Energy Profiles (WJEC GCSE Chemistry: Combined Science)

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

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

Activation energy, E_a, in energy profiles

• Energy profiles typically have a curve connecting the reactants to the products
  • This curve indicates the activation energy or minimum amount of energy required for the reaction to proceed

Energy profiles including activation energy

Energy profile of an exothermic reaction and an endothermic reaction including activation energy

• The initial increase in energy, from the reactants to the peak of the curve, represents the activation energy required to start the reaction
• The greater the initial rise; the more energy is required to get the reaction going e.g., more heat is needed