How ammonia is made from nitrogen and hydrogen, why the conditions are a compromise, and how ammonia is used to make fertilisers.
Objectives
Write the equation for the Haber process and name its raw materials and where they come from.
State the conditions used: about 450 °C, 200 atmospheres and an iron catalyst.
Explain why these conditions are a compromise between rate, yield and cost.
Describe how unreacted gases are recycled and how ammonia is used to make fertilisers.
The Haber process and fertilisers explained
Start here
Plants need nitrogen to grow, but they cannot use the nitrogen gas in the air. Farmers add fertilisers that contain nitrogen compounds. Many of these are made from ammonia, and nearly all ammonia is made by the Haber process.
This matters because fertilisers help to grow the food for billions of people. It is also a good example of how a chemist chooses conditions: the best conditions are a compromise between speed, yield and cost.
By the end you will be able to write the equation, give the raw materials and conditions, explain the compromise, and say how ammonia becomes a fertiliser.
Making ammonia
Ammonia has the formula NH₃. It is made from nitrogen and hydrogen in the Haber process. The equation is: nitrogen + hydrogen ⇌ ammonia, or N₂(g) + 3H₂(g) ⇌ 2NH₃(g).
The reaction is reversible. Nitrogen and hydrogen make ammonia, and ammonia can break down again into nitrogen and hydrogen. The forward reaction is exothermic, which means it gives out heat.
In a closed container the reaction reaches dynamic equilibrium. The forward and backward reactions go on at the same rate, so the amounts of nitrogen, hydrogen and ammonia stay constant.
Raw materials
Nitrogen is taken from the air. The air is about four-fifths nitrogen, so there is plenty of it. It is separated from oxygen by fractional distillation of liquid air.
Hydrogen is obtained from natural gas, which is mostly methane. The methane is reacted with steam to make hydrogen.
Both gases are cleaned and mixed in the ratio 1 nitrogen to 3 hydrogen, which is the ratio in the balanced equation.
The conditions
The gases are passed over an iron catalyst at a temperature of about 450 °C and a pressure of about 200 atmospheres.
The iron catalyst speeds up the reaction so that the gases reach equilibrium faster. A catalyst does not change the position of equilibrium and is not used up.
The mixture leaving the reactor contains ammonia mixed with unreacted nitrogen and hydrogen. The reaction does not go to completion, so the unreacted gases must be dealt with.
A compromise (Higher tier)
The forward reaction is exothermic. A lower temperature would push the equilibrium towards ammonia, giving a higher yield. But a low temperature makes the reaction very slow. A high temperature is fast but gives a lower yield. About 450 °C is a compromise that gives a reasonable rate and a reasonable yield.
There are 4 molecules of gas on the left (1 N₂ and 3 H₂) and only 2 on the right (2 NH₃). A high pressure pushes the equilibrium towards the side with fewer gas molecules, so it increases the yield of ammonia. But very high pressures need strong pipes and powerful pumps, which cost a lot and are dangerous. About 200 atmospheres is a compromise.
When you explain a compromise, say what is good, what is bad, and why the chosen value is in between.
Recycling the gases
The gases leaving the reactor are cooled. Ammonia has a higher boiling point than nitrogen and hydrogen, so it turns to a liquid and is removed.
The unreacted nitrogen and hydrogen are recycled back into the reactor, so none is wasted. This is how the process turns nearly all of the raw materials into ammonia even though each pass through the reactor makes only a little.
Ammonia and fertilisers
Ammonia is used to make fertilisers. For example, ammonia reacts with nitric acid to make ammonium nitrate, which supplies nitrogen to crops. (Separate Chemistry only: NPK fertilisers contain compounds of nitrogen, phosphorus and potassium, the three elements plants need most.)
Fertilisers increase crop yields, but too much can be washed into rivers where it causes problems for wildlife. Farmers need to apply the right amount.
Recap
N₂ + 3H₂ ⇌ 2NH₃. Nitrogen comes from the air and hydrogen from natural gas.
Conditions: about 450 °C, 200 atmospheres, iron catalyst. Higher tier: the temperature and pressure are compromises between rate, yield and cost.
Ammonia is liquefied by cooling. Unreacted nitrogen and hydrogen are recycled. Ammonia is used to make fertilisers.
Key points
N₂(g) + 3H₂(g) ⇌ 2NH₃(g). The forward reaction is exothermic.
Nitrogen from the air. Hydrogen from natural gas.
About 450 °C, 200 atmospheres, iron catalyst.
Unreacted nitrogen and hydrogen are recycled. Ammonia is liquefied by cooling and used to make fertilisers.
Worked examples
Example 1
State the raw materials for the Haber process and where each comes from.
The product is ammonia, NH₃, so the reactants are nitrogen and hydrogen.
Nitrogen is taken from the air.
Hydrogen is obtained from natural gas.
Answer: Nitrogen from the air and hydrogen from natural gas
Common slips
Slip: Oxygen and hydrogen. Oxygen and hydrogen make water. Ammonia needs nitrogen.
Slip: Nitrogen from natural gas. Nitrogen comes from the air. Hydrogen comes from natural gas.
Slip: Ammonia from the air. Ammonia is the product, not a raw material.
Example 2
Calculate the mass of ammonia (NH₃) that can be made from 28 tonnes of nitrogen. Relative atomic masses: N = 14, H = 1.
N₂ + 3H₂ ⇌ 2NH₃.
Mr of N₂ = 28 and Mr of NH₃ = 17, so 2NH₃ = 34.
