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Revision notes for AQA GCSE Chemistry Reaction profiles. Open the guide for explanations and worked examples. Written against the AQA GCSE Chemistry (8462) specification, so the content matches what's examinable rather than general Chemistry background.

Reaction profiles

What you'll learn

  • Why reacting particles must collide with enough energy before a reaction can happen.
  • What activation energy means and how to show it on a reaction profile.
  • How to draw reaction profiles for exothermic and endothermic reactions.
  • How to use a profile to identify the overall energy change of a reaction.

The starting point: particles must collide

Chemical reactions happen between particles: atoms, ions or molecules. For a reaction to occur, the reacting particles must collide with each other.

But not every collision leads to a reaction. A collision is only successful if the particles collide:

  • with enough energy
  • in a suitable arrangement for bonds to break and new bonds to form

For GCSE Chemistry, the most important idea here is the energy requirement.

Definition

Activation energy

The activation energy is the minimum amount of energy that reacting particles must have for a reaction to occur.

You can think of activation energy as the “energy barrier” that must be overcome before reactants can become products.

Key Idea

Successful collisions

Reacting particles can only react when they collide with at least the activation energy. If they do not have enough energy, they simply bounce apart without reacting.

Reaction profiles: energy during a reaction

A reaction profile, also called an energy level diagram, is a graph that shows how the energy changes during a chemical reaction.

Definition

Reaction profile

A reaction profile is a diagram showing the relative energies of the reactants, the products, the activation energy, and the overall energy change as a reaction takes place.

The vertical axis shows energy. The horizontal axis shows the progress of reaction — this means how far the reaction has gone from reactants to products.

Common Mistake

Progress of reaction is not time

The x-axis is usually labelled progress of reaction, not time. A reaction profile shows the energy pathway, not how quickly the reaction happens.

A reaction profile uses:

  • a starting horizontal line for the reactants
  • a curved line rising to a peak, showing the energy barrier
  • an ending horizontal line for the products
  • an arrow from the reactant level to the peak for the activation energy
  • an arrow between reactants and products for the overall energy change

Here are the two main shapes you need to recognise and draw.

Reaction profile diagrams for exothermic and endothermic reactions showing reactants, products, activation energy and overall energy change

Exothermic reaction profiles

An exothermic reaction transfers energy to the surroundings. The surroundings usually get warmer.

Definition

Exothermic reaction

An exothermic reaction is a reaction that transfers energy to the surroundings, so the temperature of the surroundings increases.

In an exothermic reaction profile:

  • the reactants start at a higher energy
  • the products finish at a lower energy
  • the overall energy change is downwards
  • the reaction releases energy overall

You may see the overall energy change written as ΔH\Delta HΔH. For exothermic reactions, ΔH\Delta HΔH is negative because the products have less energy than the reactants.

Key Idea

Exothermic profile shape

For an exothermic reaction, the products are lower than the reactants on the energy axis.

Example

Interpreting an exothermic profile

A reaction profile has reactants at 90 kJ/mol, products at 30 kJ/mol, and a peak at 140 kJ/mol. Work out the activation energy, the overall energy change, and whether the reaction is exothermic or endothermic.

  1. Find the activation energy by comparing the peak with the reactants:
Ea=140 kJ/mol−90 kJ/mol=50 kJ/mol E_a = 140\ \text{kJ/mol} - 90\ \text{kJ/mol} = 50\ \text{kJ/mol} Ea​=140 kJ/mol−90 kJ/mol=50 kJ/mol
  1. Find the overall energy change by comparing products with reactants:
ΔH=30 kJ/mol−90 kJ/mol=−60 kJ/mol \Delta H = 30\ \text{kJ/mol} - 90\ \text{kJ/mol} = -60\ \text{kJ/mol} ΔH=30 kJ/mol−90 kJ/mol=−60 kJ/mol
  1. Interpret the sign and the diagram: the products are lower in energy than the reactants, so energy has been released. The reaction is exothermic.

Endothermic reaction profiles

An endothermic reaction takes in energy from the surroundings. The surroundings usually get colder.

Definition

Endothermic reaction

An endothermic reaction is a reaction that transfers energy from the surroundings, so the temperature of the surroundings decreases.

In an endothermic reaction profile:

  • the reactants start at a lower energy
  • the products finish at a higher energy
  • the overall energy change is upwards
  • the reaction takes in energy overall

For endothermic reactions, ΔH\Delta HΔH is positive because the products have more energy than the reactants.

Key Idea

Endothermic profile shape

For an endothermic reaction, the products are higher than the reactants on the energy axis.

