5.3.3 Energy Profiles

• An energy profile or 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

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

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