[cliverlong]

Hi

Some sources I read say the standard enthalpy of neutralisation of strong and weak mono-protic acids (HCl and ethanoic) by a strong alkali are always the same: around -57kJ mol^-1. The explanation given is that the total amount of bond formation between H⁺ and OH^- is the same in both cases so enthalpy released is the same (more or less). As H⁺ from weak acid is consumed more acid molecules dissociate and react with OH^-.

However, an EdExcel A-level question has appeared twice in recent years that asks why the standard enthalpy of neutralisation of hydrocyanic acid is much less, around -12 kJ mol^-1.

The explanation given in the mark scheme is that hydrocyanic acid is a weak acid. But that contradicts with the other explanations about the behaviour of weak acids. Does anyone know the correct explanation?

Thanks

Clive

[Borek]

What about enthalpy of hydrocyanic acid dissociation.