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Amphiprotic Substances

This is part of the HSC Chemistry course under the topic Using Brønsted-Lowry Theory.

HSC Chemistry Syllabus

  • Write ionic equations to represent the dissociation of acids and bases in water, conjugate acid/base pairs in solution and amphiprotic nature of some salts, for example:
    • sodium hydrogen carbonate
    • potassium dihydrogen phosphate

What does amphiprotic mean?

Amphiprotic means the ability for a molecule to donate and accept proton(s).

This video discusses the amphiprotic property of various substances including hydrogen carbonate, dihydrogen phosphate and hydrogen phosphate.

 

     

    Amphiprotic Species

    • Amphiprotic species are ones that can act either as acids or bases. They can either gain or lose a proton.
    • The amphiprotic species will act as an acid when it reacts with a stronger base or it will act as a base if reacted with a stronger acid.
    For example, the hydrogen carbonate ion HCO3– is amphiprotic.
    • HCO3– acts as a base when it reacts with a stronger acid e.g. hydronium ion H3O+

       

      $$HCO_3^-(aq) + H_3O^+(aq) \rightarrow H_2CO_3(aq) + H_2O(l)$$

       

      • HCO3– acts as an acid when it reacts with a stronger base e.g. hydroxide ion OH–

       

      $$HCO_3^-(aq) + OH^- \rightarrow CO_3^{2-}(aq) + H_2O(l)$$

       

      Reaction Between Amphiprotic Substances and Acid/Bases

      Whether an amphiprotic substance acts as an acid or base depends on its Ka and Kb values.

      Recall: 

      • The stronger an acid, the larger the Ka
      • The stronger the base, the larger the Kb

      When hydrogen carbonate reacts with acetic acid, which has a larger Ka value, it acts as a base (proton acceptor) according to the equation:

       

      $$HCO_3^-(aq) + CH_3COOH(aq) \rightarrow H_2CO_3(aq) + CH_3COO^-(aq)$$

       

      When hydrogen carbonate reacts with ammona, which has a larger Kb value, it acts as an acid (proton donor) according to the equation:

       

      $$HCO_3^-(aq) + NH_3(aq) \rightarrow CO_3^{2-}(aq) + NH_4^+(aq)$$

       

      Amphiprotic Substances in Water

      Whether an amphiprotic substance forms an acidic or basic solution in water depends on its relative acid and base dissociation values. 

      • If an amphiprotic substance's acid dissociation constant is larger than its base dissociation constant, it produces an acidic solution.
      • If an amphiprotic substance's base dissociation constant is larger than its acid dissociation constant, it produces a basic solution.

      For example, the base dissociation constant of hydrogen carbonate in water is greater than its acid dissociation constant:

      Acid ionisation of hydrogen carbonate:

      $$HCO_3^-(aq) + H_2O(l) \rightleftharpoons CO_3^{2-}(aq) + H_3O^+(aq) \hspace{0.4cm} K_a = 5.0 \times 10^{-11}$$

       

      Base ionisation of hydrogen carbonate:

      $$HCO_3^-(aq) + H_2O(l) \rightleftharpoons H_2CO_3(aq) + OH^-(aq) \hspace{0.4cm} K_b = 2.0 \times 10^{-8}$$

       

      As a result, a solution of hydrogen carbonate is basic. 

      Amphiprotic Property of Water

      Water is also an amphiprotic substance. The equation below shows water forms an equilibrium with hydronium and hydroxide ions.

       

      $$H_2O(l) + H_2O(l) \rightleftharpoons H_3O^+(aq) + OH^-$$

       

      A very small proportion of water molecules, at 25ºC, exists as ions. In pure water, there are equal concentrations of hydrogen (hydronium) ions and hydroxide ions. Hence the solution will be neutral.

      The equilibrium constant for this reaction is Keq and does not include the water molecules, only the ions: 

       

      $$K_w = [H_3O^+][OH^-]$$

       

      This constant is given a special name. It is the ionic product of water or Kw and at 25 ºC (298 K) is 1.0 x 10–14.

       

       BACK TO MODULE 6: ACID/BASE REACTIONS