At constant volume and at 298 K, the heat of combustion of sucrose $\left[{\mathrm{C}}_{12}{\mathrm{H}}_{22}{\mathrm{O}}_{11}\left(\mathrm{s}\right)\right]$ is -1348.9 kcal/mol. If steam is produced during the combustion, the heat of reaction at constant pressure will be:

The given reaction for combustion is: $\begin{array}{c}{\mathrm{C}}_{12}{\mathrm{H}}_{22}{\mathrm{O}}_{11}\left(\mathrm{s}\right)+12{\mathrm{O}}_2\left(\mathrm{g}\right)\to 12{\mathrm{CO}}_2\left(\mathrm{g}\right)+11{\mathrm{H}}_2\mathrm{O}\left(\mathrm{g}\right);\mathrm{\Delta H}=-1348.9\mathrm{kcal}\\ \end{array}$

$∆\mathrm{n}$=number of moles of products- number of moles of reactants

          =(12+11)-12 =11 mol

$∆\mathrm{H}$= Heat of reaction at constant pressure

$\mathrm{\Delta E}\text{ =heat of reaction at constant volume; }\mathrm{R}=0.002\mathrm{kcal};\mathrm{T}=298 \mathrm{K}.$

$\text{ But }\mathrm{\Delta H}=\mathrm{\Delta E}+\mathrm{\Delta nRT}$

                  = -1348.9+(11 x0.002 x298)= -1348.9+6.556

                  = -1342.344 k cal