10.4.3 Choosing Reaction Conditions

Temperature

• Lowering Temperature: Since the forward reaction is exothermic, lowering the temperature shifts the equilibrium towards the production of more ammonia, increasing yield.
• Compromise Temperature: However, too low a temperature reduces the reaction rate, decreasing the overall yield. Thus, a compromise temperature of around 450°C is used to balance the rate and yield of ammonia production.

Pressure

• Increasing Pressure: The equilibrium shifts towards the side with fewer moles of gas, which is the side producing ammonia (2 moles versus 4 moles). Therefore, increasing pressure favours ammonia production.
• Compromise Pressure: High pressures can be expensive to maintain, so a compromise pressure of around 200 atm is used. This pressure provides a good yield while keeping costs manageable.