The Haber Process (Cambridge International IGCSE Chemistry)

Spec point — 6.2.3 · State the symbol equation for the production of ammonia in the Haber process, N2(g) + 3H2(g) ⇌ 2NH3(g) State the sources of the hydrogen (methane) and nitrogen (air) in the Haber process State the typical conditions in the Haber process as 450°C, 20000kPa /200atm and an iron catalyst

The Haber process

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

Worked Example Ammonia is produced during the Haber Process. The reaction is summarised in the diagram below.

1. Name gas A.
2. Name catalyst B used and state why it is used

Answer:

1. Gas A is hydrogen / H₂.
2. Catalyst B is iron, which is used to speed up the reaction / increase the rate of reaction.

Exam Hint Examiners often comment that while many students can state the overall reaction, they frequently lose marks by not knowing the specific details.

To secure top marks, make sure you can recall:

• The sources of the raw materials:

  • Nitrogen from the fractional distillation of liquid air
  • Hydrogen from natural gas (methane)
  • This is a specific syllabus point and is often tested

• The name of the catalyst:

  • Iron
  • Do not confuse it with catalysts from other industrial processes, like 'vanadium oxide' and 'enzymes'

Explaining the conditions in the Haber process

Spec point — 6.2.3 · Explain, in terms of rate of reaction and position of equilibrium, why the typical conditions stated are used in the Haber process, including safety considerations and economics

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-axis that

• 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

Exam Hint 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.