Reaction profiles

There are two simple ways of showing the energy changes that take place in a chemical reaction:

Energy level diagrams

An energy level diagram shows the energy of the reactants and products, and the difference in energy between them.

Exothermic reaction

The energy level of the substances decreases in an exothermic reaction. This is because 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.A downwards arrow shows that energy is given out

Endothermic reaction

The energy level of the substances increases in an endothermic reaction. This is because energy is taken in 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.An upwards arrow shows that energy is taken in

Reaction profiles

A reaction profile shows how the energy of the reactants and products changes during a reaction. It includes the activation energy, the minimum energy needed for a reaction to start. The activation energy is shown as a 'hump' in the line, which:

  • starts at the energy of the reactants
  • is equal to the difference in energy between the top of the 'hump' and the reactants

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

Exothermic reactions

The diagram shows a reaction profile for an exothermic reaction.

A reaction profile for an exothermic reaction showing the overall change in energy.A reaction profile for an exothermic reaction
Question

Describe how you can see from a reaction profile that a reaction is exothermic.

In an exothermic reaction, the energy change is negative. This is because the products have less energy than the reactants, and the arrow showing the overall change in energy points downwards.

Endothermic reactions

The diagram shows a reaction profile for an endothermic reaction.

A reaction profile for an endothermic reaction.A reaction profile for an endothermic reaction
Question

Describe how you can see from a reaction profile that a reaction is endothermic.

In an endothermic reaction, the energy change is positive. This is because the products have more energy than the reactants, and the arrow showing the overall change in energy points upwards.