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Heat energy changes in chemical reactions - EdexcelReaction profile diagrams

Exothermic reactions in solution give out energy and the temperature increases, while endothermic reactions take in energy and the temperature decreases. Bonds are broken and made in reactions.

Part of Chemistry (Single Science)Rates of reaction and energy changes

Reaction profile diagrams

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

  • an
  • a

Reaction profiles

A reaction profile shows how the of the and changes during a reaction. It includes the - 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 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 you can see from a reaction profile 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 you can see from a reaction profile 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