3.3Physical Chemistry
Reversible Reactions and Equilibria — Edexcel Linear - OL (Chemistry) Revision Notes & Topical Questions
5 min read 8 parts Lesson 6 of 9 Unit quiz
Reversible Reactions and Equilibria: Industrial Conditions — EDEXCEL LINEAR – OL (CHEMISTRY) notes: Apply the idea of dynamic equilibrium and Le Chatelier’s principle to choose conditions for the Haber and Contact processes.A reversible reaction can proceed in both directions.
- Worked example 1
- Worked example 2
- Worked example 3
- Worked example 4
- Haber process
- Sulfur is burned in air to make sulfur dioxide (S + O2 → SO2)
Section 1 of 8Reversible reactions and equilibrium
A reversible reaction can proceed in both directions.
- and the amounts of reactants and products stay constant.
Section 2 of 8Factors affecting the position
| Condition changed | Equilibrium moves towards | Rate |
|---|---|---|
| Higher temperature | Endothermic direction | Faster |
| Lower temperature | Exothermic direction | Slower |
| Higher pressure | Fewer gas molecules | Faster |
| Catalyst | No change | Faster |
| Removing a product | Products (more formed) | — |
The principle (Le Chatelier)the system acts to oppose any change imposed on it.
Section 3 of 8The Haber process
N2(g) + 3H2(g) ⇌ 2NH3(g), ΔH = −92 kJ/mol.
- so high pressure favours ammonia; very high pressure is expensive and hazardous,
- so 200 atm is used.
Section 4 of 8The Contact process
The forward reaction is exothermic and has fewer gas molecules, so the yield is high even at low pressure (about 96%); extra pressure is not economic. Sulfuric acid is used in fertilisers, detergents, paints and batteries.
Section 5 of 8Worked reasoning: choosing industrial conditions
Use a four-step structure. Step 1: write the equation with state symbols and mark the number of gas moles on each side. Step 2: state the sign of ΔH. Step 3: say which temperature and pressure favour the yield. Step 4: compare with the rate and cost, and justify the compromise.
| Question | Haber process | Contact process |
|---|---|---|
| Gas moles (left → right) | 4 → 2 | 3 → 2 |
| ΔH | Exothermic | Exothermic |
| Ideal for yield | Low T, high P | Low T, high P |
| Actual conditions | 450 °C, 200 atm, Fe | 450 °C, 2 atm, V2O5 |
| Reason for not going further | Low T too slow; high P costly and unsafe | Yield already about 96%; extra pressure not worth the cost |
Safety and the environment also matter
- ammonia fertilisers can leach into rivers causing eutrophication, and sulfur dioxide leaks cause acid rain, so plants recycle gases and scrub emissions.
- Equilibrium can also be shown with colour changes: bromine water in a closed vessel, or cobalt chloride in water and ethanol, which shifts between blue and pink on heating and cooling, giving a visual demonstration of the effect of temperature.
Section 6 of 8Worked examples
State two features of a system at dynamic equilibrium. [2]2 marks
Forward and backward reaction rates are equal, and concentrations are constant (in a closed system).
Explain why 450 °C is chosen for the Haber process. [3]3 marks
The reaction is exothermic, so low temperature gives a higher yield but is too slow; 450 °C gives an acceptable rate and yield, with an iron catalyst.
Why is the pressure in the Contact process only 2 atm? [2]2 marks
The yield is already high at low pressure, so higher pressure costs more with little benefit.
Predict the effect of removing ammonia as it forms. [2]2 marks
The equilibrium shifts right to replace it, increasing yield.
Section 7 of 8Key terms
| Term | Meaning |
|---|---|
| Reversible reaction | Reaction that can go either way |
| Dynamic equilibrium | Equal forward and backward rates |
| Le Chatelier’s principle | System opposes changes imposed on it |
| Haber process | Industrial synthesis of ammonia |
| Contact process | Industrial manufacture of sulfuric acid |
| Oleum | Fuming sulfuric acid H2S2O7 |
Section 8 of 8Exam technique
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