Reaction Pathway – IGCSE Chemistry Definition

A reaction pathway describes the route that reactants follow as they are transformed into products during a chemical reaction. On an energy level diagram, the reaction pathway is represented by the curve that connects the energy level of the reactants to the energy level of the products, passing over an energy barrier.

On an energy diagram

The reaction pathway starts at the energy level of the reactants on the left. The curve rises to a peak, representing the activation energy barrier that must be overcome for the reaction to proceed. It then falls to the energy level of the products on the right. For exothermic reactions, the products are lower in energy than the reactants. For endothermic reactions, the products are higher.

The height of the peak above the reactant energy level represents the activation energy. This is the minimum energy that colliding particles must have for a successful collision.

Effect of a catalyst

A catalyst provides an alternative reaction pathway with a lower activation energy. On the energy diagram, the catalysed pathway has a lower peak than the uncatalysed pathway. The starting and ending energy levels remain the same, so the overall energy change of the reaction is unchanged. The catalyst simply makes it easier for particles to react by reducing the energy barrier.

Exam context

Paper 4 questions frequently ask candidates to draw energy level diagrams showing the reaction pathway for both catalysed and uncatalysed reactions. Marks are awarded for correctly showing the reactant and product energy levels, the activation energy, and the lower peak for the catalysed pathway.

Worked exam question

On the energy level diagram for an exothermic reaction, sketch a second curve to show the reaction pathway when a catalyst is used. (2 marks)

The second curve should have the same start point (reactant energy level) and the same end point (product energy level) [1]. The peak of the catalysed curve should be lower than the peak of the uncatalysed curve, showing a lower activation energy [1].

Common mistakes

Candidates sometimes draw the catalysed pathway with different starting or ending energy levels. The catalyst does not change the energy of the reactants or products --- it only changes the route between them. Another error is forgetting to label the two pathways, making it unclear which is catalysed and which is not.