2025 O Level Chem P1 Q20 — Reversible Reactions / Haber Process Solution

Answer: B. Read the graph: yield is plotted against temperature (x-axis) for various pressures.

The ammonia synthesis \(\text{N}_2 + 3\text{H}_2 \rightleftharpoons 2\text{NH}_3\) is exothermic and reduces gas moles. By Le Chatelier's principle: high pressure shifts equilibrium to the right (more product), and low temperature also shifts right (favours exothermic direction). So highest yield = highest pressure + lowest temperature.

Among the four options, B (200 atm, 200 °C) is the highest pressure × lowest temperature combination, giving the highest yield on the graph.

Note: in industry, a moderate ~450 °C is used despite the lower yield because the rate is too slow at 200 °C — but the question only asks about yield, not industrial practicality.