The standard enthalpies of formation of and are -394 kJ/mol and -285.8 kJ/mol respectively. If the s — Thermodynamics and Thermochemistry Chemistry Question
Question
The standard enthalpies of formation of $CO_2(g)$ and $H_2O(l)$ are -394 kJ/mol and -285.8 kJ/mol respectively. If the standard enthalpy of combustion of propane gas ($C_3H_8$) is -2221.6 kJ/mol, what is the standard heat of formation of propane?
Answer: D
💡 Solution & Explanation
Combustion: $C_3H_8 + 5O_2 \rightarrow 3CO_2 + 4H_2O$. By Hess's Law, $\Delta H_{combustion} = [3\Delta H_f(CO_2) + 4\Delta H_f(H_2O)] - [\Delta H_f(C_3H_8) + 0]$. Thus, $-2221.6 = [3(-394) + 4(-285.8)] - \Delta H_f(C_3H_8) \Rightarrow \Delta H_f(C_3H_8) = -1182 - 1143.2 + 2221.6 = -103.6$ kJ/mol.
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