The standard enthalpies of formation of and are and respectively, and the standard enthalpy of combu — Thermodynamics and Thermochemistry Chemistry Question
Question
The standard enthalpies of formation of $CO_2(g)$ and $H_2O(l)$ are $-394 \text{ kJ mol}^{-1}$ and $-285.8 \text{ kJ mol}^{-1}$ respectively, and the standard enthalpy of combustion of propane, $C_3H_8(g)$, is $-2221.6 \text{ kJ mol}^{-1}$. What is the standard heat of formation of $C_3H_8(g)$?
💡 Solution & Explanation
The combustion reaction for propane is: $C_3H_8(g) + 5O_2(g) \rightarrow 3CO_2(g) + 4H_2O(l)$. Using Hess's Law, $\Delta H_{combustion} = \sum \Delta H_f(products) - \sum \Delta H_f(reactants) = [3 \Delta H_f(CO_2) + 4 \Delta H_f(H_2O)] - [\Delta H_f(C_3H_8) + 5 \Delta H_f(O_2)]$. Substituting given values: $-2221.6 = [3(-394) + 4(-285.8)] - [\Delta H_f(C_3H_8) + 0]$. $-2221.6 = [-1182 - 1143.2] - \Delta H_f(C_3H_8)$. $\Delta H_f(C_3H_8) = -2325.2 + 2221.6 = -103.6 \text{ kJ mol}^{-1}$.