AQA GCSE Chemistry

Revision Notes

5.1.3 Reaction Profiles

Reaction Profiles

Activation energy

  • For atoms or particles to react with each other in a chemical system they must first of all come into contact with each other in a collision
  • A number of factors come into play when analysing collisions such as energy, orientation, and number of collisions per second
  • In terms of the energy of the collision, there is a minimum amount of energy required for the collision to be successful, that is for the particles to react together
  • This minimum amount of energy is called the activation energy
  • Different reactions have different activation energies, depending on the chemical identities involved
  • Reactions which have higher activation energies require more energy to start than those with lower activation energies

Exam Tip

All reactions have an activation energy as the chemical bonds in the reactant molecules have to be broken first

Reaction profiles

  • Reaction profiles (sometimes called energy level diagrams) are graphical representations of the relative energies of the reactants and products in chemical reactions
  • The energy of the reactants and products are displayed on the y-axis and the reaction pathway is shown on the x-axis
  • Arrows on the diagrams indicate whether the reaction is exothermic (downwards pointing) or endothermic (upwards pointing)
  • The difference in height between the energy of reactants and products represents the overall energy change of a reaction
  • The initial increase in energy represents the activation energy required to start the reaction
  • The greater the initial rise then the more energy that is required to get the reaction going e.g., more heat needed

Exothermic & endothermic reactions, IGCSE & GCSE Chemistry revision notes

Reaction profile of an exothermic reaction and an endothermic reaction

  • Energy is given out in exothermic reactions
  • The energy of the products will be lower than the energy of the reactants, so the change in energy is negative
  • This is represented on the reaction profile with a downwards-arrow as the energy of the products is lower than the reactants
  • Energy is taken in endothermic reactions
  • The energy of the products will be higher than the energy of the reactants, so the change in energy is positive
  • This is represented on the reaction profile with an upwards-arrow as the energy of the products is higher than the reactants

Exam Tip

You should be able to draw clear and fully labelled reaction profile diagrams for both types of reactions and you should be able to identify a reaction from its reaction profile.

Author: Francesca

Fran has taught A level Chemistry in the UK for over 10 years. As head of science, she used her passion for education to drive improvement for staff and students, supporting them to achieve their full potential. Fran has also co-written science textbooks and worked as an examiner for UK exam boards.
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