Energy Level Diagrams (CIE AS Chemistry)

• 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 (E_a) 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’

  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

The 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

The energy level diagram for endothermic reactions

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

Answer

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

CH₄ (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 E_a and ΔH  including their values

Worked example: Determining the activation energy

The reaction pathway diagram for a reversible reaction

Answer

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

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

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

E_a (reverse reaction) = +20 kJ mol^-1