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ChemistryGCSE Higher & A Level

What is dynamic equilibrium?

A dynamic equilibrium is reached in a closed system when the forward and reverse reactions occur at the same rate, so concentrations stay constant.

Dynamic equilibrium explained

Le Chatelier's principle predicts the response to change: the system shifts to oppose an increase in concentration, pressure or temperature.
Only temperature changes the value of the equilibrium constant. Pressure and concentration shift the position, and a catalyst changes neither.

Key formula

Kc=[products][reactants]K_{c} = \dfrac{[\text{products}]}{[\text{reactants}]} at equilibrium

Worked example

In the Haber process, higher pressure shifts N2+3H2⇌2NH3\mathrm{N_2} + 3\mathrm{H_2} \rightleftharpoons 2\mathrm{NH_3} towards ammonia because there are fewer gas moles on the right.

Examiner tip

Always name the direction and the reason: 'shifts right because the forward reaction is exothermic and the system opposes the temperature decrease'.

Dynamic equilibrium: common questions

What does dynamic mean in dynamic equilibrium?
Both reactions continue at the molecular level; only the measurable concentrations stay constant.
Does a catalyst change the yield at equilibrium?
No. It speeds both directions equally, so equilibrium is reached sooner but at the same position.

Related terms

Catalyst

A catalyst speeds up a reaction by providing an alternative pathway with a lower activation energy, without being used up itself.

Rate of reaction

The rate of reaction is how quickly reactants are used up or products are formed, usually measured as a change in amount per unit time.

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