Reaction profiles

For a chemical reaction to occur between two particles, they must collide with enough energy. The minimum amount of energy needed by particles when they collide in order to cause a reaction is called the activation energy.

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. 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 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

It is usually more helpful to describe how the energy of the chemicals changes during the reaction, so a reaction profile is more useful than an energy level diagram.

A reaction profile includes the activation energy, which is the minimum energy needed by particles when they collide for a reaction to occur. 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 reactant

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 tell from a reaction profile that a reaction is exothermic.

In the profile for an exothermic reaction, the overall change is negative. You can tell this 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 tell from a reaction profile that a reaction is endothermic.

In the profile for an endothermic reaction, the overall change is positive. You can tell this because the products have more energy than the reactants, and the arrow showing the overall change in energy points upwards.

A study of the energy changes involved in chemical reactions, whether exothermic or endothermic