Question

Why does the neutralization of a strong acid like HCl with NaOH release more enthalpy per mole of water than a weak acid, like aceti acid, and a strong base?

Answer 1

Strong acids and strong bases are almost completely ionised.

HCl ------------> H+ + Cl-

Thus during the reaction of neutralization

H+ + OH- ---------> H2O

the energy liberated is totally given out.

However in case of weak acids like acetic aicd the ionisation is very less.

CH3COOH <----------> CH3COO- + H+

Thus some amount of energy liberated inneutralization process is spent to ionise the weak acid completely, so that the neutralisation is complete.

Thus the enthalpy of neutralization per mole of water in case of weak acids or weak bases is always less than those with strong acids and bases.

The difference is attirbuted to ionisation of weak acid/base.(called enthalpy of ionisation of weak acid)