Chemistry - Thermochemistry Question with Solution | TestHub

$\mathrm{HCl}⟶{\mathrm{H}}^{+}+{\mathrm{Cl}}^{-}$ Strong acid (Complete ionisation) $${\mathrm{NH}}_4\mathrm{OH}\rightleftharpoons {\mathrm{NH}}_4^{+}+{\mathrm{OH}}^{-}$$ Weak base $\Delta \mathrm{H}=x\mathrm{kJ}{\mathrm{mol}}^{-1}$ $$\begin{array}{c}{\mathrm{H}}^{+}+{\mathrm{OH}}^{-}⟶{\mathrm{H}}_2\mathrm{O}\\ \Delta \mathrm{H}=-55.90\mathrm{kJ}{\mathrm{mol}}^{-1}\end{array}$$ (from neutralisation of strong acid and strong base) From equation (i), (ii) and (iii) $$\begin{array}{rl}& {\mathrm{NH}}_4\mathrm{OH}+\mathrm{HCl}⟶{\mathrm{NH}}_4^{+}+{\mathrm{Cl}}^{-}+{\mathrm{H}}_2\mathrm{O}\\ & \Delta \mathrm{H}=-51.46\mathrm{kJ}{\mathrm{mol}}^{-1}\\ \therefore & x+(-55.90)=-51.46\\ & x=-51.46+55.90\\ & =4.44\mathrm{kJ}{\mathrm{mol}}^{-1}\end{array}$$ $\therefore$ Enthalpy of ionisation of $${\mathrm{NH}}_4\mathrm{OH}=4.44\mathrm{kJ}{\mathrm{mol}}^{-1}$$