AQA GCSE · Question 10.3 · Bonding structure and the properties of matter
The mixture of gases from the reactor cools in the condenser. Suggest why ammonia condenses but the other gases do not.
How to approach this question
1. The condenser cools the gas mixture. Condensation is the process of a gas turning into a liquid.
2. For a gas to condense, it must be cooled below its boiling point.
3. The mixture contains ammonia, nitrogen, and hydrogen. Only ammonia condenses at the temperature of the condenser.
4. What does this imply about the boiling point of ammonia compared to the boiling points of nitrogen and hydrogen?
Full Answer
Condensation occurs when a gas is cooled below its boiling point. The condenser in the Haber process is kept at a temperature that is low enough to condense ammonia, but not low enough to condense the unreacted nitrogen and hydrogen.
- Ammonia (NH₃) has a boiling point of -33 °C.
- Nitrogen (N₂) has a boiling point of -196 °C.
- Hydrogen (H₂) has a boiling point of -253 °C.
Ammonia has a much higher boiling point than nitrogen and hydrogen due to stronger intermolecular forces (hydrogen bonds). Therefore, as the mixture of gases is cooled in the condenser, ammonia turns into a liquid while the nitrogen and hydrogen remain as gases. This allows the liquid ammonia to be separated.
Common mistakes
✗ Stating ammonia has a lower boiling point.
✗ Confusing boiling point with melting point.
✗ Vague answers like "ammonia is different".
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