The graph shows the percentage yield of ammonia produced from nitrogen and hydrogen at different temperatures and pressures - HSC - SSCE Chemistry - Question 30 - 2015 - Paper 1

In the Haber process, ammonia is synthesized from nitrogen and hydrogen through the following exothermic reaction:

$N_2(g) + 3H_2(g) \rightleftharpoons 2NH_3(g) \quad \Delta H = -92 \text{ kJ}$

Temperature and Yield

The graph illustrates how the yield of ammonia varies with temperature at different pressures. It is evident that:

• Lower Temperatures: The yield of ammonia is higher at lower temperatures due to the exothermic nature of the reaction. According to Le Chatelier's principle, decreasing temperature shifts the equilibrium to the right, favoring ammonia production.
• Higher Temperatures: However, the rate of reaction is slower at lower temperatures, which means that although yields are higher, the production rate is compromised.

Pressure Considerations

From the graph, it can also be inferred:

• Higher Pressures: Increasing pressure increases the yield of ammonia. The graph shows that at higher pressures (500 atm), the yield rises significantly compared to lower pressures (200 atm).
• Optimal Conditions: In practice, the optimal condition for the Haber process balances pressure and temperature, typically at around 200-300 atm and approximately 400-450°C. This condition maximizes yield while allowing a reasonable reaction rate.

In summary, the Haber process operates under high pressure and moderate temperatures to optimize both yield and reaction rate, as demonstrated by the graph.