Chapter 6, Problem 95RQ

The value for the standard heat of combustion, $\Delta H°$ combustion, for sucrose, ${\text{C}}_{\text{l2}}{\text{H}}_{\text{22}}{\text{O}}_{\text{11}}\text{, is—5}\text{.65 }\times {\text{ 10}}^{\text{3}}{\text{ kJ mol}}^{\text{-1}}$. Write the thermochemical equation for the combustion of 1 mol of sucrose and calculate the value of $\Delta H_{\text{f}}^{\text{°}}$ for this compound. The sole products of combustion are ${\text{CO}}_{\text{2}}\left(\text{g}\right)$ and ${\text{H}}_{\text{2}}\text{O}\left(l\right)$. Use data in Table 6.2 as necessary.

compound (from its elements). We can obtain the enthalpy of formation of two moles of water ( $\Delta H°$ for the second equation) simply by multiplying the $\Delta H_{\text{f}}^{\text{°}}$ value for 1 mol of ${\text{H}}_{\text{2}}\text{O}$ by the factor 2 mol ${\text{H}}_{\text{2}}\text{O}\left(l\right)$.

$\frac{-285.9\text{ kJ}}{1{\text{ mol H}}_{\text{2}}\text{O}\left(l\right)}\times 2{\text{ mol H}}_{\text{2}}\text{O}\left(l\right)=-571.8\text{ kJ}$