Entropy and Spontaneity (Additional Higher Level) · Paper 2 — Free Chemistry HL Practice Questions

Using the standard molar entropy data below, calculate $\Delta G^\circ$ for the reaction at $298\,\text{K}$. Species — $S^\circ\,/\,\text{J K}^{-1}\text{mol}^{-1}$ $\text{N}_2(g)$ — $191.6$ $\text{H}_2(g)$ — $130.7$ $\text{NH}_3(g)$ — $192.8$ [3 marks]

(i) Explain why the Haber–Bosch process is operated at approximately $700\,\text{K}$ rather than $298\,\text{K}$, even though $\Delta G^\circ$ is negative at $298\,\text{K}$. [2]
(ii) Using your answer to (b) and the expression $\Delta G^\circ = \Delta H^\circ - T\Delta S^\circ$, estimate $\Delta G^\circ$ at $700\,\text{K}$ and evaluate whether the reaction remains thermodynamically feasible at this temperature. [2 marks]