28 tonnes of N₂ makes 34 tonnes of NH₃.
Answer: 34 tonnes
Common slips
Slip: 14 tonnes. Use the whole of N₂ (28), not a single atom.
Slip: 17 tonnes. The equation gives 2NH₃, so double the Mr of NH₃.
Slip: 56 tonnes. That doubles the mass of nitrogen. The equation gives 28 t of N₂ plus 6 t of H₂ making 34 t of NH₃.
Example 3
The reaction N₂ + 3H₂ ⇌ 2NH₃ is exothermic. Explain why a temperature of 450 °C is used, and say what effect the iron catalyst has on the yield.
A lower temperature moves the equilibrium to the right (exothermic direction), so the yield would be higher.
But the reaction would be too slow at low temperatures.
450 °C is a compromise that gives a reasonable rate and a reasonable yield.
The catalyst speeds up the reaction but does not change the equilibrium position, so it has no effect on the yield.
Answer: 450 °C is a compromise between a good yield (lower temperature) and a fast rate (higher temperature). The catalyst does not change the yield
Common slips
Slip: A high temperature gives a higher yield. For an exothermic forward reaction a higher temperature lowers the yield.
Slip: The catalyst increases the yield. A catalyst only makes equilibrium reached faster. It does not change the yield.
Slip: 450 °C is used because it is the lowest temperature. It is used because it is a compromise, not because it is the lowest.
Slip: The temperature is chosen to make the reaction stop. The reaction does not stop. It reaches equilibrium.
Questions and answers
The Haber process and fertilisers practice questions for GCSE Combined Science Chemistry. Try each one, then open the answer. Easy questions come first.
Question 1 Easy
How many molecules of hydrogen react with one molecule of nitrogen in the Haber process?
Show answer
Answer: 3
Look at the balanced equation N₂ + 3H₂ ⇌ 2NH₃.
There are 3H₂.
Question 2 Easy
What is the catalyst in the Haber process?
Show answer
Answer: iron
The Haber process uses a metal catalyst.
It is iron.
Question 3 Easy
Where does the nitrogen for the Haber process come from?
Show answer
Answer: the air
Four-fifths of the air is nitrogen.
It is taken from the air.
Question 4 Easy
What temperature is used in the Haber process, in °C?
Show answer
Answer: 450
The temperature is a compromise.
It is about 450 °C.
Question 5 Easy
What pressure is used in the Haber process, in atmospheres?
Show answer
Answer: 200
The pressure is high but not extreme.
It is about 200 atmospheres.
Question 6 Medium
Name the product of the Haber process.
Show answer
Answer: ammonia
The reactants are nitrogen and hydrogen.
The product is ammonia.
Question 7 Medium
Where does the hydrogen for the Haber process come from?
Show answer
Answer: natural gas
Hydrogen is made from methane and steam.
Methane is the main part of natural gas.
Question 8 Medium
What happens to the unreacted nitrogen and hydrogen?
Show answer
Answer: they are recycled
Only some of the gases react each time.
The rest is recycled into the reactor.
Exam-style questions
Longer questions in the style of a written paper, with the marking points. These are Yumio practice questions.
Exam-style question 15 marks
(a) Name the two raw materials used to make ammonia in the Haber process and say where each is obtained. [2 marks] (b) State the temperature, the pressure and the catalyst used in the Haber process. [3 marks]
Show answer and marking points
Answer: (a) nitrogen from the air and hydrogen from natural gas (b) about 450 °C, about 200 atmospheres and an iron catalyst
(a) nitrogen from the air (1)
(a) hydrogen from natural gas (methane) (1)
(b) 450 °C (1)
(b) 200 atmospheres (1)
(b) iron catalyst (1)
Exam-style question 24 marks
(a) Explain why the Haber process is run at 450 °C and not at a much lower temperature. [2 marks] (b) Explain why the unreacted gases are recycled. [1 mark] (c) State the effect of the catalyst on the position of equilibrium. [1 mark]
Show answer and marking points
Answer: (a) a lower temperature would give a higher yield but the reaction would be too slow, so 450 °C is a compromise (b) only some of the gases react so recycling uses all of the raw materials (c) none, it does not change the position of equilibrium
(a) lower temperature is too slow, or rate is too low (1)
(a) 450 °C is a compromise between rate and yield (1)
(b) only some of the gas reacts or to avoid wasting raw materials (1)
(c) no effect on the position of equilibrium (1)
Common mistakes
Saying the catalyst increases the yield
A catalyst speeds up the reaction but does not change the position of equilibrium.
Saying the hydrogen comes from the air
Nitrogen comes from the air. Hydrogen comes from natural gas.
Saying the reaction goes to completion
It is reversible and reaches equilibrium, so the unreacted gases are recycled.
Writing N₂ + H₂ → NH₃
Balance it and show it is reversible: N₂ + 3H₂ ⇌ 2NH₃.
Saying a low temperature is used because it is cheaper
450 °C is a compromise: low temperatures give more ammonia but the reaction is too slow.
Questions students ask
Is The Haber process and fertilisers on the Foundation or Higher tier?
The Haber process and fertilisers is on both Foundation and Higher papers for GCSE Combined Science Chemistry, aimed at around grade 6.
What is a common mistake on The Haber process and fertilisers?
Saying the catalyst increases the yield. A catalyst speeds up the reaction but does not change the position of equilibrium.
Where are the answers to the The Haber process and fertilisers questions?
Open Show answer under each question on this page. The questions are Yumio practice questions, not past exam questions.