Dynamic equilibrium and Le Chatelier's principleEdexcel International A Level Chemistry: Revision notes

Section 1

Reversible reactions and dynamic equilibrium

In a reversible reaction the products can react to re-form the reactants, shown by ⇌. In a closed system the reaction reaches dynamic equilibrium, when:

the rate of the forward reaction equals the rate of the backward reaction
the concentrations of reactants and products remain constant

The reactions have not stopped. Both continue at the same rate, which is why it is called dynamic.

Key termsreversible reactiondynamic equilibriumclosed system

Common mistake

Do not say the concentrations are equal at equilibrium. They are constant, but usually unequal.

Section 2

Le Chatelier's principle

Le Chatelier's principle: when a change is made to a system at equilibrium, the position of equilibrium shifts to oppose that change. A shift to the right means more products, so a higher yield. A shift to the left means more reactants.

Key termsLe Chatelier's principleposition of equilibrium

Section 3

Temperature, pressure and concentration

Temperature: an increase shifts the position in the endothermic direction (absorbs heat). A decrease shifts it in the exothermic direction.
Pressure (gases): an increase shifts the position to the side with fewer moles of gas. If the moles are equal, there is no effect.
Concentration: increasing a reactant shifts the position to the right. Removing a product shifts it to the right.

Example: $N_2O_4 \rightleftharpoons 2NO_2$ , $\Delta H = +57$ kJ mol⁻¹. Heating shifts the position right (darker brown). Raising the pressure shifts it left (1 mole against 2).

Key termsendothermic directionmoles of gas

Exam tip

For pressure, count only gas molecules in the balanced equation, then say which side has fewer.

Section 4

Catalysts and equilibrium

A catalyst has no effect on the position of equilibrium or the equilibrium yield. It increases the rates of the forward and backward reactions equally, so equilibrium is reached faster.

Key termscatalyst

Section 5

Industrial compromises

Industry needs a good yield and a good rate at an acceptable cost. For an exothermic reaction, low temperature gives a higher yield but a slow rate. A moderate temperature is chosen as a compromise. A high pressure raises yield (if fewer gas moles on the right) and rate, but compressors and strong vessels are expensive and hazardous, so a moderate pressure is used. Recycling unreacted reactants raises the overall conversion.

When evaluating data, compare the gain in yield with the extra cost or reduction in rate, then give a justified conclusion.

Key termscompromiseyield

That's the notes covered.

Carry on to the next subtopic.

Exam questions on Dynamic equilibrium and Le Chatelier's principle

Dinitrogen tetroxide is a colourless gas that is in equilibrium with brown nitrogen dioxide: N₂O₄(g) ⇌ 2NO₂(g), ΔH = +57 kJ mol⁻¹. A sample of the equilibrium mixture is sealed in a glass tube so that the depth of brown colour can be observed.
The tube is compressed at constant temperature so that the pressure increases. Predict the change in the position of equilibrium and justify your answer.2 marks
Ethanol is manufactured by the reversible reaction of ethene with steam: C₂H₄(g) + H₂O(g) ⇌ C₂H₅OH(g), ΔH = −45 kJ mol⁻¹. The reaction is carried out in a closed reactor at about 570 K and 65 atm using a solid acid catalyst.
The reactor uses a solid acid catalyst. State and explain the effect of the catalyst on the position of equilibrium and on the time taken to reach equilibrium.2 marks
Sulfur trioxide is made in the Contact process by the reversible reaction 2SO₂(g) + O₂(g) ⇌ 2SO₃(g), ΔH = −196 kJ mol⁻¹. The plant uses a vanadium(V) oxide catalyst at about 720 K and a pressure of 1 to 2 atm.
Explain why a temperature of about 720 K is a compromise for this process.3 marks

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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).