C2aL9 The Haber Process

Key Words

Activation energy - energy needed to get a reaction going

Contact Process - an industrial process to make sulphur trioxide.

Exothermic - Gives out heat.

Nitrate - compounds that contain the NO₃^- ion

Optimum conditions - conditions that give the best yield in the most economical way.

Sustainable development - development that does not need large amounts of non-renewable resources.

Volatile - liquid that readily changes into a gas.

Grade E

Growing plants need nitrogen.  Ammonia, NH₃, is an important chemical in the production of nitrates fertilisers.  However it is more complicated than simply putting nitrogen and hydrogen together.

It is a reversible reaction:

Nitrogen + hydrogen D ammonia

N_{2 (g)} + 3H_{2 (g)}  D 2NH_{3 (g)} 

The industrial process to make ammonia is called the Haber process, after the German chemist Fritz Haber.

The conditions are:

• iron catalyst;
• temperature of about 450 ^oC
• pressure of 200 atmospheres.

These conditions give a yield of about 30 %.  This might seem low, but they are the optimum conditions, which ensures that ammonia is made at a reasonable rate, and costs are kept low.  This is essential for sustainable development.

Grade C

The chemical reaction for the Haber process is this: 

N_{2 (g)} + 3H_{2 (g)}  D 2NH_{3 (g)} 

On the reactants side there are four molecules and on the products side, there are two molecules.  If we increase the pressure, the equilibrium will favour the side with fewer molecules.  The yield is increased with higher pressure.

The reaction is exothermic.  We need to give the nitrogen and hydrogen molecules enough energy for them to collide and react.  However, if we increase the temperature too much, the yield is reduced, as the equilibrium will favour the reactants.

The catalyst increases the rate of reaction by lowering the activation energy, but has no affect on the yield of the ammonia.

For a fast rate, we need:

• High temperature;
• High pressure;
• Catalyst.

For a high yield:

• Low temperature;
• high pressure.

The catalyst has NO effect on the yield.

Grade A

The conditions in the Haber Process are really a compromise. 

• If we made the pressure really high, we would need massively heavy equipment to do the reaction.  200 atmospheres takes some containing as it is.
• A certain temperature is needed to get the reaction to go, but the yield is reduced as the temperature goes up.  So 450 ^oC gives an optimum yield, although some of the product will change back into reactants.  These are recycled.
• The catalyst speeds up the rate of reaction.  This is vital for the process to be profitable.

In the reaction to make ammonia the activation energy is high because the two nitrogen atoms in the nitrogen molecule are held together by a triple bond.

The triple bond is hard to break, so a catalyst and a high temperature are needed.

Ammonia is not a good fertiliser:

• it is very alkaline;
• it is volatile;
• it reacts with acids to form ammonium salts that are not very soluble in water, so don't get taken up by plants.

In the Contact Process where sulphuric acid is made, the optimum conditions are:

• 450 ^oC;
• 2 atmospheres pressure (not that high);
• vanadium pentoxide catalyst.