Question

The heat of neutralization of HCl by NaOH is -55.9 $\dfrac{{kJ}}{{mol}}$. If the heat of neutralization of HCN by NaOH is -12.1 $\dfrac{{kJ}}{{mol}}$.
The energy of dissociation of HCN is:
A. -43.8 kJ
B. 43.8 kJ
C. 68 kJ
D. -68 kJ

Answer 1

Hint: The enthalpy of neutralization for a strong acid and a strong base is always a constant value. However, the heat of neutralisation for a weak acid and a strong base is not constant as some of the heat evolved is used to ionise the weak acid.

Complete step by step answer:
The enthalpy of neutralization is the amount of heat when one gram equivalent of the acid is completely neutralized by a base in dilute solution. The enthalpy of neutralization is always constant for a strong acid and a strong base. This is because all strong acids and strong bases are completely ionized in dilute solution. The enthalpy of neutralization is a negative value as the neutralization reaction is always an exothermic reaction.
The neutralization reaction of a strong acid with a strong base is the combination of one equivalent of hydrogen ions with one equivalent of hydroxyl ions. The chemical reaction for the same is:
${H^ + } + O{H^ - } \to HOH + 55.9kJ$
A weak acid does not ionise completely in water. Some amount of heat of neutralization is used to ionise undissociated molecules. The heat of neutralization in case of weak acids is equal to the sum of bond dissociation energy of the weak acid and the heat evolved in the case of a strong acid and a strong base.
$\Delta {H_{HCN}} = \Delta {H_{bond\,dissociation}} + ( - 55.9)$
$- 12.1 = \Delta {H_{bond\,dissociation}} + ( - 55.9)$
$\Delta {H_{bond\,dissociation}} = 43.8kJ$
Hence, the correct answer is (B).

Note: Remember that the heat of neutralization is a negative entity as the heat is released in the process. Bond dissociation energy is a positive entity as the energy is provided to break a bond.