• Weak acids do not ionise fully in a solution and therefore the [H+] is not proportional to the acid concentration
  • Ethanoic acid is an example of a weak acid, in 1 mol dm-3 solution only about 4 in every thousand ethanoic acid molecules are dissociated into ions and therefore the degree of dissociation is 4/1000
  •  

    CH3COOH(aq) ⇋ H+(aq) + CH3COO(aq)

     

    Ethanoic acid

    Hydrogen Ions

    Ethanoate Ions

    Before dissociation

    1000

    0

    0

    After dissociation

    996

    4

    4

    The Dissociation of Weak acids

    • If a weak acid of HA is dissociated:

    HA(aq) ⇋H+(aq) + A(aq)

    • The equilibrium constant (Kc) is formed:

    Kc = [H+(aq)][A(aq)]

    [HA(aq)]

    • For a weak acid, this is usually given the Ka symbol known as the acid dissociation constant

    Ka = [H+(aq)][A(aq)]

    [HA(aq)]

    • The greater the value of Ka the further the equilibrium is to the right, as more acid which is dissociated and the stronger it is
    • Acid dissociation constants:

    Acid

    Ka (mol dm-3

    Hydrocyanic

    4.9 x 10-10

    Benzoic

    6.3 x10-5

    Ethanoic

    1.7 x10-5

    Chloroacetic

    1.3 x10-5

    Units for Ka

    • The units for Ka are determined by the amount of molecules in the reaction

    e.g. Ka = mol dm-3 x mol dm-3 = mol dm-3

    mol dm-3