Optimising Yield in Haber Process (Higher Tier)

Conditions for Haber Process

The Haber process synthesises ammonia from nitrogen and hydrogen gases. The reaction is reversible and requires specific conditions to optimise the yield of ammonia:

• Temperature: Approximately 4506C
• Pressure: Around 200 atmospheres (atm)
• Catalyst: Finely divided iron is used to speed up the reaction

These conditions are carefully chosen to balance the rate of reaction and the amount of ammonia produced.

Effect of Temperature on Yield

Increasing the temperature in the Haber process has two main effects:

• Decreases the yield of ammonia: Higher temperatures favour the reverse reaction, reducing the amount of ammonia formed.
• Increases the rate of reaction: Molecules have more energy, so collisions happen more frequently and with greater energy, speeding up the reaction.

Because of this trade-off, a compromise temperature of about 4506C is used to achieve a reasonable yield in a practical time.

For instance, if the temperature were too low, the reaction would produce more ammonia but take too long to reach equilibrium. If too high, the reaction would be fast but produce less ammonia.

Example: If the temperature is increased from 4006C to 5006C, the rate of reaction increases but the equilibrium yield of ammonia decreases, so 4506C is chosen as a compromise.

Effect of Pressure on Yield

Pressure also affects the Haber process in two key ways:

• Higher pressure increases ammonia yield: The reaction produces fewer gas molecules (4 moles of reactants form 2 moles of ammonia), so increasing pressure shifts the reaction towards ammonia formation.
• Higher pressure increases the rate of reaction: Gas molecules are forced closer together, increasing collision frequency and speeding up the reaction.

However, very high pressures (around 200 atm) are expensive to maintain and require strong, safe equipment to prevent accidents.

Therefore, pressure is set as high as economically and safely possible to maximise yield and rate.

Example: Increasing pressure from 100 atm to 200 atm roughly doubles the ammonia yield, but the cost and safety risks also increase significantly.

Effect of Catalyst

An iron catalyst is used in the Haber process to:

• Speed up the reaction by lowering the activation energy
• Allow the reaction to reach equilibrium faster

Importantly, the catalyst does not affect the position of equilibrium or the final yield of ammonia; it only helps the system reach equilibrium more quickly.

Example: Without a catalyst, the reaction might take several hours to reach equilibrium, but with iron catalyst, it happens in minutes.

Optimising Yield Summary

Optimising the yield of ammonia in the Haber process involves balancing several factors:

• Temperature: A compromise temperature (~4506C) balances a reasonable yield with a practical reaction rate.
• Pressure: High pressure (~200 atm) increases yield and rate but raises costs and safety risks.
• Catalyst: Iron catalyst speeds up the reaction without changing the yield.
• Economic and safety considerations: Conditions must be chosen to maximise profit and ensure safe operation.

This balance ensures ammonia is produced efficiently on an industrial scale.

Learning example: If the ammonia yield at 300 atm is 35%, what would you expect the yield to be at 150 atm, assuming a linear relationship between pressure and yield?

Answer: Since pressure is halved from 300 atm to 150 atm, the yield would roughly halve from 35% to about 17.5%.