The purified gases are passed over a catalyst of iron at a high…

The idea

In the Haber process, purified nitrogen and hydrogen are passed over an iron catalyst at about 450°C and about 200 atmospheres. Some nitrogen and hydrogen react to form ammonia, but because the reaction is reversible, some ammonia breaks down again into nitrogen and hydrogen, so an equilibrium mixture leaves the reactor. The mixture is cooled; ammonia liquefies and is removed, while unreacted nitrogen and hydrogen remain as gases and are recycled back into the reactor. Recycling improves the overall yield because it gives unreacted gases further chances to react. The word equation is nitrogen + hydrogen ⇌ ammonia, and the balanced symbol equation is N₂ + 3H₂ ⇌ 2NH₃.

What earns the marks

• Purified nitrogen and hydrogen are passed over an iron catalyst.
• The temperature used is about 450°C.
• The pressure used is about 200 atmospheres.
• The reaction is reversible, so ammonia can break down into nitrogen and hydrogen.
• Cooling the mixture liquefies ammonia, which is then removed.
• Unreacted nitrogen and hydrogen are recycled back into the reactor.
• The word equation is nitrogen + hydrogen ⇌ ammonia.
• The balanced symbol equation is N₂ + 3H₂ ⇌ 2NH₃.

Common mistakes and how to put them right

• Writing the equation with a single arrow instead of the reversible arrow.

  Put it right: Use ⇌ in the equation.

• Saying ammonia is removed by filtration.

  Put it right: Ammonia is liquefied by cooling and then removed as a liquid.

• Forgetting to recycle unreacted gases.

  Put it right: Remaining nitrogen and hydrogen are returned to the reactor.

Exam tips

• Quote the conditions as about 450°C and about 200 atmospheres, including the word 'about' because they are approximate.
• Name iron as the catalyst and state that it speeds up the reaction without being used up.
• When describing the process, use the sequence react, cool, liquefy, remove, recycle to keep your answer ordered.