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21.13: Strong and Weak Bases and Base Ionization Constant \(\left( K_\text{b} \right)\)

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    Strong and Weak Bases and Base Ionization Constant, \(K_\text{b}\)

    As with acids, bases can either be strong or weak, depending on their extent of ionization. A strong base is a base, which ionizes completely in an aqueous solution. The most common strong bases are soluble metal hydroxide compounds such as potassium hydroxide. Some metal hydroxides are not as strong simply because they are not as soluble. Calcium hydroxide is only slightly soluble in water, but the portion that does dissolve also dissociates into ions.

    A weak base is a base that ionizes only slightly in an aqueous solution. Recall that a base can be defined as a substance, which accepts a hydrogen ion from another substance. When a weak base such as ammonia is dissolved in water, it accepts an \(\ce{H^+}\) ion from water, forming the hydroxide ion and the conjugate acid of the base, the ammonium ion.

    \[\ce{NH_3} \left( aq \right) + \ce{H_2O} \left( l \right) \rightleftharpoons \ce{NH_4^+} \left( aq \right) + \ce{OH^-} \left( aq \right)\]

    The equilibrium greatly favors the reactants and the extent of ionization of the ammonia molecule is very small.

    An equilibrium expression can be written for the reactions of weak bases with water. Because the concentration of water is extremely large and virtually constant, the water is not included in the expression. A base ionization constant \(\left( K_\text{b} \right)\) is the equilibrium constant for the ionization of a base. For ammonia the expression is:

    \[K_\text{b} = \frac{\left[ \ce{NH_4^+} \right] \left[ \ce{OH^-} \right]}{\left[ \ce{NH_3} \right]}\]

    The numerical value of \(K_\text{b}\) is a reflection of the strength of the base. Weak bases with relatively higher \(K_\text{b}\) values are stronger than bases with relatively lower \(K_\text{b}\) values. The table below is a listing of base ionization constants for several weak bases.

    Table 21.13.1: Base Ionization Constants at \(25^\text{o} \text{C}\)
    Name of Base Ionization Equation \(K_\text{b}\)
    Methylamine \(\ce{CH_3NH_2} + \ce{H_2O} \rightleftharpoons \ce{CH_3NH_3^+} + \ce{OH^-}\) \(5.6 \times 10^{-4}\)
    Ammonia \(\ce{NH_3} + \ce{H_2O} \rightleftharpoons \ce{NH_4^+} + \ce{OH^-}\) \(1.8 \times 10^{-5}\)
    Pyridine \(\ce{C_5H_5N} + \ce{H_2O} \rightleftharpoons \ce{C_5H_5NH^+} + \ce{OH^-}\) \(1.7 \times 10^{-9}\)
    Acetate ion \(\ce{CH_3COO^-} + \ce{H_2O} \rightleftharpoons \ce{CH_3COOH} + \ce{OH^-}\) \(5.6 \times 10^{-10}\)
    Fluoride ion \(\ce{F^-} + \ce{H_2O} \rightleftharpoons \ce{HF} + \ce{OH^-}\) \(1.4 \times 10^{-11}\)
    Urea \(\ce{H_2NCONH_2} + \ce{H_2O} \rightleftharpoons \ce{H_2NCONH_3^+} + \ce{OH^-}\) \(1.5 \times 10^{-14}\)

    Notice that the conjugate base of a weak acid is also a strong base. For example, the acetate ion has a small tendency to accept a hydrogen ion from water to form acetic acid and the hydroxide ion.

    Summary

    • Strong and weak bases are defined.
    • The base equilibrium constant is stated.

    Contributors

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