Reversible reactions: The direction of reversible reactions can be changed by changing the conditions.

The idea

A reversible reaction can proceed in both the forward and reverse directions, shown by the ⇌ symbol. The direction in which the reaction appears to go is not fixed: it depends on the conditions. If you change a condition such as temperature, pressure or concentration, you change the balance between the two directions, so the reaction may shift towards making more products or more reactants. For example, heating hydrated copper(II) sulfate drives off water to form white anhydrous copper(II) sulfate and water vapour; adding water to the white solid reverses this, regenerating the blue hydrated salt and releasing heat. In a closed system the reaction eventually reaches equilibrium, where forward and reverse rates are equal, but the position of that equilibrium still moves when conditions change.

What earns the marks

• A reversible reaction is represented by the ⇌ symbol, meaning the forward and reverse reactions occur simultaneously rather than the reaction simply stopping.
• Changing a condition such as temperature, pressure or concentration alters the relative rates of the forward and reverse reactions, so the direction of the overall change can be reversed.
• In a closed system, reversible reactions can reach dynamic equilibrium, where the rates of the forward and reverse reactions are equal and the concentrations of reactants and products remain constant.
• Changing conditions shifts the position of equilibrium, so the yield of product or reactant changes even though the reaction remains reversible.
• A concrete example is the hydration and dehydration of copper(II) sulfate: heating blue hydrated copper(II) sulfate produces white anhydrous copper(II) sulfate plus water vapour, and adding water reverses the change.
• The direction observed depends on which set of conditions is applied, so a statement about direction must be linked to the specific conditions used.

Common mistakes and how to put them right

• Thinking a reversible reaction stops when it reaches equilibrium.

  Put it right: Both forward and reverse reactions continue at equal rates, so it is dynamic.

• Assuming the ⇌ symbol means the reaction can only go one way at a time.

  Put it right: Both directions occur together, and conditions decide which direction dominates.

• Writing that heating always pushes a reversible reaction forwards.

  Put it right: Heating favours the endothermic direction, which may be the reverse reaction depending on the reaction.

• Treating an open system as if it reaches the same equilibrium as a closed system.

  Put it right: Escaping gases can prevent true equilibrium being reached.

Exam tips

• Always name the specific condition you are changing, such as temperature or pressure, and state how the direction of the reaction changes as a result.
• Use the ⇌ symbol correctly in equations and explain that it shows reversibility, not that the reaction has stopped.
• When describing equilibrium, include the idea that forward and reverse rates are equal and that concentrations stay constant in a closed system.
• Link any example to observable evidence, such as a colour change, so your explanation shows how the direction change can be detected.