Dynamic equilibrium and Le Chatelier's principleEdexcel A-Level Chemistry: Revision notes
Section 1
Reversible reactions and dynamic equilibrium
Many reactions are reversible: the products can react to re-form the reactants, shown by the sign ⇌. In a closed system (nothing can enter or leave) a reversible reaction reaches dynamic equilibrium.
At dynamic equilibrium:
- the forward and reverse reactions are both still taking place
- they take place at equal rates
- so the concentrations of reactants and products stay constant, and macroscopic properties such as colour and pressure do not change
The same equilibrium mixture is reached whether you start from the reactants alone, the products alone, or a mixture of both.
Do not say the reactions have stopped, or that the concentrations are equal. At equilibrium the concentrations are constant, not necessarily equal.
Section 2
Le Chatelier's principle
Le Chatelier's principle predicts what happens when a system at equilibrium is disturbed: if a change is made to the conditions, the position of equilibrium shifts to oppose the change.
The position of equilibrium describes how far the reaction has gone: a shift to the right gives more products, a shift to the left gives more reactants. The principle tells you the direction of the shift, not its size, and the opposition is only partial.
Section 3
Effect of concentration, pressure and temperature
For a homogeneous system (everything in one phase):
- Concentration: increasing the concentration of a species shifts the equilibrium in the direction that uses it up; decreasing it shifts the equilibrium to replace it. Removing a product as it forms pulls the equilibrium to the right.
- Pressure (gases only): increasing the pressure shifts the equilibrium towards the side with fewer moles of gas; decreasing it favours the side with more. If both sides have the same number of moles of gas, pressure has no effect on the position.
- Temperature: increasing the temperature shifts the equilibrium in the endothermic direction, because that absorbs heat and opposes the rise; decreasing it favours the exothermic direction.
Example: N₂(g) + 3H₂(g) ⇌ 2NH₃(g), ΔH = −92 kJ mol⁻¹. High pressure favours NH₃ (4 mol of gas to 2 mol); low temperature favours NH₃ (forward is exothermic).
For a justified prediction give three steps: state the change, state which direction opposes it, state what happens to the amount of product.
Count moles of gas only. Solids and liquids are ignored when predicting the effect of pressure.
Section 4
Catalysts and rate
A catalyst provides an alternative reaction route with a lower activation energy. It speeds up the forward and reverse reactions equally, so:
- equilibrium is reached faster
- the position of equilibrium and the yield do not change
A temperature change is different: it changes the position of equilibrium and also changes the rate of both reactions, so the two effects can pull in opposite directions.
Section 5
Industrial compromises
Industry wants a high yield at a high rate at low cost. These aims often conflict, so conditions are a compromise.
Haber process (N₂ + 3H₂ ⇌ 2NH₃, exothermic): about 450 °C, 200 atm, iron catalyst. A lower temperature would give a higher yield but too slow a rate. A higher pressure would raise the yield but cost more in strong equipment and compression energy. Unreacted gases are recycled because the conversion per pass is low.
Contact process (2SO₂ + O₂ ⇌ 2SO₃, exothermic): about 450 °C, 1–2 atm, vanadium(V) oxide catalyst. The conversion is already high at low pressure, so extra pressure is not worth the cost.
When evaluating data, compare the equilibrium yield with the rate at each temperature or pressure, then justify the chosen conditions by weighing yield, rate, energy and equipment costs.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Dynamic equilibrium and Le Chatelier's principle
- A sealed glass tube contains a mixture of colourless dinitrogen tetroxide and brown nitrogen dioxide at a constant temperature of 298 K: N₂O₄(g) ⇌ 2NO₂(g), ΔH = +57 kJ mol⁻¹. At the start the tube contained only N₂O₄. After some minutes the brown colour stops getting darker and then stays the same.Explain what is meant by dynamic equilibrium in this tube.2 marks
- Ethanoic acid reacts with ethanol in a sealed flask at 60 °C in the presence of a few drops of concentrated sulfuric acid: CH₃COOH(l) + C₂H₅OH(l) ⇌ CH₃COOC₂H₅(l) + H₂O(l). The enthalpy change for the reaction is close to zero. Equal amounts of the two reactants are mixed and the mixture is left until its composition stops changing.Water is removed from the mixture as it forms, using a dehydrating agent. Use Le Chatelier's principle to explain the effect on the yield of ethyl ethanoate.2 marks
- In the Haber process nitrogen and hydrogen react in a 1 : 3 mole ratio over an iron catalyst: N₂(g) + 3H₂(g) ⇌ 2NH₃(g), ΔH = −92 kJ mol⁻¹. In a pilot study at 200 atm the equilibrium mixture contained about 62% ammonia at 300 °C and about 18% ammonia at 500 °C. The iron catalyst works too slowly below about 400 °C. The industrial plant operates at about 450 °C and 200 atm, and unreacted gases are recycled.Use the data to explain why the plant operates at about 450 °C rather than 300 °C.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).