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Energy Level DiagramsOxford AQA IGCSE Chemistry: Revision notes

Section 1

What does an energy level diagram show?

An energy level diagram is a simple graph plotting energy (vertical axis) against the progress of the reaction (horizontal axis). It shows:

  • The energy of the reactants (starting level)
  • The energy of the products (finishing level)
  • The activation energy — the size of the 'hump' between reactants and the peak of the curve
  • The overall energy change (ΔH) of the reaction — the vertical distance between the reactant and product energy levels
Key termsenergy level diagramoverall energy change

Section 2

How does an exothermic reaction look on the diagram?

In an exothermic reaction, the products have less energy than the reactants, because energy has been transferred to the surroundings.

  • The product energy level is drawn below the reactant energy level
  • ΔH is negative
  • Examples: combustion, many oxidation reactions, neutralisation
Key termsexothermic
Exam tip

Always state whether the product line is above or below the reactant line — examiners award marks for this, not just the word 'exothermic'.

Section 3

How does an endothermic reaction look on the diagram?

In an endothermic reaction, the products have more energy than the reactants, because energy has been taken in from the surroundings.

  • The product energy level is drawn above the reactant energy level
  • ΔH is positive
  • Examples: thermal decomposition reactions
Key termsendothermic

Section 4

What is activation energy?

The activation energy is the minimum amount of energy that colliding particles must have for a reaction to occur. On the diagram it is shown as the height of the curve above the reactants' energy level — the 'hump' the reaction pathway must climb over before it can fall to the products' energy level (or continue rising, for an endothermic reaction).

Key termsactivation energy
Think of it like this

Think of activation energy as a small hill a ball must be pushed over before it can roll down (exothermic) or up (endothermic) to its final energy level.

Section 5

How does a catalyst affect the diagram?

A catalyst provides an alternative reaction pathway with a lower activation energy. On the diagram, this is shown as a lower, flatter hump between the reactants and products.

  • The overall energy change (ΔH) stays exactly the same — only the activation energy is reduced
  • More particles will have enough energy to react, so the reaction rate increases
Key termscatalyst
Common mistake

Do not draw a catalyst as changing the energy of the reactants or products — it only changes the height of the activation energy hump.

Must Know

  • Energy level diagrams show reactant energy, product energy, activation energy, and overall energy change (ΔH)
  • Exothermic: products lower than reactants, ΔH negative
  • Endothermic: products higher than reactants, ΔH positive
  • Activation energy is the minimum energy needed for particles to react, shown as the hump
  • A catalyst lowers the activation energy hump but does not change ΔH
  • The overall energy change is the vertical gap between reactant and product levels

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