The enthalpies of neutralization of NaOH and ${\mathrm{NH}}_4\mathrm{OH}$ with HCl are 57.30 and 51.43 kJ respectively.  The enthalpy of ionization of ${\mathrm{NH}}_4\mathrm{OH}$ is

The basic reaction that occurs when a strong acid is neutralized by a strong base is 

${\mathrm{H}}^{+}+{\mathrm{OH}}^{-}\to {\mathrm{H}}_2\mathrm{O}+\mathrm{x}$

Hence, the amount of heat liberated x is the same when any strong acid is neutralized by any strong base.  However if either the acid or base is weak this amount is less since the weak acid/base is to be ionized by absorbing heat.  Thus the difference in the two quantities is equal to the heat of ionization of weak acid / base. 

Heat of ionization of ${\mathrm{NH}}_4\mathrm{OH}$

= 57.30 – 51.43 = +5.87 kJ ${\mathrm{mol}}^{-1}$.