What is a reversible reaction, and how is the Haber process used to make ammonia?
Reversible reactions and dynamic equilibrium, how changing conditions shifts the position of equilibrium, and the conditions used in the Haber process to make ammonia for fertilisers.
A focused CCEA GCSE Double Award Science (Chemistry Unit C2) answer on reversible reactions, covering dynamic equilibrium, how changing temperature, pressure and concentration shift the equilibrium, and the conditions used in the Haber process to make ammonia.
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What this dot point is asking
CCEA Double Award wants reversible reactions and dynamic equilibrium, how changing conditions shifts the equilibrium, and the conditions used in the Haber process and why. The Haber process ties the rate and equilibrium ideas together in one industrial example.
Reversible reactions
An example is heating hydrated copper sulfate (blue) to drive off water and form anhydrous copper sulfate (white); adding water reverses it.
Dynamic equilibrium
Changing the conditions shifts the position of equilibrium - whether there is more product or more reactant. For example, increasing the pressure shifts the equilibrium towards the side with fewer gas molecules, and changing the temperature favours either the exothermic or the endothermic direction.
The Haber process
The Haber process makes ammonia (used for fertilisers) from nitrogen and hydrogen:
The nitrogen comes from the air and the hydrogen from natural gas. The conditions are:
- About 450 degrees Celsius (a compromise temperature).
- About 200 atmospheres pressure.
- An iron catalyst.
The ammonia is removed by cooling it to a liquid, and the unreacted nitrogen and hydrogen are recycled, which improves the overall yield. The high pressure is chosen because the forward reaction makes fewer gas molecules (four molecules of reactant become two of product), so a high pressure shifts the equilibrium towards the ammonia and also speeds the reaction; the limit is the cost and safety of building equipment for very high pressures.
Examples in context
Example 1. Feeding the world. Ammonia from the Haber process is turned into fertilisers that boost crop yields, which has been vital for feeding a growing population. This is why the industrial conditions matter so much: they have to balance cost, rate and yield.
Example 2. Recycling the unreacted gases. Because only some of the nitrogen and hydrogen react each pass, the leftover gases are recycled back through the reactor. This makes the process efficient overall even though the single-pass yield is modest, showing how industry manages an equilibrium.
Try this
Q1. What does the double arrow in an equation mean? [1 mark]
- Cue. The reaction is reversible.
Q2. Name the catalyst used in the Haber process. [1 mark]
- Cue. Iron.
Exam-style practice questions
Practice questions written in the style of CCEA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
CCEA-style3 marksWrite the word equation for the Haber process and state where the two raw materials come from.Show worked answer →
Equation plus sources for three marks.
Nitrogen + hydrogen gives ammonia (a reversible reaction).
The nitrogen comes from the air.
The hydrogen comes from natural gas (or from cracking hydrocarbons). Markers reward the reversible equation and both sources.
CCEA-style4 marksState the conditions used in the Haber process and explain why a catalyst is used.Show worked answer →
Conditions plus the role of the catalyst for four marks.
The conditions are about 450 degrees Celsius, about 200 atmospheres pressure, and an iron catalyst.
The temperature is a compromise: higher temperature speeds the reaction but lowers the yield.
The high pressure increases the yield and the rate.
The iron catalyst speeds up the reaction without being used up, so equilibrium is reached faster. Markers reward the three conditions and the catalyst speeding the reaction.
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Sources & how we know this
- CCEA GCSE Science Double Award specification — CCEA (2017)