Energy Level Diagrams (Edexcel IGCSE Chemistry)

Spec point — 3.5C

Energy level diagrams

• 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 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 enthalpy change of a reaction

Energy level diagrams

Energy level diagram 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 level diagram 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 level diagram with an upwards arrow as the energy of the products is higher than the reactants

Exam Hint You should be able to draw clear and fully labelled energy level diagrams for both types of reactions and you should be able to identify a reaction from its energy level diagram.

Reaction profiles, covered in a later topic, are similar to energy level diagrams but also show how the energy changes during the course of a reaction. These can be used to indicate the activation energy and enthalpy change of the reaction.