Chemistry Subject: Heat of Neutralization

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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, e.g., Neutralization reactions are always exothermic reaction and the value of H is (–ve).

HCl(aq.) + NaOH (aq.) –> NaCl(aq.) + H2O

$\Delta H=-13.7\,kcal$

The heat of neutralization of a strong acid against a strong base is always constant . $(13.7\,kcal\,or\,57\,KJ\,mole^{-1})$ It is because in dilute solutions all strong acids and bases ionize completely and thus the heat of neutralization in such cases is actually the heat of formation of water from H+ and OH– ions, i.e.,

$H^{+}+OH^{-}\rightarrow H_{2}O:\,\Delta H=-13.7\,kcal$

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 $(13.7\,kcal\,mole^{-1})$ $(57\,KJ\,mole^{-1})$ .

For example,