Reaction profiles

Breaking and making bonds

Energy is transferred when bonds are broken or are formed. During a chemical reaction:

  • energy is taken in to break bonds in the reactants
  • energy is released when new bonds are made in the products

The difference in the amounts of energy involved determines the type of reaction. A reaction is:

  • exothermic - if more energy is released in forming bonds in the products than is taken in when breaking bonds in the reactants
  • endothermic - if less energy is released in forming bonds in the products than is taken in when breaking bonds in the reactant

Reaction profile diagrams

A reaction profile shows how the energy in 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 tell from a reaction profile that a reaction is exothermic.

In the reaction for an exothermic reaction, the overall energy 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 reaction for an endothermic reaction, the overall energy 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.