Equilibrium — AQA GCSE Chemistry
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Equilibrium explained
In a closed system, no reactants or products can escape.
Read the full explanation
A reversible reaction therefore continues in both directions, and eventually the rates of the forward and reverse reactions become equal. At this point the reaction is at equilibrium. Equilibrium does not mean the reaction has stopped: both reactions continue, but at exactly the same rate, so the amounts of reactants and products remain constant. The concentrations of reactants and products become steady, although they are not necessarily equal to each other. For example, in a sealed container, hydrogen and iodine can react to form hydrogen iodide while hydrogen iodide decomposes back to hydrogen and iodine; once the two rates match, the mixture is at equilibrium.
Your focus
- Define equilibrium in terms of equal forward and reverse reaction rates in a closed system.
- Explain why equilibrium is described as dynamic rather than static.
- Describe the observable constancy of reactant and product amounts at equilibrium.
Equilibrium exam tips
Marking Points
- States that equilibrium requires a closed system in which reactants and products cannot escape.
- Explains that at equilibrium the forward and reverse reactions continue to occur.
- Identifies that the rates of the forward and reverse reactions are exactly equal at equilibrium.
- Recognises that the amounts or concentrations of reactants and products remain constant at equilibrium.
- Distinguishes dynamic equilibrium from a stopped reaction.
- Applies the idea to a named reversible reaction in a sealed container.
Examiner Tips
- 💡Always mention that the system is closed when explaining how equilibrium is reached.
- 💡Use the phrase exactly the same rate when describing the forward and reverse reactions at equilibrium.
- 💡Explain that constant concentrations, not equal concentrations, are observed at equilibrium.
Common Mistakes
- Thinking that the reaction stops at equilibrium; the correction is that both forward and reverse reactions continue at equal rates.
- Assuming the amounts of reactants and products must be equal at equilibrium; the correction is that they remain constant but are not necessarily equal.
- Believing equilibrium can be reached in an open system where products escape; the correction is that a closed system is needed to prevent escape of reactants and products.