The Haber process

1. The Haber process is used to make ammonia, $\mathrm{N}{\mathrm{H}}_3$, which is mostly used to make fertilisers.
2. Nitrogen and hydrogen react in a reversible reaction: ${\mathrm{N}}_2(\mathrm{g})+3{\mathrm{H}}_2(\mathrm{g})\rightleftharpoons 2N{\mathrm{H}}_3(\mathrm{g})$.
3. One mole of nitrogen reacts with three moles of hydrogen to form two moles of ammonia.
4. Because the reaction is reversible, the gases never react completely and some ammonia always breaks down again into nitrogen and hydrogen.

Raw materials for ammonia

1. Nitrogen comes from the air, which is about $78\%$ nitrogen.
2. The nitrogen is separated from the other gases in air by fractional distillation of liquid air.
3. Hydrogen comes from methane, $\mathrm{C}{\mathrm{H}}_4$, the main substance in natural gas.
4. Methane is reacted with steam to release the hydrogen it contains.
5. The nitrogen and hydrogen are mixed in a ratio of $1:3$ by volume, matching the ratio in the equation.

Haber process conditions

1. Temperature: $450{}^{\circ }C$.
2. Pressure: $20000kPa$, which is $200atm$.
3. Catalyst: iron.
4. Under these conditions only a fraction of the nitrogen and hydrogen, around $15\%$, is converted into ammonia each time the gases pass over the catalyst.
5. The gas mixture leaving the reactor is cooled, so the ammonia condenses to a liquid and is removed.
6. The unreacted nitrogen and hydrogen are recycled back into the reactor.

The Contact process

1. The Contact process is used to make sulfuric acid, ${\mathrm{H}}_2\mathrm{S}{\mathrm{O}}_4$.
2. Its key step converts sulfur dioxide into sulfur trioxide in a reversible reaction: $2S{\mathrm{O}}_2(\mathrm{g})+{\mathrm{O}}_2(\mathrm{g})\rightleftharpoons 2S{\mathrm{O}}_3(\mathrm{g})$.
3. Two moles of sulfur dioxide react with one mole of oxygen to form two moles of sulfur trioxide.
4. Like the Haber process, this step never goes to completion because the reaction is reversible.

Raw materials for sulfur trioxide

1. Sulfur dioxide is made by burning sulfur in air: $\mathrm{S}(\mathrm{s})+{\mathrm{O}}_2(\mathrm{g})\to \mathrm{S}{\mathrm{O}}_2(\mathrm{g})$.
2. Sulfur dioxide is also made by roasting sulfide ores, which means heating them strongly in air, such as $2ZnS(\mathrm{s})+3{\mathrm{O}}_2(\mathrm{g})\to 2ZnO(\mathrm{s})+2S{\mathrm{O}}_2(\mathrm{g})$.
3. Oxygen comes from the air.

Contact process conditions

1. Temperature: $450{}^{\circ }C$.
2. Pressure: $200kPa$, which is $2atm$.
3. Catalyst: vanadium(V) oxide, ${\mathrm{V}}_2{\mathrm{O}}_5$.
4. The sulfur trioxide formed is then used to make sulfuric acid.

Comparing the two processes

1. Product: the Haber process makes ammonia, whereas the Contact process step makes sulfur trioxide for sulfuric acid.
2. Raw materials: the Haber process uses nitrogen from air and hydrogen from methane, whereas the Contact process uses sulfur dioxide from burning sulfur or roasting sulfide ores, and oxygen from air.
3. Temperature: both processes use $450{}^{\circ }C$.
4. Pressure: the Haber process uses $20000kPa$ ($200atm$), whereas the Contact process uses only $200kPa$ ($2atm$).
5. Catalyst: the Haber process uses iron, whereas the Contact process uses vanadium(V) oxide.