10.4.1a The Haber process

The Haber process makes ammonia for fertilisers

1. The Haber process manufactures ammonia, ${NH}_3$, on an industrial scale.
2. Ammonia is used to make nitrogen-based fertilisers, which raise crop yields.
3. Producing ammonia this way is vital for growing enough food for a rising population.

Raw materials: nitrogen from the air, hydrogen from natural gas

1. Nitrogen is obtained from the air, which is about 78% nitrogen.
2. Hydrogen is obtained from natural gas, which is mainly methane.
3. The two gases are purified before they enter the reactor.

The reaction conditions

1. The purified gases are passed over an iron catalyst.
2. The reactor runs at a temperature of about 450 degrees Celsius.
3. It also runs at a high pressure of about 200 atmospheres.
4. The reaction is reversible: $N_2+3H_2\rightleftharpoons 2{NH}_3$.

Separating and recycling the gases

1. The reaction never uses up all the nitrogen and hydrogen, so the mixture leaving the reactor contains all three gases.
2. The mixture is cooled so that the ammonia condenses into a liquid and is removed.
3. The unreacted nitrogen and hydrogen are recycled back into the reactor.
   1. Recycling means very little of the raw material is wasted.

What the ammonia is used for

1. Most of the ammonia is used to make nitrogen-based fertilisers, such as ammonium salts.
2. Ammonia is also used to manufacture nitric acid.
3. These products support farming, so the Haber process underpins the world's food supply.

10.4.1b Equilibrium and rate in the Haber process

The Haber process conditions are a compromise between yield and rate

1. The Haber reaction, $N_2+3H_2\rightleftharpoons 2{NH}_3$, is reversible and reaches a dynamic equilibrium in a closed system.
2. The forward reaction, which makes ammonia, is exothermic.
3. The chosen conditions balance a good equilibrium yield against a fast enough rate and a sensible cost.

Pressure: a high pressure increases the yield

1. The forward reaction turns 4 molecules of gas into 2 molecules of gas.
2. Increasing the pressure shifts the equilibrium position towards the side with fewer gas molecules, which is the ammonia, so the yield rises.
3. A higher pressure also speeds up the rate, because the particles are closer together and collide more often.
4. Very high pressures need thick, expensive pipes and are dangerous, so about 200 atmospheres is used as a compromise.

Temperature: a low temperature increases the yield but slows the reaction

1. Because the forward reaction is exothermic, lowering the temperature shifts the equilibrium towards the ammonia, raising the yield.
2. A lower temperature also slows the rate, so the ammonia would be made too slowly to be useful.
3. About 450 degrees Celsius is a compromise: it gives a lower yield but at a workable rate.
   1. The unreacted gases are recycled, so a lower yield each pass is acceptable.

The catalyst speeds up the reaction without changing the yield

1. The iron catalyst provides a pathway with a lower activation energy, so the reaction is faster.
2. The catalyst lets equilibrium be reached more quickly.
3. It does not change the equilibrium position, so it does not change the yield of ammonia.

Recycling gases and reading yield graphs

1. Unreacted nitrogen and hydrogen are recycled as feedstock, which increases the overall amount of ammonia made.
2. On a graph of yield against temperature, the yield falls as the temperature rises.
3. On a graph of yield against pressure, the yield rises as the pressure increases.
4. To explain the conditions, read the yield off the graph and describe the compromise between yield, rate and cost.