Entropy and Spontaneity (Additional Higher Level) · Paper 1 — Free Chemistry HL Practice Questions

For the Haber process, $\text{N}_2\text{(g)} + 3\text{H}_2\text{(g)} \rightleftharpoons 2\text{NH}_3\text{(g)}$, $\Delta H = -92.6 \ \text{kJ mol}^{-1}$. The reaction is spontaneous at 298 K but becomes non-spontaneous at sufficiently high temperatures. Which statement correctly accounts for this behaviour?

At 298 K, ammonium nitrate dissolves endothermically in water: $\text{NH}_4\text{NO}_3\text{(s)} \rightarrow \text{NH}_4^+\text{(aq)} + \text{NO}_3^-\text{(aq)}$, with $\Delta H = +25.7 \text{ kJ mol}^{-1}$ and $\Delta S = +108 \text{ J K}^{-1}\text{ mol}^{-1}$. Which statement about the spontaneity of this process at 298 K is correct?

At 298 K, the combustion of methane has $\Delta H = -890 \ \text{kJ mol}^{-1}$ and $\Delta S = -242 \ \text{J K}^{-1} \text{mol}^{-1}$. What is the value of $\Delta G$ at this temperature, and what does it indicate about spontaneity?

The thermal decomposition of ammonium chloride is represented by the equation: $$\text{NH}_4\text{Cl}(s) \rightarrow \text{NH}_3(g) + \text{HCl}(g) \quad \Delta H^\circ = +176 \text{ kJ mol}^{-1}$$ The reaction is non-spontaneous below approximately 340 °C but becomes spontaneous above this temperature. Which statement correctly explains this behaviour in terms of entropy changes?