Topic 3 of 3
Equilibrium in industry
Explain the choices in an ammonia plant.
A-Level 8873, revised syllabus (2026-2027)
Choose a useful production rate and yield
Industrial conditions combine equilibrium, kinetics and cost.
The Haber process makes ammonia: N2(g) + 3H2(g) ⇌ 2NH3(g), with an exothermic forward reaction. A representative school description uses an iron catalyst, about 450 °C and a pressure of about 200 atm. Actual plants vary; the reasoning behind the conditions matters more than treating one pair of numbers as universal.
| Choice | Chemical reason | Practical trade-off |
|---|---|---|
| High pressure | Compression favours 2 mol of product gas over 4 mol of reactant gas; it also increases gas concentrations. | Compression consumes energy and stronger pressure-resistant equipment costs more. Indefinitely high pressure is not economical. |
| Moderately high temperature | A higher temperature increases reaction rate, but decreases Kc for the exothermic forward reaction. | A compromise gives an acceptable rate while retaining a useful equilibrium yield. Very low temperature gives a favourable equilibrium but a slow approach to it. |
| Iron catalyst | Lowers activation barriers and accelerates approach to equilibrium. | Allows an economically useful rate; it does not increase the equilibrium yield at fixed temperature. |
| Cool the outlet and separate ammonia | Ammonia is condensed and removed from the gas stream. | Separation allows product collection and leaves unreacted nitrogen and hydrogen for recycling. |
| Recycle unreacted gases | Gases not converted on one pass return to the reactor. | Raises the overall conversion of fresh feed, although conversion in one pass remains limited by the operating equilibrium. |
- Feed and compress
Supply purified nitrogen and hydrogen in an approximately 1:3 mole ratio.
- React over iron
Use high pressure and a moderately high temperature; the outlet still contains reactants as well as ammonia.
- Cool and separate
Condense ammonia for collection.
- Recycle
Return unreacted nitrogen and hydrogen to the reaction loop.
Worked example
Read an industrial data comparison
Two runs use the same catalyst and pressure. At 400 °C the equilibrium NH3 proportion is higher than at 500 °C, but the initial production rate is lower. Explain both observations.
- Ammonia formation is exothermic. The lower temperature favours the forward equilibrium and gives a larger Kc.
- At the lower temperature, a smaller fraction of collisions has enough energy to react, so the rate constant is smaller.
- The manufacturer must compare the amount made per unit time, equipment and energy costs, not maximise the equilibrium percentage in isolation.
Higher equilibrium yield and faster reaction are separate advantages that can favour different temperatures.