Reaction Profiles (AQA GCSE Chemistry)

Reaction profiles

Activation energy, E_a

• For atoms or particles to react with each other in a chemical system they must first come into contact with each other in a collision

• Several factors come into play when analysing collisions such as:

  • Energy

  • Orientation

  • 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 and given the symbol E_a

• 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

Energy profiles

• Energy profiles 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 progress of the reaction 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

Energy profile diagrams

Energy profiles 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 energy profile with a downwards arrow as the energy of the products is lower than the reactants

• Energy is taken in 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 energy profile with an upwards arrow as the energy of the products is higher than the reactants

Exothermic reaction profile & endothermic reaction profile

Reaction profile of an exothermic reaction and an endothermic reaction