Question

The reaction below represents the Haber process for the industrial production of ammonia, \[ N_2(g) + 3H_2(g) \rightleftharpoons 2NH_3(g); \Delta H^\circ = -92 \, {kJ} \] The optimum conditions of temperature and pressure are chosen as a compromise between those that favour a high yield of ammonia and those that favour a fast rate of production. Economic considerations are also important. Which statement is correct?

• A. A higher temperature would ensure a higher yield and a faster rate
• B. A lower pressure would ensure a higher yield at a lower cost
• C. A lower temperature would ensure a higher yield and a faster rate
• D. A higher pressure would ensure a higher yield at a higher cost

Hint:

For exothermic reactions like the Haber process, increasing pressure and lowering temperature favour the formation of products, but economic and practical factors must be considered for optimal conditions.

Answer 1

The Correct Option is D

The Haber process is an equilibrium reaction, where \( \Delta H^\circ = -92 \, \text{kJ} \), indicating that the reaction is exothermic. According to Le Chatelier's Principle, increasing the pressure favours the side of the reaction with fewer moles of gas, which in this case is the production of ammonia. The balanced reaction is: \[ N_2(g) + 3H_2(g) \rightleftharpoons 2NH_3(g) \] Here, 4 moles of reactants (N2 and H2) are converted into 2 moles of products (NH3), so increasing the pressure shifts the equilibrium towards the production of ammonia, thereby increasing the yield. Additionally, a higher temperature increases the rate of the reaction but decreases the yield, as the forward reaction is exothermic. If the temperature is too high, the reverse reaction becomes more favored. Therefore, a compromise temperature is used in practice, which provides a balance between a reasonable rate of reaction and an acceptable yield. Thus, the correct statement is (d) A higher pressure would ensure a higher yield at a higher cost, as higher pressure favours ammonia production but requires more energy and equipment to sustain.