Question

The heat of neutralization of HCl by NaOH is -55.9 kJ mol${}^{-1}$. If the heat of neutralization of HCN by NaOH is -12.1 kJ/mol, the energy of dissociation of HCN is:

• A. -43.8 kJ
• B. 43.8 kJ
• C. 68 kJ
• D. -68 kJ

Answer 1

The correct option is B 43.8 kJ

Since neutralization is an exothermic process, so the sign of its heat is -ve.
As we know, for weak acid (like HCN), some amount of heat is used to dissociate it, i.e., heat for dissociation (for weak acid) + heat during neutralisation= heat during neutralisation for strong acid with strong base, NaOH.

Which means for this case, putting the values, we get,
Heat of dissociation (for HCN) + 12.1 = 55.9
$\therefore$ Heat of dissociation (for HCN) = 55.9 - 12.1 = 43.8 kJ