Reaction Pathway Diagrams
- A reaction pathway diagram shows the energies of the reactants, the transition state(s) and the products of the reaction with time
- They are sometimes called energy profile diagrams
- 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’
Example reaction pathway diagram
The reaction pathway 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 reaction pathway diagram for exothermic reactions
Exothermic reaction pathway diagrams always have the products lower in energy 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
The reaction pathway diagram for endothermic reactions
Endothermic reaction pathway diagrams always have the products higher in energy than the reactants
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 reaction pathway 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
Determine, using the reaction pathway diagram, the Ea for the forward and backward 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
- 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 the transition state.
- The enthalpy change of the reaction is the energy difference from reactants to products.
- Remember to label the axis of the reaction pathway diagrams!
