AQA GCSE Chemistry

Revision Notes

10.4.3 Choosing Reaction Conditions

Higher Tier Only

Choosing Reaction Conditions

Economic Considerations

  • Like all industries, companies that manufacture and sell chemical goods do so to make a profit
  • Part of the industrial process is the economic decision on how and where to design and implement a manufacturing site
  • The availability and cost of raw materials is a major consideration which must be studied well before any decisions are taken
  • In the Haber Process the raw materials are readily available and inexpensive to purify:
    • Nitrogen – from the air
    • Hydrogen– from natural gas
  • If the cost of extraction of raw materials is too high or they are unavailable then the process is no longer economically viable
  • Many industrial processes require huge amounts of heat and pressure which is very expensive to maintain
  • Production energy costs are also a factor to be considered carefully and alongside the raw materials issue

Temperature: 450ºC

  • 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

  • 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
  • higher pressure would favour the forward reaction as it will try to decrease the pressure by creating fewer 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

Conditions for Haber process, IGCSE & GCSE Chemistry revision notes

Choosing the conditions for the Haber Process

Catalyst

  • The presence of a catalyst does not affect the position of equilibrium but it does increase the rate at which equilibrium is reached
  • This is because the catalyst increases the rate of both the forward and backward reactions by the same amount (by providing an alternative pathway requiring lower activation energy)
  • As a result, the concentration of reactants and products is nevertheless the same at equilibrium as it would be without the catalyst.
    So a catalyst is used as it helps the reaction reach equilibrium quicker
  • It allows for an acceptable yield to be achieved at a lower temperature by lowering the activation energy required
  • Without it the process would have to be carried out at an even higher temperature, increasing costs and decreasing yield as the higher temperature decomposes more of the NH3 molecules

Dynamic-Equilibrium, IGCSE & GCSE Chemistry revision notes

Diagram showing the effect of catalyst on equilibrium position

Exam Tip

The reaction conditions chosen for the Haber process are not ideal in terms of the yield but do provide balance between product yield, reaction rate and production cost. These are called compromise conditions as they are chosen to give a good compromise between the yield, rate and cost.

Author: Francesca

Fran has taught A level Chemistry in the UK for over 10 years. As head of science, she used her passion for education to drive improvement for staff and students, supporting them to achieve their full potential. Fran has also co-written science textbooks and worked as an examiner for UK exam boards.
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