${\mathrm{H}}^{+}+{\mathrm{OH}}^{-}⟶{\mathrm{H}}_2\mathrm{O};\mathrm{\Delta }{H}^{\circ }=-57\mathrm{kJ}/\mathrm{mol}$

$\mathrm{\Delta }{H}_{\text{ionisation }}^{\circ }\left[\mathrm{HCN}\right]=45\mathrm{kJ}/\mathrm{mol}$

If 200mL of  $\frac{1}{10}\mathrm{MBa}(\mathrm{OH}{)}_2$ solution is mixed in 500mL of $\frac{1}{10}\mathrm{M} \mathrm{HCN}$ solution, then, heat evolved will be :

$\begin{array}{llc}\mathrm{Ba}(\mathrm{OH}{)}_2+2\mathrm{HCN}⟶\mathrm{Ba}(\mathrm{CN}{)}_2+2{\mathrm{H}}_2\mathrm{O}& & \\ 20(\mathrm{L}.\mathrm{R}) 50 \mathrm{\Delta }H=-24\mathrm{kJ}/\mathrm{mol}& & \\ \text{ millimole }\text{ millimole }& & \end{array}$

Heat evolved will be $=24\times 20\times {10}^{-3}$

                                       $$\begin{gathered} =48\times {10}^{-2}\mathrm{kJ} \\ =480\text{ Joule } \end{gathered}$$