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The Haber Process

In this activity, students will learn how the Haber process is used to produce ammonia. Students will also learn the reversible nature of the reaction.

'The Haber Process' worksheet

Key stage:  KS 4

GCSE Subjects:   Chemistry: Single Subject, Chemistry: Combined Science

GCSE Boards:   Pearson Edexcel

Curriculum topic:   Extracting Metals and Equilibria

Curriculum subtopic:   Reversible Reactions and Equilibria

Difficulty level:  

Worksheet Overview

QUESTION 1 of 10

Ammonia is an incredibly useful chemical. It is used to make nitrogen-based fertilisers. Without those, it would be impossible to grow enough food to feed all the people in the world. So how do we make it?

Ammonia has the chemical formula NH3. Both hydrogen and nitrogen are simple to get; we can obtain nitrogen from the air, and hydrogen from methane. The reaction to make ammonia is then
N2 (g) + 3 H2 (g) ⇌ 2 NH3 (g).
Remember that ⇌ means a reversible reaction;  at a molecular level, we see both the making and breakup of ammonia all the time. Since the reaction is reversible, we need to pick the right temperature and pressure conditions to make ammonia.


What conditions would maximise the yield?
The forward reaction (nitrogen + hydrogen → ammonia) is exothermic, so that direction is favoured at lower temperatures. The forward reaction also reduces the volume of the gas (there are four molecules on the left hand side, and two on the right hand side), so that direction is favoured at high pressures. So the best way to convert as much hydrogen and nitrogen into ammonia as possible is to run the reaction at as high a pressure and as low a temperature as possible. Real life is a bit more complicated than this.


What conditions are actually used and why?
The Haber process is an example of an important principle, which most chemistry textbooks don’t mention;
Chemical factories are there to make profits, not chemicals
If a company can’t make a product for less money than they can sell it for, they won’t make it for long. It’s really important that chemical factories can make things cheaply.
This means that real chemical factories making ammonia use the Haber process  running at about 200 atmospheres (so 200 times higher than the pressure of the atmosphere). In theory, they could increase the pressure even more, and increase the yield. The disadvantage of this is that high pressures are expensive to make. If you have ever seen a pressure cooker, you will have seen how the metal walls have to be much thicker and stronger than for a normal saucepan. Making a chemical works strong enough to withstand pressure higher than 200 atmospheres is very expensive, and not viable when making ammonia.
The temperature used for the Haber process is about 450 °C. Lower temperatures would increase the yield some more,  but reduce the rate of reaction. That means that it would take more time to make a certain amount of ammonia, and for businesses, time costs money. The temperature used is a compromise between the need to have a good yield and a good rate of reaction.
When the Haber process is used in chemical factories, a catalyst (iron) is used to speed up the reaction.
Although the conditions only convert about 40 % of the reactants into ammonia, the leftover hydrogen and nitrogen are not wasted. Once the reaction has reached equilibrium, the reacting mixture is cooled down. The ammonia turns to liquid, and is stored. The remaining hydrogen and nitrogen are collected, and run through the Haber process again.
The Haber process makes an important chemical. It’s also a useful one to think about when you are learning chemistry, because it recaps ideas about reversible reactions, rates of reaction and industrial chemistry. Now you know how it works, try the questions to see how well you understand it.

What is the link between the Haber process and food production

Ammonia is a food additive

Ammonia is used to make fertiliser

Ammonia keeps food fresh

Match up these half-sentences about ammonia production.

Column A

Column B

The chemical formula for ammonia is
N2
The reactant we get from the air is
H2
The reactant we get from methane is
NH3.

What type of reaction occurs between hydrogen + nitrogen and ammonia?

exothermic

endothermic

reversible

neutralisation

Think about the forward reaction in the Haber process;

nitrogen + hydrogen → ammonia

Tick two of these statements about the forward reaction which are correct.

The reaction is endothermic.

The reaction is exothermic.

The reaction is favoured at high temperatures.

The reaction is favoured at low temperatures.

What temperature is actually used for the Haber process, and why? Pick a temperature and a reason.

The reaction is endothermic.

The reaction is exothermic.

The reaction is favoured at high temperatures.

The reaction is favoured at low temperatures.

