Reversible reactions and equilibrium — AQA GCSE Chemistry revision

Reversible reactions

• A reversible reaction can proceed in both the forward and backward directions, shown with a double-headed arrow (⇌).
• Energy changes in reversible reactions are opposite in each direction — if the forward reaction is exothermic, the reverse is endothermic by the same amount.
• Heating hydrated copper sulfate to remove water and then adding water back to reform it is a classic example of a reversible reaction.
• In a closed system, no matter can enter or leave, which is required to reach true equilibrium.

Dynamic equilibrium

• At equilibrium, the forward and backward reactions occur at exactly the same rate.
• The concentrations of reactants and products remain constant at equilibrium, but they are not necessarily equal.
• Equilibrium is described as dynamic because both reactions are still happening, just at matching rates.
• Equilibrium can only be reached in a closed system where nothing escapes or is added.

Key terms

Reversible reaction

A reaction that can proceed in both the forward and reverse directions.

Dynamic equilibrium

A state where forward and reverse reaction rates are equal, so concentrations stay constant.

Closed system

A system where no reactants or products can escape or enter.

Le Chatelier's principle

The idea that a system at equilibrium shifts to oppose any imposed change.

Shift in equilibrium

A change in the relative amounts of reactants and products in response to a disturbance.

Forward reaction

The reaction that proceeds from reactants to products.

Common exam traps

• Equilibrium does not mean equal concentrations of reactants and products — it means constant, unequal amounts.
• Equilibrium can only be reached in a closed system, not an open one where substances escape.
• Increasing temperature always favours the endothermic direction, regardless of whether that's forward or backward.

Practice questions with answers

1. 1. What symbol shows a reaction is reversible?

   • • →
   • • ⇌
   • • =
   • • ≠

   Answer: ⇌

   A double-headed arrow shows the reaction can go both forward and backward.

2. 2. What is true about the rates of forward and backward reactions at equilibrium?

   Answer: They are equal

   At dynamic equilibrium, both reactions occur at the same rate.

3. 3. In what type of system can equilibrium be established?

   • • Open system
   • • Closed system
   • • Any system
   • • Vacuum only

   Answer: Closed system

   A closed system prevents substances from escaping, allowing equilibrium to form.

4. 4. If the forward reaction in a reversible process is exothermic, what is the backward reaction?

   Answer: Endothermic

   The reverse of an exothermic reaction is always endothermic, releasing/absorbing the same amount of energy.

5. 5. According to Le Chatelier's principle, what happens if you increase the concentration of a reactant?

   • • Equilibrium shifts towards reactants
   • • Equilibrium shifts towards products
   • • No change occurs
   • • The reaction stops

   Answer: Equilibrium shifts towards products

   The system counteracts the increase by using up the extra reactant to form more product.

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