The standard enthalpies of combustion of , , and are , , and respectively. What is the standard enth — Thermodynamics and Thermochemistry Chemistry Question
Question
The standard enthalpies of combustion of $C_2H_4(g)$, $C_2H_6(g)$, and $H_2(g)$ are $-1409.5 \text{ kJ mol}^{-1}$, $-1558.3 \text{ kJ mol}^{-1}$, and $-285.6 \text{ kJ mol}^{-1}$ respectively. What is the standard enthalpy of hydrogenation of ethene?
Answer: A
💡 Solution & Explanation
The specific hydrogenation reaction is $C_2H_4(g) + H_2(g) \rightarrow C_2H_6(g)$. By Hess's Law using combustion data, $\Delta H_{rxn} = \sum \Delta H_c(reactants) - \sum \Delta H_c(products) = [\Delta H_c(C_2H_4) + \Delta H_c(H_2)] - \Delta H_c(C_2H_6) = [-1409.5 - 285.6] - [-1558.3] = -1695.1 + 1558.3 = -136.8 \text{ kJ mol}^{-1}$.
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