What is a reversible reaction, and how is the Haber process optimised?
Reversible reactions and dynamic equilibrium, how changing conditions shifts the position of equilibrium, and the conditions used in the Haber process to make ammonia.
A CCEA GCSE Chemistry answer on reversible reactions and equilibrium, covering dynamic equilibrium in a closed system, how changing temperature, pressure and concentration shift the equilibrium, and the conditions used in the Haber process to make ammonia as a compromise.
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What this dot point is asking
CCEA wants you to explain reversible reactions and dynamic equilibrium, describe how changing temperature, pressure and concentration shifts the position of equilibrium, and apply these ideas to the conditions chosen for the Haber process.
Reversible reactions and dynamic equilibrium
At equilibrium the reaction has not stopped; both directions continue, but at equal rates, so nothing appears to change. The "position" of equilibrium tells you whether there is more product or more reactant in the mixture.
Shifting the equilibrium
These rules let chemists choose conditions that push a reversible industrial reaction towards the product they want.
The Haber process
The forward reaction is exothermic, so a low temperature would give a higher yield, but the reaction would be too slow. A high pressure favours ammonia (4 gas molecules become 2), but very high pressures are expensive and dangerous. The chosen conditions are a compromise: a moderate temperature for a reasonable rate and yield, a high but practical pressure, and an iron catalyst to speed things up without changing the yield. Unreacted gases are recycled.
Worked example
Examples in context
Example 1. Feeding the world. The ammonia from the Haber process is turned into nitrogen fertilisers that have hugely increased crop yields worldwide. The careful balancing of equilibrium and rate in this one process underpins modern food production.
Example 2. Recycling unreacted gas. Because only some of the nitrogen and hydrogen react each pass, the unreacted gases are cooled to remove ammonia and then recycled back through the reactor. This makes the process efficient despite the modest yield, a practical response to the equilibrium being incomplete.
Try this
Q1. State what is meant by dynamic equilibrium. [2 marks]
- Cue. The forward and backward reactions occur at the same rate, so concentrations stay constant in a closed system.
Q2. State 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 20194 marksState the raw materials and the conditions used in the Haber process, and write the equation for the reaction.Show worked answer →
Markers want the reactants, the conditions and a balanced equation.
The raw materials are nitrogen (from the air) and hydrogen (from natural gas).
The conditions are a temperature of about 450 degrees C, a pressure of about 200 atmospheres, and an iron catalyst.
The equation, with the reversible arrow, is:
Markers reward nitrogen and hydrogen, the three conditions (about 450 degrees C, about 200 atm, iron catalyst), and the balanced reversible equation.
CCEA 20214 marksThe forward reaction in the Haber process is exothermic. Explain why a moderate temperature of about 450 degrees C is used rather than a much lower temperature.Show worked answer →
Markers want the conflict between yield and rate, resolved as a compromise.
Because the forward reaction is exothermic, a lower temperature would shift the equilibrium towards the products and give a higher yield of ammonia.
However, at a low temperature the reaction would be very slow, so it would take too long to make ammonia.
A moderate temperature of about 450 degrees C is therefore a compromise: the yield is lower than at low temperature, but the reaction is fast enough to make ammonia at a useful rate, so more is produced in a given time.
Markers reward that low temperature gives a higher yield but is too slow, and that 450 degrees C is a compromise between yield and rate.
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Sources & how we know this
- CCEA GCSE Chemistry specification (1110) — CCEA (2017)