CIE A Level Chemistry

Revision Notes

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First teaching 2020

Last exams 2024

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1.5.2 Energy Level Diagrams

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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 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-level-diagram

  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

 energy-level-diagram

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

 5-1-2-endo-reaction-profile

The energy level diagram for endothermic reactions

Worked example: Drawing energy level diagrams of the combustion of methane

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

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

energy-level-diagram

 

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

energy-level-diagram

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

energy-level-diagram

Worked example: Determining the activation energy

Chemical Energetics Worked Example - Determining the activation energy, downloadable AS & A Level Chemistry revision notes

1-6-2-we-2

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

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 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!

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Author: Francesca

Fran studied for a BSc in Chemistry with Forensic Science, and since graduating taught A level Chemistry in the UK for over 11 years. She studied for an MBA in Senior Leadership, and has held a number of roles during her time in Education, including Head of Chemistry, Head of Science and most recently as an Assistant Headteacher. In this role, she used her passion for education to drive improvement and success for staff and students across a number of subjects in addition to Science, supporting them to achieve their full potential. Fran has co-written Science textbooks, delivered CPD for teachers, and worked as an examiner for a number of UK exam boards.