The Haber process and the use of NPK fertilisers (chemistry only)

10.4.1 The Haber process

Used to manufacture ammonia to produce nitrogen-based fertilisers


Nitrogen + hydrogen ⇌ ammonia

N(g) + 3H(g)  2NH(g)

Purified gases passed over… (conditions)

  • Iron catalyst
  • High temp – 450°C
  • High pressure – 200atm

Explain why iron is used in the reactor for the Haber process (2)

  • As a catalyst
  • So the reaction speeds up

Describe how the ammonia is separated from the other gases (2)

  • Cooled
  • Ammonia condenses / liquefies

What happens to the mixture of unreacted gases (nitrogen and hydrogen)? (1)


The graph shows the percentage yield of ammonia using different conditions.

Use the graph above to suggest the conditions that produce the greatest yield of ammonia. (1)

  • 200 °C and 1000 atmospheres

Use the graph above to suggest and explain why the conditions used to produce ammonia in the Haber process are a temperature of 450 °C and a pressure of 200 atmospheres. (5)

  • The reaction is reversible
  • Forward reaction is exothermic so increased temperature lowers the yield of ammonia
  • A lower temperature would decrease rate of reaction
  • A higher pressure would increase the yield of ammonia coz the forward reaction produces the least number of (gaseous) molecules / moles
  • Higher pressures would involve high cost / energy

10.4.2 Production and uses of NPK fertilisers

NPK fertilisers

  • Compounds: nitrogen, phosphorus & potassium
  • Used to improve agricultural production
  • Industrial production of NPK fertilisers can be achieved using various raw materials in serval integrated process
    • Formulations of various salts containing appropriate % of elements

Why do farmers use ammonium nitrate on their fields? (1)

  • Fertiliser

Explain what fertilisers are used for (2)

  • Put on soil or for plants
  • For growth

Ammonia – used to manufacture ammonium salts & nitric acid

Potassium chloride, potassium sulfate and phosphate rock – obtained by mining

Phosphate rock

  • Cannot be used directly as a fertiliser.
  • Treated with nitric acid or sulfuric acid to produce soluble salts → use as fertilisers

In this question you will be assessed on using good English, organising information clearly and using specialist terms where appropriate.

Farmers use ammonium nitrate as a fertiliser for crops.

Rainwater dissolves ammonium nitrate in the soil.

Some of the dissolved ammonium nitrate runs off into rivers and lakes.

 Below shows three graphs A, B and C. The graphs show information about the use of ammonium nitrate as a fertiliser. A hectare is a measurement of an area of land.

Suggest how much ammonium nitrate farmers should use per hectare.

Give reasons for your answer.

Use information from graphs A, B and C (6)


  • Using fertiliser improves yield.
  • Yield improved most up to about 200 kg (per ha) of fertiliser.
  • Yield only increased slightly above about 200 kg (per ha).


  • About 200 kg of fertiliser gives the most profit.
  • Above about 200 kg (per ha) of fertiliser profit declines.

Run off:

  • Run off is at low levels until about 300 kg (per ha) of fertiliser.
  • Above about 300 kg (per ha) of fertiliser, run off increases.

Examples of linking of ideas:

  • Overall 200 kg gives high crop yield and most profit.
  • In conclusion 200 kg gives high crop yield and low run off.
  • 200 kg gives most profit and low run off.

Examples of compromise:

  • Profits go down after about 200 kg (per ha) of fertiliser because cost of fertiliser is not covered by increased yield.
  • 200 kg gives the highest profit although it is not the highest yield.
  • 500 kg gives the best yield but has the most runoff.

Level 3 (5 – 6 marks):

Suggestion with reasons from all three graphs, and linking of ideas which may explain a compromise