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Activation energy

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

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

High and low activation energy

In order for two substances to react, their 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 shows how the energy in the and 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 reaction.

A reaction profile for an exothermic reaction showing the overall change in energy.
Figure caption,
A reaction profile for an exothermic reaction

Question

Describe how a reaction profile shows that a reaction is exothermic.

Endothermic reactions

The diagram shows a reaction profile for an reaction.

A reaction profile for an endothermic reaction.
Figure caption,
A reaction profile for an endothermic reaction

Question

Describe how a reaction profile shows that a reaction is endothermic.

The effect of catalysts

A is a substance that:

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

A catalyst allows an alternative 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 catalyst
Figure caption,
A reaction profile for a reaction with and without a catalyst