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3.1.4Physical Chemistry

Energy Level Diagrams — Edexcel International GCSE Science Double Award 4SD0 - Chemistry Revision Notes & Topical Questions

5 min read 5 parts Lesson 6 of 19

📚 Edexcel International GCSE Science Double Award 4SD0 - Chemistry Hub / Topical Revision
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Part 1 of 5Drawing Energy Level Diagrams

3.1.4 Energy Level Diagrams

An energy level diagram compares the chemical energy of the reactants and products on one axis.

1
Draw a vertical axis labelled energy, and a horizontal axis labelled as the progress of the reaction.
2
Draw a horizontal line for the reactants at their energy level, and a second horizontal line for the products.
3
If the products are lower than the reactants, draw a downward arrow between the two lines to show an exothermic reaction.
4
If the products are higher than the reactants, draw an upward arrow to show an endothermic reaction, and label the arrow with the enthalpy change.

Part 2 of 5Exothermic and Endothermic Diagrams

The relative heights of the two energy levels show whether energy is released or taken in.

Exothermic
  • The products are at a lower energy than the reactants, so energy is given out and ΔH is negative.
Endothermic
  • The products are at a higher energy than the reactants, so energy is taken in and ΔH is positive.
Energy change
  • The energy change equals the energy of the products minus the energy of the reactants.
Identification
  • A diagram can be identified as exothermic or endothermic by comparing the two levels directly.
Reaction typeProducts relative to reactantsSign of ΔH
ExothermicLowerNegative
EndothermicHigherPositive

Part 3 of 5Reading the Diagram

Reading the Diagram
Reactant level
  • The starting energy of the chemicals is shown at the left-hand side of the diagram.
Product level
  • The final energy of the chemicals is shown at the right-hand side of the diagram.
Enthalpy change
  • The vertical difference between the two levels gives the size of the enthalpy change.
Later topic
  • Reaction profiles add the activation energy as a peak between reactants and products, and are studied later.

Part 4 of 5Worked Example

Question

  • A reaction has reactants at an energy of 150 kJ/mol and products at an energy of 80 kJ/mol.
  • Calculate the enthalpy change and state whether the reaction is exothermic or endothermic.

  1. Enthalpy change ΔH = energy of products − energy of reactants = 80 − 150 = −70 kJ/mol.
  2. The value is negative, so energy is released to the surroundings.
  3. The products are lower than the reactants on the diagram, so the reaction is exothermic.

Answer

  • ΔH = −70 kJ/mol;
    • the reaction is exothermic.

Part 5 of 5Exam Checklist & Tips

3.1.4 – Key Points for the Exam

  • Energy level diagram A diagram showing the relative energies of reactants and products in a reaction.
  • Reactants The starting substances in a chemical reaction.
  • Products The substances formed by a chemical reaction.
  • Enthalpy change, ΔH The energy change in a reaction, negative for exothermic and positive for endothermic.

Common Mistakes

  • Drawing the products higher than the reactants for an exothermic reaction.
  • Forgetting to label the axes, or failing to show the arrow for the enthalpy change.
  • Calculating ΔH as reactants minus products, which reverses the sign.
  • Confusing an energy level diagram with a reaction profile, which also shows the activation energy.

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