Reaction profiles: Reaction profiles can be used to show the relative energies of reactants and products, the activation energy and the overall energy change of a reaction.

The idea

A reaction profile is a graph of energy on the vertical axis against progress of reaction on the horizontal axis. The curve starts at the energy level of the reactants and ends at the energy level of the products, so the vertical gap between these two levels gives the overall energy change. The highest point on the curve is the transition state; the vertical distance from the reactants' level up to that peak is the activation energy, the minimum energy colliding particles need to react. In an exothermic reaction the products sit lower than the reactants, so energy is released and the overall energy change is negative. In an endothermic reaction the products sit higher, so energy is taken in and the change is positive.

For example: Burning methane has products below reactants, whereas thermal decomposition of calcium carbonate has products above reactants.

What earns the marks

• A reaction profile plots energy vertically against progress of reaction horizontally, with the curve beginning at the reactant energy level and finishing at the product energy level.
• The overall energy change is the vertical difference between the reactant level and the product level; it is negative for an exothermic reaction and positive for an endothermic reaction.
• The activation energy is the vertical distance from the reactant energy level to the highest point of the curve, representing the minimum energy particles need to react.
• The peak of the curve represents the transition state or activated complex, where bonds in the reactants are being broken and new bonds are forming.
• Adding a catalyst lowers the peak of the curve, reducing the activation energy, while leaving the reactant and product energy levels unchanged.
• On a labelled profile, arrows or brackets should clearly identify which gap represents activation energy and which represents the overall energy change.

Common mistakes and how to put them right

• Measuring activation energy from the products' level to the peak instead of from the reactants' level to the peak.

  Put it right by starting the activation energy measurement at the reactant energy level.

• Confusing the overall energy change with the activation energy.

  Put it right by remembering that overall energy change compares reactants with products, while activation energy compares reactants with the peak.

• Assuming a catalyst changes the overall energy change.

  Put it right by stating that a catalyst lowers only the activation energy and leaves the reactant and product levels, and therefore the overall energy change, unchanged.

Exam tips

• Label both axes and both energy gaps on any profile you draw, using a bracket or double-headed arrow for each gap so the examiner can see exactly what you mean.
• State the sign of the overall energy change explicitly: negative for exothermic and positive for endothermic, linking the sign to whether products are lower or higher than reactants.
• When asked to compare two profiles, quote the direction of the difference, for example that the catalysed curve has a lower peak but the same start and end levels.