Reaction Profiles (AQA GCSE Chemistry: Combined Science)

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

Exothermic reaction profile & endothermic reaction profile

Reaction profile of an exothermic reaction and an endothermic reaction