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 as the reaction proceeds ( called 'extent of the reaction' below)
- The transition state is a stage during the reaction at which chemical bonds are partially broken and formed
- The transition state is very unstable – a molecule in the transition state 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’
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. ∆H is negative because the system (reactants) loses energy to the surroundings.
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. The system (reactant substances) have gained energy so ∆H is positive.
Worked example
Drawing energy level diagrams of the combustion of methane
The Ea and ΔH for the complete combustion of methane are +2653 kJ mol-1 and -890 kJ mol-1 respectively.
Draw the 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
Determining the activation energy
ΔH for a reaction is +70 kJ mol-1 and Ea for the reverse reaction is +20 kJ mol-1.
Use the reaction pathway diagram below to determine the Ea for the forward 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 the transition state.
The enthalpy change of the reaction is the energy difference from reactants to products.
Remember: Label the axis of the energy level diagrams!
