Activation energy

When a fuel burns, a spark is needed to start the reaction. A minimum amount of energy is needed in order for a fuel to burn.

The minimum amount of energy needed for a reaction to take place is called the activation energy.

High and low activation energy

In order for two substances to react, their particles must collide with enough energy. The particles in a substance have a range of different energies.

  • A low activation energy means that a lot of the particles will collide with enough energy to react. The reaction will be fast.
  • A high activation energy means that far fewer particles will collide with enough energy. The reaction will be slow.

Reaction profiles

A reaction profile shows how the energy in the reactants and products changes during a reaction. It includes the activation energy.

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 a reaction profile shows that a reaction is exothermic.

In the reaction 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 a reaction profile shows that a reaction is endothermic.

In the reaction 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.

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