The Haber Process (CIE IGCSE Chemistry)

Extended tier only

• Ammonia is manufactured in an exothermic reaction called the Haber process which occurs in five stages:
• Stage 1
  • H₂ is obtained from methane
  • N₂ is obtained from the air
  • They are pumped into the compressor through a pipe
• Stage 2 
  • Inside the compressor, the gases are compressed to around 20 000 kPa or 200 atmospheres 
• Stage 3
  • The pressurised gases are pumped into a tank containing layers of an iron catalyst at a temperature of 450 °C
  • Some of the hydrogen and nitrogen react to form ammonia:

N₂ (g) + 3H₂ (g) ⇌ 2NH₃ (g)

• Stage 4
  • Unreacted H₂ and N₂ and the ammonia product pass into a cooling tank
  • The ammonia is liquefied and removed to pressurised storage vessels
• Stage 5
  • The unreacted H₂ and N₂ gases are recycled back into the system and start over again

The production of ammonia by the Haber process

Extended tier only

• 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 graph below illustrates the effects of changing temperature and pressure on the yield of ammonia obtained

The yield of ammonia produced changes with changes made to temperature and pressure

• From the graph:
  • As the pressure increases, the percentage yield increases 
    • This is shown by following any of the curved lines  
  • As the temperature decreases, the percentage yield increases 
    • This is shown by following any vertical line upwards from the x-axisthat 
• The actual conditions used must be chosen depending on a number of economical, chemical and practical considerations

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

• High temperature favours the reverse reaction as it is endothermic
  • So, a higher yield of reactants will be made
• Low temperature favours the forward reaction as it is exothermic
  • So, a higher yield of products will be made
• However, at low temperature the rate of reaction is very slow
• So, 450 ºC is a compromise temperature between having a lower yield of products but being made more quickly

Pressure: 200 atm

• Low pressure favours the reverse reaction as there are more moles of gaseous reactant
  • So, a higher yield of reactants will be made
• High pressure favours the forward reaction as there are fewer moles of gaseous product
  • So, a higher yield of products will be made
• However, high pressure can be dangerous and very expensive equipment is needed
• So, 200 atmospheres is a compromise pressure between a lower yield of products being made safely and economically

Catalyst: Iron

• 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