CIE A Level Chemistry复习笔记1.5.2 Energy Level Diagrams

Energy Level Diagrams

  • An energy level diagram is a diagram that shows the energies of the reactants, the transition state(s) and the products of the reaction with time
  • The transition state is a stage during the reaction at which chemical bonds are partially broken and formed
  • The transition state is very unstable – it cannot be isolated and is higher in energy than the reactants and products
  • The activation energy (Ea) is the energy needed to reach the transition state
  • We can define the activation energy as ‘the minimum amount of energy needed for reactant molecules to have a successful collision and start the reaction’

Chemical Energetics Energy Level Diagram, downloadable AS & A Level Chemistry revision notes

The energy level diagram for the reaction of hydrogen with chlorine to form hydrogen chloride gas

Exothermic reaction

  • In an exothermic reaction, the reactants are higher in energy than the products
  • The reactants are therefore closer in energy to the transition state
  • This means that exothermic reactions have a lower activation energy compared to endothermic reactions

1.5-Chemical-Energetics-Energy-Level-Diagram-of-Exothermic-ReactionsThe energy level diagram for exothermic reactions

Endothermic reaction

  • In an endothermic reaction, the reactants are lower in energy than the products
  • The reactants are therefore further away in energy to the transition state
  • This means that endothermic reactions have a higher activation energy compared to exothermic reactions

1.5-Chemical-Energetics-Energy-Level-Diagram-of-Endothermic-ReactionsThe energy level diagram for endothermic reactions

Worked example: Drawing energy level diagrams of the combustion of methane



  • Step 1: The chemical equation for the complete combustion of methane is:

CH(g) + 2O2 (g) → CO2 (g) + 2H2O (l)

  • Step 2: Combustion reactions are always exothermic (ΔH is negative) so the reactants should be drawn higher in energy than the products


  • Step 3: Draw the curve in the energy level diagram clearly showing the transition state


  • Step 4: Draw arrows to show the Ea and ΔH  including their values


Worked example: Determining the activation energy



The reaction pathway diagram for a reversible reaction


The Ea  is the energy difference from the energy level of the reactants to the top of the ‘hump’

Ea (forward reaction) = (+70 kJ mol-1) + (+ 20 kJ mol-1 ) = +90 kJ mol-1

As the question is asking for the reverse reaction the Ea  is the energy difference from the energy level of the products to the ‘hump’

Ea (reverse reaction) = +20 kJ mol-1

Exam Tip

The activation energy is the energy difference from reactants to transition state.The enthalpy change of the reaction is the energy difference from reactants to products.Remember to label the axis of the energy level diagrams!