Everything you need to help your 14–16 learners with this tricky topic

Equilibrium is an important process in industry, eg for making ammonia using the Haber process which is a reversible reaction. Manufacturers want to make as much product as possible, minimising the quantity of starting materials left over. They also want to minimise energy use when conducting reactions at high temperatures and pressures.

To make reversible chemical reactions as efficient and sustainable as possible, manufacturers need to understand equilibrium. Because the equilibrium position – the concentrations of substances present at equilibrium – affects the yield of the product.

Reversible reactions

Most chemical reactions are irreversible, eg combustion, where the reaction only takes place in one direction. 

However, many chemical reactions are reversible: the products can react together to reform the original reactants. A reversible reaction is represented by a double arrow symbol: ⇌.

EiC-Equilibrium-Final-1

Source: © Dan Bright

Many chemical reactions are reversible: the products can react together to reform the original reactants. A reversible reaction is represented by a double arrow symbol: ⇌. The forwards reaction: A + B → C + D, and the reverse reaction: C + D → A + B, are occurring at the same time. If the forwards reaction is exothermic, the reverse reaction will be endothermic and vice versa. This is because the same amount of energy is transferred in both reactions

An example of a reversible reaction. Click through the gallery to see how the reaction reaches equilibrium

Source: © Dan Bright

In dynamic equilibrium, the forwards and reverse reactions occur at the same rate in a closed system. The concentrations of substances at equilibrium are constant ie, they are not changing. This means that a reaction is not immediately at equilibrium

Source: © Dan Bright

At the start of the reaction, the concentrations of the initial reactants, A and B, are at their highest, so the rate of the forwards reaction is highest. When A and B react, their concentrations decrease, decreasing the rate of the forwards reaction. C and D are produced, increasing their concentrations and, therefore, the rate of the reverse reaction

Source: © Dan Bright

Eventually, the rates of the forwards and reverse reactions become equal and equilibrium is reached. Particles are still reacting, but as A and B react to produce C and D, C and D react to produce A and B, maintaining a constant concentration

Source: © Dan Bright

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Infographic poster, fact sheet and student worksheet. Display the poster in your classroom or on a projector. Alternatively, print it and use as a handout.

The accompanying storyboard activity asks learners to describe and sequence a dynamic equilibrium reaction using words and images.

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Two conical flasks containing a blue liquid - one is labelled open with arrows entering and leaving the flask and the other is labelled closed with a rubber bung preventing any particles entering or leaving the system

Source: © Dan Bright

Dynamic equilibrium can only happen in a closed system. This is a system or reaction vessel where nothing can enter or leave

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Did you know …?

In 2020, the Haber process – a reversible reaction – produced over 176 million tonnes of ammonia for fertiliser and other uses. 

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Emma is an assistant subject leader of science at Horizon Community College in Barnsley. She won the RSC Early Career Prize for Excellence in Secondary and Further Education in 2021.View full profile

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