Reaction profiles - Higher tier

Energy level diagrams

An energy level diagram shows whether a reaction is exothermic or endothermic. It shows the energy in the reactants and products, and the difference in energy between them.

Exothermic reaction

The energy level decreases in an exothermic reaction, as energy is given out to the surroundings.

Graph of energy against progress of reaction. Horizontal line represents reactants energy. Drops vertically as energy released. Horizontal line at a lower level represents products' energy.

Endothermic reaction

The energy level increases in an endothermic reaction, as energy is taken from the surroundings.

Graph of energy against progress of reaction. A horizontal line represents the reactants' energy. It rises vertically as energy is absorbed. A horizontal line at a higher level then represents the products' energy.

Reaction profile

A reaction profile is similar to an energy level diagram, but it includes the activation energy – the minimum energy that particles need when they collide for a reaction to occur. The activation energy is a hump in the line, which:

  • starts to rise from the same energy as the reactants
  • is equal to the difference in energy between the top of the hump and the reactants.

The overall change in energy is the difference between the energy of the reactants and products.

Exothermic reactions

Exothermic reaction graph

Endothermic reactions

Endothermic reaction graph