Think about the forward reaction,

N2 (g) + 3 H2 (g) → 2 NH3.

How does the forward reaction affect the volume occupied by the chemicals?

Increased volume

Unchanged volume

Decreased volume

Pick the pressure used for the Haber process, and the reason that pressure is used.

Increased volume

Unchanged volume

Decreased volume

The Haber process uses a catalyst; what is this made from?

When we do the Haber process at 450 °C and 200 atmospheres, most of the hydrogen and nitrogen do not react. What happens to them?

They are returned to the atmosphere

They are burnt

They are recycled

Fill in the gaps in this sentence:

They are returned to the atmosphere

They are burnt

They are recycled

  • Question 1

What is the link between the Haber process and food production

CORRECT ANSWER
Ammonia is used to make fertiliser
EDDIE SAYS
Fertilisers are incredibly important to grow crops. We can produce fertilisers organically (e.g. animal dung, or compost), but the Haber process is the way we can make enough fertiliser to grow enough crops to feed everyone.
  • Question 2

Match up these half-sentences about ammonia production.

CORRECT ANSWER

Column A

Column B

The chemical formula for ammonia ...
NH3.
The reactant we get from the air ...
N2
The reactant we get from methane ...
H2
EDDIE SAYS
Although nitrogen and hydrogen are both abundant, neither is totally simple to obtain. For nitrogen, we have to cool air enough to be able to do fractional distillation on it. Extracting hydrogen from methane involves reacting methane with steam at high pressures.
  • Question 3

What type of reaction occurs between hydrogen + nitrogen and ammonia?

CORRECT ANSWER
reversible
EDDIE SAYS
The reaction is reversible, which is why the equation has ⇌ instead of an arrow.
  • Question 4

Think about the forward reaction in the Haber process;

nitrogen + hydrogen → ammonia

Tick two of these statements about the forward reaction which are correct.

CORRECT ANSWER
The reaction is exothermic.
The reaction is favoured at low temperatures.
EDDIE SAYS
The reaction is exothermic. You can prove that with bond energy calculations, but it's simpler to remember and state it for now. Le Chatelier's principle says that equilibrium moves to undo an external change. So we make more ammonia by reducing the temperature; the energy released by the exothermic reaction undoes the cooling.
  • Question 5

What temperature is actually used for the Haber process, and why? Pick a temperature and a reason.

CORRECT ANSWER
EDDIE SAYS
We get the highest yield at the lowest possible temperature. Unfortunately, chemical reactions are slower at low temperatures. At room temperature, the reaction would take far too long for the factory to wait to make a profit. Running at 450 °C reduces the yield, but increases the speed of the process a lot.
  • Question 6

Think about the forward reaction,

N2 (g) + 3 H2 (g) → 2 NH3.

How does the forward reaction affect the volume occupied by the chemicals?

CORRECT ANSWER
Decreased volume
EDDIE SAYS
On the reactants side, there are four molecules, and on the products side there are two molecules. This means that the volume of the gas will be halved by the forward reaction.
  • Question 7

Pick the pressure used for the Haber process, and the reason that pressure is used.

CORRECT ANSWER
EDDIE SAYS
Equipment able to withstand very high pressures is expensive. It needs to be made of thick sheets of metal, strongly welded together.
  • Question 8

The Haber process uses a catalyst; what is this made from?

CORRECT ANSWER
iron
Fe
EDDIE SAYS
Different reactions need different catalysts. The structure of iron matches the ammonia structure well, which is why it is a useful catalyst.
  • Question 9

When we do the Haber process at 450 °C and 200 atmospheres, most of the hydrogen and nitrogen do not react. What happens to them?

CORRECT ANSWER
They are recycled
EDDIE SAYS
This is the reason that the lowish yield from the Haber process doesn\'t matter. Once we cool the reacting mixture (so the ammonia turns to liquid), we can recycle the remaining hydrogen and nitrogen.
  • Question 10

Fill in the gaps in this sentence:

CORRECT ANSWER
EDDIE SAYS
This isn't always true (it's possible to make medicines on a charitable basis) but it's true for most of the chemical industry, and it's important to keep in mind to explain why different chemical factories operate the way they do.
---- OR ----

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