Amphiprotic Substances

  • 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

                                                              

     

    Amphiprotic Species

    • Species that can act either as acids or bases are known as amphiprotic. 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+

     

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

     

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

     

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

     

    The stronger the base, the larger the Kb

     

    Similarly, when reacted with a stronger acid, the hydrogen carbonate will act as a base, the stronger an acid, the larger the Ka

     

    As we can see, the Ka of the acetic acid is greater than that of the hydrogen carbonate which is why when we get the reaction between the acetic acid and the hydrogen carbonate, it acts as a base.

    The equation below shows there is an equilibrium in water. A very small proportion of water molecules, at 25ºC, exists as ions. In a neutral solution (just water with nothing added to it) compare the concentration of hydrogen ions and hydroxide ions.There will be equal concentrations of hydrogen (hydronium) ions and hydroxide ions. Hence the solution will be neutral.

     

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

     

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

     

    $$K_{eq} = [H^+][OH^-] $$

     

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