CGPET 2018 Solved Paper

When a strong acid will react with a strong base, nearly 13.7 kcal mol ‌−1 heat comes out from the reaction.
But when a weak acid (like HCN) go for neutralisation, some heat is used up to dissociate the HCN molecules.
∵ Given, only 2.9kcal∕mol heat is coming out, means.
13.7−2.9=+10.8kcal∕mol heat is used upto dissociates the HCN molecules.
∴(b) is the correct option.