Chemistry - Thermochemistry Question with Solution | TestHub

(i) ${\mathrm{C}}_{(\mathrm{s})}+{\mathrm{O}}_{2(\mathrm{g})}\to {\mathrm{CO}}_{2(g)}$; $\Delta {\mathrm{H}}_i=-94\mathrm{kcal}/\mathrm{mole}$ (ii) $2{\mathrm{H}}_{2(g)}+{\mathrm{O}}_{2(g)}\to 2{\mathrm{H}}_2{\mathrm{O}}_{(l)}$; $\Delta {\mathrm{H}}_{\mathrm{ii}}=-68\times 2\mathrm{kcal}/\mathrm{mole}$ (iii) ${\mathrm{CH}}_{4(g)}+2{\mathrm{O}}_{2(g)}\to {\mathrm{CO}}_{2(g)}+$ $2{\mathrm{H}}_2{\mathrm{O}}_{(l)};\Delta {\mathrm{H}}_{\mathrm{iii}}=-213\mathrm{kcal}/\mathrm{mol}$ (iv) ${\mathrm{C}}_{(s)}+2{\mathrm{H}}_{2(g)}\to {\mathrm{CH}}_{4(\mathrm{g})}$; $\Delta {\mathrm{H}}_{\mathrm{IV}}=$ ? By applying Hess's law we can compute $\Delta {\mathrm{H}}_{\mathrm{iv}}$. $\begin{array}{rl}& \therefore \Delta {\mathrm{H}}_{\mathrm{iv}}=\Delta {\mathrm{H}}_{\mathrm{i}}+\Delta {\mathrm{H}}_{\mathrm{ii}}-\Delta {\mathrm{H}}_{\mathrm{iii}}\\ & =(-94-68\times 2+213)\mathrm{kcal}\\ & =-17\mathrm{kcal}\end{array}$