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1.8.2Physical Chemistry

Enthalpy Level Diagrams — Edexcel AS Chemistry Revision Notes & Topical Questions

5 min read 6 parts Lesson 34 of 42

📚 Edexcel AS Chemistry Hub / Topical Revision
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Section 1 of 6Reading an Enthalpy Profile

1.8.2 Enthalpy Level Diagrams
Vertical axis
  • Enthalpy increases upwards, so the position of reactants and products shows which has more energy.
Horizontal axis
  • The progress of the reaction runs from reactants on the left to products on the right.
ΔH
  • The vertical difference between the reactants and products levels.
Activation energy
  • The vertical height from the reactants up to the top of the curve.

Section 2 of 6Exothermic and Endothermic Diagrams

Exothermic and Endothermic Diagrams
Exothermic
  • Products lie below reactants; ΔH is negative and the arrow for ΔH points downwards.
Endothermic
  • Products lie above reactants; ΔH is positive and the arrow points upwards.
Activation energy
  • Both types need energy to start because bonds must be broken before new ones form.
Labelling
  • Label axes, reactants, products, ΔH and Ea with arrows and the correct species and states.
ReactionReactants vs productsSign of ΔH
Combustion of fuelsProducts lowerNegative
Thermal decomposition of CaCO₃Products higherPositive

Section 3 of 6Catalysts and Diagrams

1
A catalyst provides an alternative reaction pathway with a lower activation energy.
2
On the diagram, draw a second curve with a lower peak but the same start and end levels.
3
The enthalpy change ΔH is unchanged because the reactants and products are the same.
4
More particles have enough energy to react, so the rate increases.
5
The catalyst is not used up and does not change the position of equilibrium.

Section 4 of 6Forward and Reverse Reactions

Reverse enthalpy change
  • The ΔH of the reverse reaction has the same size as the forward reaction but the opposite sign.
Forward activation energy
  • If ΔH is negative, the reverse reaction has a higher activation energy than the forward reaction.
Calculation
  • Use Ea(reverse) = Ea(forward) − ΔH(forward), keeping the sign of ΔH in the working.
Check
  • The reverse activation energy of an exothermic reaction must be larger than the forward value.

Section 5 of 6Worked Example

Question

  • A reaction has Ea = 120 kJ mol⁻¹ and ΔH = −50 kJ mol⁻¹.
  • Calculate the activation energy of the reverse reaction.

  1. The forward reaction is exothermic, so products are 50 kJ below reactants.
  2. The reverse reaction must climb from the product level to the same peak.
  3. Ea(reverse) = 120 + 50 = 170 kJ mol⁻¹.

Answer Ea(reverse) = 170 kJ mol⁻¹

Section 6 of 6Exam Checklist & Tips

1.8.2 – Key Points for the Exam

  • Activation energy Minimum energy colliding particles need for a reaction to occur.
  • Enthalpy profile A diagram showing enthalpy as a reaction progresses.
  • Catalyst A substance that speeds up a reaction by lowering Ea and is regenerated.
  • Transition state The highest-energy arrangement of atoms during a reaction.

Common Mistakes

  • Drawing the products at the same height as the reactants.
  • Placing ΔH on the diagram without an arrow showing its direction.
  • Changing the product level when drawing a catalysed curve.
  • Labelling activation energy from the product to the peak.

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