Equilibrium: When a reversible reaction occurs in apparatus which prevents the escape of reactants and products, equilibrium is reached when the forward and reverse reactions occur at exactly the same rate.

The idea

In a closed system, where no reactants or products can escape, a reversible reaction can reach dynamic equilibrium. At equilibrium, the forward and reverse reactions continue to occur, but they do so at exactly the same rate. Because the rates are equal, the concentrations of reactants and products remain constant over time, even though both reactions are still happening. This is not a static state: individual molecules or ions are constantly reacting in both directions. For example, in the Haber process for ammonia, nitrogen and hydrogen react to form ammonia while ammonia decomposes back to nitrogen and hydrogen. In a sealed container, equilibrium is reached when the rate of ammonia formation equals the rate of ammonia decomposition, so the amounts of all three gases stay constant. Equilibrium can only be reached if the apparatus prevents the escape of reactants and products.

What earns the marks

• Defines equilibrium as the state reached in a closed system when the forward and reverse reactions occur at exactly the same rate.
• Explains that equilibrium requires a closed system so that reactants and products cannot escape.
• States that at equilibrium the concentrations of reactants and products remain constant, but both reactions continue to occur.
• Distinguishes dynamic equilibrium from a reaction that has stopped, noting that forward and reverse processes are still taking place.
• Applies the concept to a named reversible reaction, such as the Haber process, and explains how equal rates lead to constant amounts.
• Interprets rate–time or concentration–time graphs to identify when equilibrium is established.

Common mistakes and how to put them right

• Saying that at equilibrium the reaction has stopped.

  Put it right: Equilibrium is dynamic; both forward and reverse reactions continue at equal rates.

• Thinking equilibrium can be reached in an open system where products escape.

  Put it right: Equilibrium requires a closed system that prevents escape of reactants and products.

• Assuming the amounts of reactants and products must be equal at equilibrium.

  Put it right: The rates are equal, but the concentrations or amounts do not have to be equal.

• Confusing equilibrium with completion of the reaction.

  Put it right: At equilibrium, both reactants and products are present and interconverting.

Exam tips

• Always state that equilibrium is dynamic and that the forward and reverse rates are equal, not just that the amounts are constant.
• Mention the need for a closed system whenever you explain how equilibrium is reached.
• When using a graph, identify the point where the curve becomes horizontal as the time equilibrium is reached, and explain that this shows constant concentrations.
• Avoid saying 'the reaction stops' at equilibrium; instead say 'the reaction continues in both directions at the same rate'.