AQA A Level chemistry - AS Unit 2: Section 3.2.3 Equilibria

3.2.3 Equilibria - The dynamic nature of equilibria

Students should:

know that many chemical reactions are reversible
understand that for a reaction in equilibrium, although the concentrations of reactants and products remain constant, both forward and reverse reactions are still proceeding at equal rates

Dynamic equilibrium

If our body of liquid is placed in a closed container the higher energy particles will continue to leave but now they will be held in the same place (the air space above the liquid surface). Occasionally, due to collisions, some of these vapour particles will lose energy and rejoin the body of liquid. As more particles gather in the air space there will be more possibility of particles losing energy and returning to the liquid.

We now have two processs happening. Liquid particles turning to vapour and vapour particles turning to liquid. Eventually the rate at which the particles move from the liquid to the vapour phase will equal the rate at which the particles move from the vapour phase to the liquid. When this situation is arrived at the concentration of vapour particles in the air space will be constant even though there is movement in both directions. We call this Dynamic Equilibrium.

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Chemical reactions

Many chemical reactions are reversible. If the energy required for the reverse reaction is not too different from that required for the forward reaction a dynamic equilibrium can be established in which both reactions are proceeding at the same rate.

In a chemical equilibrium the forward reaction is happening at the same rate as the reverse reaction and the concentration of the reactants and products does not change (note: the concentrations are NOT equal)

reactants products

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