Sometimes tested C11.3

Interpreting Energy Level Diagrams

Energy level diagrams are visual representations of the energy changes during a chemical reaction. For the ESAT, you must be able to read these diagrams to determine if a reaction releases or absorbs energy and calculate the overall enthalpy change.

Part of the ESAT Chemistry syllabus — revision for the Engineering and Science Admissions Test (ESAT), the UAT-UK admissions test for Cambridge, Imperial, Oxford and UCL.

Key points

  • The vertical y-axis represents potential energy or enthalpy (H).
  • Reactants are always on the left, and products are on the right.
  • If the products are at a lower energy level than the reactants, the reaction is exothermic and releases energy.
  • If the products are at a higher energy level than the reactants, the reaction is endothermic and absorbs energy.
  • The overall enthalpy change (ΔH) is the vertical difference in energy between the products and the reactants.
  • The 'hump' between reactants and products represents the activation energy, which affects the reaction rate but not the overall ΔH.

Diagram

Endothermic reaction profileReaction profile: reactants to products, products higher than reactants. energyprogress of reactionreactantsproducts
Reading an energy diagram: if products are lower in energy than reactants, the reaction is exothermic (ΔH negative). If products are higher, the reaction is endothermic (ΔH positive).

Formulae

ΔH = H(products) - H(reactants)

To calculate the overall enthalpy change of a reaction using the absolute enthalpy values of the products and reactants.

Definitions

Energy Level Diagram
A graph plotting the enthalpy of a chemical system against the progress of the reaction, showing the relative energies of reactants and products.
Enthalpy Change (ΔH)
The overall heat energy change in a reaction at constant pressure. A negative ΔH signifies an exothermic reaction, while a positive ΔH signifies an endothermic one.
Exothermic
A process that releases heat energy into the surroundings. On a diagram, the products are lower in energy than the reactants.
Endothermic
A process that absorbs heat energy from the surroundings. On a diagram, the products are higher in energy than the reactants.

Worked example

A reaction has reactants with a starting enthalpy of -20 kJ mol-1. The products are formed with an enthalpy of +70 kJ mol-1. According to an energy level diagram for this reaction, what is the enthalpy change (ΔH) in kJ mol-1, and is the reaction exothermic or endothermic?

  1. 1

    Identify the enthalpy of the reactants:

    H(reactants) = -20 kJ mol-1.

  2. 2

    Identify the enthalpy of the products:

    H(products) = +70 kJ mol-1
  3. 3

    Use the formula ΔH = H(products) - H(reactants).

  4. 4

    Substitute the values:

    ΔH = (+70) - (-20)
  5. 5

    Calculate the result:

    ΔH = 70 + 20 = +90 kJ mol-1
  6. 6

    Interpret the sign:

    Since ΔH is positive, the reaction absorbs energy and is endothermic.

Answer: +90 kJ mol-1, endothermic

Common mistakes

  • ×Making a sign error. Always calculate ΔH as 'products minus reactants'. A common mistake is to do 'reactants minus products' or just find the difference without considering the sign.
  • ×Confusing the activation energy (the peak of the diagram) with the overall enthalpy change (the difference between the start and end levels).

No-calculator tips

  • ✓Before any calculation, just look at the diagram. If the product line is higher than the reactant line, the answer for ΔH must be positive (endothermic). If it's lower, the answer must be negative (exothermic).
  • ✓Be very careful subtracting negative numbers. Remember that subtracting a negative is the same as adding a positive, e.g., 50 - (-30) = 50 + 30 = 80.

Read this topic in the official UAT-UK ESAT guide →

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