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Last exams 2024

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Energy Profiles (DP IB Chemistry: SL)

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Energy Profiles

  • An energy profile or 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’

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

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

 5-1-2-exo-reaction-profile

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

The enthalpy of combustion for methane is -890 kJ mol-1 and the activation energy is +2653 kJ mol-1Draw 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

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

Use the energy level diagram below to identify the activation energy, Ea , for the reaction1-6-2-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

Examiner 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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Stewart

Author: Stewart

Expertise: Chemistry Lead

Stewart has been an enthusiastic GCSE, IGCSE, A Level and IB teacher for more than 30 years in the UK as well as overseas, and has also been an examiner for IB and A Level. As a long-standing Head of Science, Stewart brings a wealth of experience to creating Exam Questions and revision materials for Save My Exams. Stewart specialises in Chemistry, but has also taught Physics and Environmental Systems and Societies.