Chemistry - Thermochemistry Question with Solution | TestHub

Given ${\mathrm{H}}_2(\mathrm{g})+\frac{1}{2}{\mathrm{O}}_2(g)⟶{\mathrm{H}}_2\mathrm{O}(l)$; $\Delta {\mathrm{H}}^{\circ }=-285.9\mathrm{kJ}{\mathrm{mol}}^{-1}\dots (1)$ ${\mathrm{H}}_2(\mathrm{g})+\frac{1}{2}{\mathrm{O}}_2(g)⟶{\mathrm{H}}_2\mathrm{O}(g)$; $\Delta {\mathrm{H}}^{\circ }=-241.8\mathrm{kJ}{\mathrm{mol}}^{-1}\dots (2)$ We have to calculate ${\mathrm{H}}_2\mathrm{O}(l)⟶{\mathrm{H}}_2\mathrm{O}(\mathrm{g});\Delta {\mathrm{H}}^{\circ }=\text{ ? }$ On substracting eqn. $(2)$ from eqn. $(1)$ we get ${\mathrm{H}}_2\mathrm{O}(l)⟶{\mathrm{H}}_2\mathrm{O}(\mathrm{g})$; $\Delta {\mathrm{H}}^{\circ }=-241.8-(-285.9)$ $=44.1\mathrm{kJ}{\mathrm{mol}}^{-1}$