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Reaction Pathway Diagrams (CIE AS Chemistry)

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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

1-6-23-2-1-energy-level-diagram-for-hcl

  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 

5-1-2-exo-reaction-profile

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 

5-1-2-endo-reaction-profile

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

Chemical Energetics Step 2 - Drawing energy level diagrams of the combustion of methane, downloadable AS & A Level Chemistry revision notes

  • Step 3: Draw the curve in the energy level diagram clearly showing the transition state

Chemical Energetics Step 3 - Drawing energy level diagrams of the combustion of methane, downloadable AS & A Level Chemistry revision notes

  • Step 4: Draw arrows to show the Ea and ΔH  including their values

1-6-2-we-1

Worked example

Determining the activation energy

Determine, using the reaction pathway diagram, the Ea for the forward and backward reaction.

1-6-2-we-2

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

Examiner 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!

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Caroline

Author: Caroline

Expertise: Physics Lead

Caroline graduated from the University of Nottingham with a degree in Chemistry and Molecular Physics. She spent several years working as an Industrial Chemist in the automotive industry before retraining to teach. Caroline has over 12 years of experience teaching GCSE and A-level chemistry and physics. She is passionate about creating high-quality resources to help students achieve their full potential.