Dynamic EquilibriaEdexcel GCSE Chemistry: Revision notes

The Haber process is the industrial reaction between nitrogen (extracted from the air) and hydrogen (obtained from natural gas) to produce ammonia. It is a reversible reaction, shown using the symbol ⇌:

N2 + 3H2 ⇌ 2NH3

Because the reaction is reversible and takes place in a sealed, closed system, it can reach a dynamic equilibrium — ammonia decomposes back into nitrogen and hydrogen at the same time as it is being formed.

Dynamic equilibrium is reached in a closed system when the forward and reverse reactions occur at exactly the same rate, so the concentrations of reactants and products remain constant (though both reactions are still happening).

Conditions for the Haber process:

• Temperature: 450 °C
• Pressure: 200 atmospheres
• Catalyst: iron

(Higher tier) Changing temperature, pressure or concentration shifts the position of equilibrium (which side is favoured) and also affects the rate at which equilibrium is reached:

• Higher pressure: favours the side with fewer gas molecules, and reaches equilibrium faster
• Higher temperature and use of a catalyst: both increase the rate at which equilibrium is reached (a catalyst does not shift the position of equilibrium, only speeds up reaching it)
• Concentration changes shift the equilibrium to oppose the change (e.g. removing a product encourages more to be made)

For the Haber process specifically, industrial conditions are chosen as a compromise, balancing:

• Cost of raw materials and energy supplies (high pressure and high temperature are expensive to generate and maintain)
• Achieving an acceptable yield of ammonia in an acceptable time

450 °C is a compromise temperature: lower temperature would give a higher equilibrium yield of ammonia (since the forward reaction is exothermic) but the rate would be too slow; higher temperature would give a faster rate but a lower yield. 200 atmospheres balances a reasonably high yield against the cost and engineering difficulty of generating very high pressures. The iron catalyst does not change the yield but speeds up how quickly equilibrium (and therefore an economically useful reaction rate) is reached.

Ammonia produced in the Haber process is used to make fertilisers, which contain nitrogen, phosphorus and potassium compounds to promote plant growth.

Ammonia reacts with nitric acid to produce ammonium nitrate, a salt used directly as a fertiliser:

ammonia + nitric acid → ammonium nitrate

Ammonium sulfate can similarly be prepared:

• In the laboratory, on a small scale, by reacting ammonia solution with dilute sulfuric acid
• Industrially, on a much larger scale, using continuous processes, larger quantities of concentrated reactants, and closer control of temperature and pressure to maximise output and efficiency