Fe is used as a heterogeneous catalyst in the Haber process.

The idea

The Haber process makes ammonia: N₂(g) + 3H₂(g) ⇌ 2NH₃(g), an exothermic reversible reaction run at roughly 450 °C and about 200 atm over solid iron. Because the reactants are gases and the catalyst is a solid, this is heterogeneous catalysis. Nitrogen and hydrogen are adsorbed onto active sites on the iron surface, the strong N≡N triple bond is weakened, the molecules react, and ammonia is desorbed. The catalyst raises the rate only. It does not shift the position of equilibrium, so it does not increase the yield; it allows an acceptable yield to be reached quickly at a lower temperature than would otherwise be needed. Sulfur impurities poison the iron, so the gases are purified first.

What earns the marks

• The balanced equation N₂ + 3H₂ ⇌ 2NH₃ written with an equilibrium sign.
• Identifying iron as the catalyst and the catalysis as heterogeneous, with a solid catalyst and gaseous reactants.
• Stating that the catalyst increases the rate but does not change the position of equilibrium or the yield.
• Typical conditions of about 450 °C and about 200 atm.

Common mistakes and how to put them right

• Claiming the iron catalyst increases the yield of ammonia.

• Writing the equation with a single arrow, so losing the reversible nature of the process.

• Naming the catalyst as iron(III) oxide in the reaction itself rather than iron.

• Describing the process as homogeneous because everything happens inside one reactor.

Exam tips

• Separate rate from yield in every equilibrium answer, because a catalyst affects only the first.
• Quote the conditions with units and note that the temperature is a compromise.
• Say which species is the solid and which are gases when justifying the classification.