The heats of neutralisation of CH3COOH, HCOOH, HCN and H2S are -12.8, -13.2, -3.2 and -3.9 kcal per equivalent. Arrange these acids in increasing order of strength.
HCN<H2S<CH3COOH<HCOOH
Heat of neutralization per equivalent ∝ Acidic strength, the maximum value being the enthalpy of formation of water. Every equivalent of an acid would form 1 mole of water if it were a strong acid. As an acid gets weaker, this value decreases as fewer water molecules are formed - as fewer protons are furnished by weaker acids.