Reaction profiles

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

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.

The effect of catalysts

A catalyst is a substance that:

  • speeds up the rate of a reaction
  • does not alter the products of the reaction
  • is unchanged chemically and in mass at the end of the reaction

A catalyst allows an alternative reaction pathway that has a lower activation energy than the uncatalysed reaction. This is seen in the reaction profile as a lower 'hump' representing the activation energy.

A reaction profile for a reaction with and without a catalystA reaction profile for a reaction with and without a catalyst