C2aL17 The Haber Process

Key Words

Ammonia - NH₃.  A pungent liquid (readily turns to a gas) which is corrosive and toxic.

Exothermic - Gives out heat.

Fritz Haber - a German chemist who devised a way of making ammonia.

Haber Process - industrial process to make ammonia.

Hydrogen - H₂, a very light and flammable gas.

Liquefies - turns into a liquid.

Nitrogen - N₂, an unreactive gas forming 79 % of the atmosphere

Reversible reaction - reaction that can go forwards or backwards.

Yield - how much of a product is made from a reaction

Grade E

Growing plants need nitrogen.  Ammonia, NH₃, is an important chemical in the production of 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.

Grade C

The schematic for the Haber process is like this:

Remember that the reaction is between gases and is reversible. 

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.

Notice that not all the reactants are used.  Un-reacted hydrogen and nitrogen are recycled.

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

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