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 what is termed a collision.
  • A number of factors come into play when analysing collisions such as energy, orientation, number of collisions, etc.
  • 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.

Identifying Reactions by Profile

  • 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.

Reaction Profile Diagrams

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.

AQA GCSE Chemistry Notes

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Morgan Curtin Chemistry

Author: Morgan

Morgan’s passion for the Periodic Table begun on his 10th birthday when he received his first Chemistry set. After studying the subject at university he went on to become a fully fledged Chemistry teacher, and now works in an international school in Madrid! In his spare time he helps create our fantastic resources to help you ace your exams.