Given the standard molar entropies () at 298 K for , , , and . Calculate the standard entropy change — Thermodynamics and Thermochemistry Chemistry Question
Question
Given the standard molar entropies ($S^\circ$) at 298 K for $CH_4(g) = 186.2$, $O_2(g) = 205.0$, $CO_2(g) = 213.6$, and $H_2O(l) = 69.9 \text{ J K}^{-1}\text{mol}^{-1}$. Calculate the standard entropy change ($\Delta S^\circ$) for the combustion of methane.
Answer: A
💡 Solution & Explanation
The reaction is $CH_4(g) + 2O_2(g) \rightarrow CO_2(g) + 2H_2O(l)$. $\Delta S^\circ = \sum S^\circ(products) - \sum S^\circ(reactants) = [213.6 + 2(69.9)] - [186.2 + 2(205.0)] = [213.6 + 139.8] - [186.2 + 410.0] = 353.4 - 596.2 = -242.8$ J/K-mol.
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