Choosing the Haber Process Conditions (WJEC GCSE Chemistry)

Higher tier

What are the conditions used in the Haber Process?

• Chemists' knowledge of the energy changes and factors affecting reaction rates can be used to predict the best possible conditions to make the most ammonia in the fastest possible time
• However, sometimes those conditions are contradictory and choices have to be made between factors that improve the yield of ammonia and those that speed up the reaction
• The most economical conditions have to take into account the energy and production costs as well as how quickly the product is made 

Conditions

Temperature: 450ºC

• A higher temperature would favour the reverse reaction as it is endothermic (takes in heat) so a higher yield of reactants would be made
• If a lower temperature is used it favours the forward reaction as it is exothermic (releases heat) so a higher yield of products will be made
• However at a lower temperature the rate of reaction is very slow
• So 450ºC is a compromise temperature between having a lower yield of products but being made more quickly

Pressure: 200 atm

• A lower pressure would favour the reverse reaction as the system will try to increase the pressure by creating more molecules (4 molecules of gaseous reactants) so a higher yield of reactants will be made
• A higher pressure would favour the forward reaction as it will try to decrease the pressure by creating less molecules (2 molecules of gaseous products) so a higher yield of products will be made
• However high pressures can be dangerous and very expensive equipment is needed
• So 200 atm is a compromise pressure between a lower yield of products being made safely and economically

Catalyst

• A catalyst of iron is used to speed up the reaction

Choosing the conditions for the Haber Process