactive material. In more recent times, compounds such as ammonium bifluoride have been used. Whichever chemical is employed, the artist must be very careful not to get any on their skin.

    Strong and Weak Acids and Acid Ionization Constant

    Acids are classified as either strong or weak, based on their ionization in water. A strong acid is an acid which is completely ionized in an aqueous solution. Hydrogen chloride \(\left( \ce{HCl} \right)\) ionizes completely into hydrogen ions and chloride ions in water.

    \[\ce{HCl} \left( g \right) \rightarrow \ce{H^+} \left( aq \right) + \ce{Cl^-} \left( aq \right)\]

    weak acid is an acid that ionizes only slightly in an aqueous solution. Acetic acid (found in vinegar) is a very common weak acid. Its ionization is shown below.

    \[\ce{CH_3COOH} \left( aq \right) \rightleftharpoons \ce{H^+} \left( aq \right) + \ce{CH_3COO^-} \left( aq \right)\]

    The ionization of acetic acid is incomplete, and so the equation is shown with a double arrow. The extent of ionization of weak acids varies, but is generally less than \(10\%\). A \(0.10 \: \text{M}\) solution of acetic acid is only about \(1.3\%\) ionized, meaning that the equilibrium strongly favors the reactants.

    Weak acids, like strong acids, ionize to yield the \(\ce{H^+}\) ion and a conjugate base. Because \(\ce{HCl}\) is a strong acid, its conjugate base \(\left( \ce{Cl^-} \right)\) is extremely weak. The chloride ion is incapable of accepting the \(\ce{H^+}\) ion and becoming \(\ce{HCl}\) again. In general, the stronger the acid, the weaker its conjugate base. Likewise, the weaker the acid, the stronger its conjugate base.

    Acid Conjugate Base
    Table 21.12.1: Relative Strengths of Acids and their Conjugate Bases
    Strong Acids  
    \(\ce{HCl}\) (hydrochloric acid) (strongest) \(\ce{Cl^-}\) (chloride ion) (weakest)
    \(\ce{H_2SO_4}\) (sulfuric acid) \(\ce{HSO_4^-}\) (hydrogen sulfate ion)
    \(\ce{HNO_3}\) (nitric acid) \(\ce{NO_3^-}\) (nitrate ion)
    Weak Acids  
    \(\ce{H_3PO_4}\) (phosphoric acid) \(\ce{H_2PO_4^-}\) (dihydrogen phosphate ion)
    \(\ce{CH_3COOH}\) (acetic acid) \(\ce{CH_3COO^-}\) (acetate ion)
    \(\ce{H_2CO_3}\) (carbonic acid) \(\ce{HCO_3^-}\) (hydrogen carbonate ion)
    \(\ce{HCN}\) (hydrocyanic acid) (weakest) \(\ce{CN^-}\) (cyanide ion) (strongest)

    Strong acids are \(100\%\) ionized in solution. Weak acids are only slightly ionized. Phosphoric acid is stronger than acetic acid and so is ionized to a greater extent. Acetic acid is stronger than carbonic acid, and so on.

    The Acid Ionization Constant, \(K_\text{a}\)

    The ionization for a general weak acid, \(\ce{HA}\), can be written as follows:

    \[\ce{HA} \left( aq \right) \rightarrow \ce{H^+} \left( aq \right) + \ce{A^-} \left( aq \right)\]

    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.

    \[K_\text{a} = \frac{\left[ \ce{H^+} \right] \left[ \ce{A^-} \right]}{\left[ \ce{HA} \right]}\]

    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 21.12.2: 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

    \(1.3 \times 10^{-2}\)

    Oxalic acid

    \(\ce{H_2C_2O_4} \rightleftharpoons \ce{H^+} + \ce{HC_2O_4^-}\)

    \(\ce{HC_2O_4} \rightleftharpoons \ce{H^+} + \ce{C_2O_4^{2-}}\)

    \(6.5 \times 10^{-2}\)

    \(6.1 \times 10^{-5}\)

    Phosphoric acid

    \(\ce{H_3PO_4} \rightleftharpoons \ce{H^+} + \ce{H_2PO_4^-}\)

    \(\ce{H_2PO_4^-} \rightleftharpoons \ce{H^+} + \ce{HPO_4^{2--}}\)

    \(\ce{HPO_4^{2-}} \rightleftharpoons \ce{H^+} + \ce{PO_4^{3-}}\)

    \(7.5 \times 10^{-3}\)

    \(6.2 \times 10^{-8}\)

    \(4.8 \times 10^{-13}\)

    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

    \(\ce{H_2CO_3} \rightleftharpoons \ce{H^+} + \ce{HCO_3^-}\)

    \(\ce{HCO_3^-} \rightleftharpoons \ce{H^+} + \ce{CO_3^{2-}}\)

    \(4.2 \times 10^{-7}\)

    \(4.8 \times 10^{-11}\)

    Hydrocyanic acid \(\ce{HCN} \rightleftharpoons \ce{H^+} + \ce{CN^-}\) \(4.9 \times 10^{-10}\)

    Summary

    • Strong and weak acids are defined.
    • The acid ionization constant \(\left( K_\text{a} \right)\) is defined.

    Contributors

    • CK-12 Foundation by Sharon Bewick, Richard Parsons, Therese Forsythe, Shonna Robinson, and Jean Dupon.