The Haber Process

Booklet by Dr. E. Kruiswijk, 01-11-2005

1

The Manufacture of Ammonia Introduction

Ammonia (NH3) is a small but very important molecule. We use it to make

fertilizers, as well as explosives and plastics. It was first made from its

elements (nitrogen and hydrogen) in 1909 by a German chemist called

Fritz Haber. This development, followed by its scaling up to an industrial

process by Carl Bosch, helped German preparations for the First World

War, which started in 1914. The Haber process, as we now call it, is still

used to make ammonia and helps feed the world by providing the starting

material for millions of tonnes of fertilizer made each year.

The Haber process - reversibility and equilibrium

In chemical reactions, the substances you start off with (the reactants)

change into new substances (the products). But in some chemical

reactions, the products can change back again into the original reactants.

Reactions like this that can go in both directions are called reversible

reactions. The diagram shows a reversible reaction that you have seen

many times before.

2

Q: Copy and complete the sentences.

To change litmus from red to blue, you add __________.

To change litmus from blue to red, you add __________.

The reaction is __________ so we can show it like this:

blue litmus red litmus

An important reversible reaction: the Haber process

Without nitrogen fertiliser, farmers couldn't produce so much food. The

first step in making nitrogen fertiliser is to make ammonia from the

nitrogen in the air, and hydrogen. Almost all the ammonia used throughout

the world is manufactured by the process developed by Fritz Haber at

the beginning of the twentieth century. This and the next few spreads

look at the Haber process in detail. The reaction used in the Haber

process is a reversible reaction. It can go both ways.

3

Q: Look at the diagram. For the Haber process:

(a) write down the equation for the forward reaction;

(b) write down the equation for the reverse reaction;

(c) write down an equation which shows both reactions at the same time.

Equilibrium - a key idea

When the Haber process reaction begins, there will be many reacting

molecules of nitrogen and hydrogen but few ammonia molecules. This

means that the forward reaction will be fast, but the reverse reaction

will be slow.

As the reaction continues, the number of nitrogen and hydrogen

molecules will decrease and the number of ammonia molecules will

increase. So the rate of the forward reaction will decrease and the rate

of the reverse reaction will increase.

Eventually a point is reached where the rate of the forward reaction and

the rate of the reverse reaction are equal. This point is called equilibrium.

4

Q: Copy and complete the sentences.

the forward and reverse reactions

ow much of the product there is in the mixture at equilibrium depends

n the Haber process, we need to know what conditions will give a good

re (25 °C and 1 atmosphere), the

e of the nitrogen is air. The source of the hydrogen is natural

CH4 + H2O 3H2 + CO

At equilibrium, the ____________ of

are equal. For the reactants and products to reach equilibrium, they must

be in a __________system. At equilibrium, in the Haber process reaction

vessel there will be three substances:__________,________

and__________.

H

on the particular reaction and on the reaction conditions (that is, the

temperature and pressure).

I

yield of ammonia. The next spread looks at this.

Under normal temperature and pressu

amount of ammonia at equilibrium is about 1 %.

The sourc

gas and steam:

5

Using your knowledge:

This word equation shows a reversible reaction:

iron + steam iron oxide + hydrogen

(a) Explain what is meant by a reversible reaction.

(b)When the reaction begins, the reverse reaction (right to left) is slow.

Explain why.

6

(c) In a closed system, the reaction will reach equilibrium. Explain what is

happening at equilibrium.

Changing the position of equilibrium

The reaction used in the Haber process is reversible:

N2 + 3H2 -> 2NH3

Like other reversible reactions, this reaction reaches an equilibrium in a

closed system. At equilibrium, the forward and reverse reactions occur at

the same rate. In a reversible reaction such as the Haber process, it is

possible to change the position of equilibrium by changing the conditions

under which the reaction takes place.

Q: Write down two ways in which we can change the conditions inside the

reaction vessel used in the Haber process.

In the Haber process, ammonia is produced in the forward reaction:

N2 + 3H2 -> 2NH3

7

Q: Copy and complete the sentence.

To produce more ammonia, we need to choose conditions that will favour

the_________ reaction.

Increasing the pressure

If we increase the pressure on an equilibrium mixture of nitrogen,

hydrogen and ammonia, the rate of the forward reaction increases more

than the rate of the reverse reaction. So a new equilibrium is reached

which contains more ammonia than before.

