The value of heat of combustion per unit of cyclopropane is –697 kJ/mol. Calculate its strain energy — Thermodynamics and Thermochemistry Chemistry Question
Question
The value of heat of combustion per $\text{CH}_2$ unit of cyclopropane is –697 kJ/mol. Calculate its strain energy. Given: the heats of formation of C(g), H(g), carbon dioxide and water are 715, 218, –393 and –285 kJ/mol, respectively, and C–C and C–H bond energies are 356 and 408 kJ/mol, respectively.
💡 Solution & Explanation
Experimental $\Delta H_c$ for cyclopropane ($\text{C}_3\text{H}_6$) $= 3 \times (-697) = -2091\text{ kJ/mol}$. Experimental $\Delta H_f = 3(-393) + 3(-285) - (-2091) = -2034 + 2091 = +57\text{ kJ/mol}$. Theoretical formation energy from atoms $= 3(356) + 6(408) = 1068 + 2448 = 3516\text{ kJ/mol}$ released. Theoretical $\Delta H_f = 3(715) + 6(218) - 3516 = 3453 - 3516 = -63\text{ kJ/mol}$. Strain Energy $= \text{Experimental } \Delta H_f - \text{Theoretical } \Delta H_f = 57 - (-63) = 120\text{ kJ/mol}$. Format as 0120.