The standard enthalpies of formation of , , and solid glucose () at are , , and respectively. Calcul — Thermodynamics and Thermochemistry Chemistry Question
Question
The standard enthalpies of formation of $CO_2(g)$, $H_2O(l)$, and solid glucose ($C_6H_{12}O_6(s)$) at $25^\circ\text{C}$ are $-400 \text{ kJ mol}^{-1}$, $-300 \text{ kJ mol}^{-1}$, and $-1300 \text{ kJ mol}^{-1}$ respectively. Calculate the absolute magnitude of the standard enthalpy of combustion per gram of glucose at $25^\circ\text{C}$ in $\text{kJ g}^{-1}$. (Molar mass of glucose = $180 \text{ g mol}^{-1}$, round answer to two decimal places)
💡 Solution & Explanation
The stoichiometric combustion reaction is: $C_6H_{12}O_6(s) + 6O_2(g) \rightarrow 6CO_2(g) + 6H_2O(l)$. Using Hess's Law, $\Delta H_c = \sum \Delta H_f(products) - \sum \Delta H_f(reactants) = [6(-400) + 6(-300)] - [-1300 + 0] = [-2400 - 1800] + 1300 = -4200 + 1300 = -2900 \text{ kJ mol}^{-1}$. Heat released strictly per gram = $\frac{2900 \text{ kJ}}{180 \text{ g}} = 16.111 \text{ kJ g}^{-1}$.