Energy Profiles
- An energy profile 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’
Energy profile diagram
The energy profile 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
Diagram to show an energy profile for an exothermic reaction
The potential energy of the products is lower than the reactants
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
Diagram to show an energy profile for an endothermic reaction
The potential energy of the products is lower than the reactants
Worked example
The enthalpy of combustion for methane is -890 kJ mol-1 and the activation energy is +2653 kJ mol-1.
Draw a labelled energy level diagram for this reaction.
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 Ea and ΔH including their values
Worked example
Use the energy level diagram below to identify the activation energy, Ea , for the reaction.
The reaction pathway diagram for a reversible reaction
Answer:
- 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
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!