Q: (a) Which reaction in the Haber process, forward or reverse, is

favoured by raising the pressure?

(b) Why is this reaction favoured?

8

Increasing the temperature

The energy changes in a reversible reaction are also reversible:

N2 + 3H2 2NH3 ΔH = -93 kJ

N2 + 3H2 2NH3 ΔH = +93 kJ

REMEMBER

If ΔH is negative, the reaction is exothermic.

If ΔH is positive, the reaction is endothermic.

If we increase the temperature of an equilibrium mixture of nitrogen,

hydrogen and ammonia, the equilibrium will shift so as to reduce the

temperature. So the rate of the endothermic reaction is increased.

Q: (a) Which reaction in the Haber process, forward or reverse, is

favoured by increasing the temperature?

(b) Why is this reaction favoured?

9

Putting it all together

The graph shows the percentage of reacting gases converted into

ammonia, at different temperatures and pressures.

Q: (a) From the graph, under what conditions of temperature and

pressure is the yield of ammonia greatest?

(b) Suggest a combination of temperature and pressure that would give an

even higher yield.

10

Using your knowledge

Copy this table and complete it for the reaction used in the Haber

process.

Conditions for

the reaction

Effect on rate of

forward reaction

compared with reverse

reaction (increased or

decreased)

Yield of ammonia

(high or low)

high temperature

low temperature

high pressure

low pressure

Q: If a forward reaction is endothermic, what is the effect of increasing

the temperature on the yield of the product? Explain your answer.

Economic and environmental considerations

All manufacturers try to make their products as economically as possible.

Ammonia is usually manufactured at a temperature of about 400 °C and a

pressure of about 200 atmospheres.

Q: (a) How do these conditions compare with those for highest yield as

shown on the graphs? (b) Estimate the yield under these conditions.

11

The reasons for using these conditions are mainly economic. The reaction

vessel is made from reinforced steel and may be 20 metres high with a

mass up to 200 tonnes. The cost of making the reaction vessel is high, but

it would be much higher if it was built to withstand higher pressures. It

would need to be even thicker and stronger.

Running costs would be greater at higher pressures, as the reacting gases

would have to be pumped to a higher pressure. Operating at a higher

pressure also increases safety risks. For these reasons, an optimum

pressure of 200 atmospheres is used. ('Optimum' means the most

favourable when all factors are considered.)

12

Q: Explain why 200 atmospheres is the optimum pressure.

Although the yield of ammonia is higher when the temperature of

reaction is lower, the rate of reaction is also lower. Slow production

increases the costs of manufacture considerably.

For this reason, an optimum temperature of 400 °C is used.

Q: Explain why 400°C is the optimum temperature.

An iron catalyst is used to increase the rate of reaction. The catalyst

increases the rates of both forward and reverse reactions equally in this

reversible reaction.

Q: Copy and complete the sentences.

To raise the _________ above 200 atmospheres or to reduce the

_______ below 400 °C, would mean that the _________ in running costs

would outweigh the advantages of the _______ yield.

13

Looking at the whole process

Reactants are put in and products are removed continuously over a long

period of time. So we call this a continuous process.

Energy released from the exothermic reaction, and from hot ammonia

leaving the system, is used to heat up incoming reactants and produce

steam to drive turbines.

The plant can be operated by a small number of staff.

The ammonia plant and other plants which use ammonia for manufacture

of nitric acid and fertilisers are usually on the same site. This reduces

transport costs and delivery times.

Research is carried out into more-efficient processes, so making energy

savings.

14

Q: The hydrogen used in the reaction is expensive to produce. How does

the process ensure that none of it is wasted?

Using your knowledge

The table shows the yield of ammonia as a percentage at different

temperatures and pressures in the Haber process.

Pressure

(atm)

100 °C 300

°C

500 °C

25 91.7% 27.4% 2.9%

100 96.7% 52.5% 10.6%

400 99.4% 79.7% 31.9%

(a) Under what conditions (shown in the table) would the maximum yield

of ammonia be obtained?

(b) At room temperature and pressure the yield of ammonia is only about

1%. Describe and explain how the yield is affected by the conditions used

in the manufacturing process.

(c) Why is the pressure used lower than the pressure that gives the

maximum yield?

15

Summary

16