Calculate (in J/K) for the chemical reaction: ; . The standard entropies of , and are 6.0, 130.6 and — Thermodynamics and Thermochemistry Chemistry Question
Question
Calculate $\Delta S_{\text{univ}}$ (in J/K) for the chemical reaction: $\text{C(graphite)} + 2\text{H}_2\text{(g)} \to \text{CH}_4\text{(g)}$; $\Delta H^\circ_{300} = -75.0\text{ kJ}$. The standard entropies of $\text{C (graphite)}$, $\text{H}_2\text{(g)}$ and $\text{CH}_4\text{(g)}$ are 6.0, 130.6 and 186.2 J/K-mol, respectively.
💡 Solution & Explanation
System entropy change $\Delta S_{\text{sys}} = S^\circ(\text{CH}_4) - [S^\circ(\text{C}) + 2S^\circ(\text{H}_2)] = 186.2 - [6.0 + 2(130.6)] = 186.2 - 267.2 = -81.0\text{ J/K}$. Surrounding entropy change $\Delta S_{\text{surr}} = -\Delta H/T = -(-75000)/300 = +250\text{ J/K}$. Total entropy change $\Delta S_{\text{univ}} = \Delta S_{\text{sys}} + \Delta S_{\text{surr}} = -81.0 + 250 = +169\text{ J/K}$. Format as 0169.