At 300 K, the standard enthalpies of formation of , and are and , respectively. The enthalpy of comb — Thermodynamics and Thermochemistry Chemistry Question
Question
At 300 K, the standard enthalpies of formation of $\text{C}_6\text{H}_5\text{COOH(s)}$, $\text{CO}_2\text{(g)}$ and $\text{H}_2\text{O(l)}$ are $-408, -393$ and $-286\text{ kJ/mol}$, respectively. The enthalpy of combustion of benzoic acid(s) at 300 K is
Answer: A,D
💡 Solution & Explanation
Reaction: $\text{C}_6\text{H}_5\text{COOH} + 7.5\text{O}_2 \to 7\text{CO}_2 + 3\text{H}_2\text{O}$. $\Delta H_c = 7(-393) + 3(-286) - (-408) = -3201\text{ kJ/mol}$ (constant pressure). $\Delta n_g = 7 - 7.5 = -0.5$. $\Delta U_c = \Delta H_c - \Delta n_g RT = -3201 - (-0.5 \times 8.314 \times 10^{-3} \times 300) = -3201 + 1.247 = -3199.753\text{ kJ/mol}$ (constant volume).
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