Question

Give reasons for the following.

Heat of neutralisation value for the reaction of $HCl$ and ${NH}_{4}OH$ is less than $13.7kcal/mole$

Answer 1

It is the amount of heat evolved (i.e., change in enthalpy) when one equivalent of an acid is neutralized by one equivalent of a base in fairly dilute solution. Neutralization reactions are always exothermic reaction and the value of $\Delta H$ is (ve). The heat of neutralization of a strong acid against a strong base is always constant ($13.7kcal/mole$ or $57kJ/mole$). In case of neutralization of a weak acid or a weak base against a strong base or acid respectively, since a part of the evolved heat is used up in ionizing the weak acid or base, it is always less than $13.7kcal/mole$. $N{H}_{4}OH$ is a weak base and hence some heat is utilized to ionize the weak base.