Example

Interpreting an endothermic profile

A reaction profile has reactants at 40 kJ/mol, products at 85 kJ/mol, and a peak at 120 kJ/mol. Work out the activation energy, the overall energy change, and identify the reaction type.

  1. Calculate the activation energy from the reactant level to the peak:
Ea=120 kJ/mol−40 kJ/mol=80 kJ/mol E_a = 120\ \text{kJ/mol} - 40\ \text{kJ/mol} = 80\ \text{kJ/mol} Ea​=120 kJ/mol−40 kJ/mol=80 kJ/mol
  1. Calculate the overall energy change from reactants to products:
ΔH=85 kJ/mol−40 kJ/mol=+45 kJ/mol \Delta H = 85\ \text{kJ/mol} - 40\ \text{kJ/mol} = +45\ \text{kJ/mol} ΔH=85 kJ/mol−40 kJ/mol=+45 kJ/mol
  1. Use the product level to classify the reaction: the products are higher than the reactants, so energy has been taken in. The reaction is endothermic.

The activation energy arrow

The activation energy arrow is measured from the energy level of the reactants up to the highest point of the curve.

This is true for both exothermic and endothermic reactions.

Common Mistake

Measuring activation energy from zero

Do not draw the activation energy arrow from the bottom of the graph. It starts at the reactant energy level and ends at the top of the curve.

The peak of the curve represents the point where the reaction has enough energy for old bonds to break and new bonds to form. You do not need A-level terms for this at GCSE — just understand that it is the top of the energy barrier.

Overall energy change

The overall energy change is the difference between the energy of the reactants and the energy of the products.

Definition

Overall energy change

The overall energy change is the energy difference between the reactants and products in a reaction.

You can calculate it using:

ΔH=energy of products−energy of reactants \Delta H = \text{energy of products} - \text{energy of reactants} ΔH=energy of products−energy of reactants

So:

  • if products are lower than reactants, ΔH\Delta HΔH is negative → exothermic
  • if products are higher than reactants, ΔH\Delta HΔH is positive → endothermic
Tip

Quick direction check

Look only at the product level compared with the reactant level. Products lower means exothermic. Products higher means endothermic.

Drawing reaction profiles

In the exam, you may be asked to draw a simple reaction profile. You do not need exact numerical values unless the question gives them. The key is to include the correct relative energy levels and labels.

How to draw one

  1. Draw axes: energy on the y-axis and progress of reaction on the x-axis.
  2. Draw a horizontal line for the reactants.
  3. Draw a horizontal line for the products:
    • lower than reactants for exothermic
    • higher than reactants for endothermic
  4. Draw a curved line from reactants to products, going up to a peak first.
  5. Label the activation energy from the reactant level to the peak.
  6. Label the overall energy change between the reactant and product levels.
Example

Drawing an endothermic profile

Draw a reaction profile for an endothermic reaction and label the activation energy and overall energy change.

  1. Place the reactants on a lower horizontal energy level because endothermic reactions start lower than they finish.

  2. Place the products on a higher horizontal energy level because the products have gained energy overall.

  3. Draw a curve that rises above both reactants and products before falling to the product level, because the reaction must still overcome an activation energy barrier.

  4. Add an activation energy arrow from the reactant level up to the peak, then add an overall energy change arrow from the reactant level up to the product level.

Why reaction profiles are useful

Reaction profiles let you see three important things quickly:

  • whether a reaction is exothermic or endothermic
  • how much energy is needed to start the reaction
  • the overall energy difference between reactants and products

They are especially useful because they separate two ideas that students often mix up:

  • Activation energy is the energy needed to start the reaction.
  • Overall energy change is the energy transferred overall between reactants and products.
Common Mistake

Mixing up activation energy and overall energy change

The activation energy is the arrow to the peak. The overall energy change is the arrow between reactants and products.

Catalysts and reaction profiles

A catalyst is a substance that increases the rate of a chemical reaction without being used up. Catalysts are covered more fully in the rates of reaction topic, but they connect nicely to reaction profiles.

A catalyst provides a different pathway for the reaction with a lower activation energy.

The reactants and products stay at the same energy levels, so the overall energy change does not change.

Key Idea

Catalysts lower the barrier

A catalyst lowers the activation energy, but it does not change the energy of the reactants, the energy of the products, or the overall energy change.

Common Mistake

Catalysts do not make reactions more exothermic

A catalyst can make a reaction happen faster, but it does not increase the amount of energy released or taken in overall.

Exam technique

In the exam

  1. Check whether the products are above or below the reactants before deciding if the reaction is endothermic or exothermic.
  2. Draw activation energy from the reactant level to the peak, not from the bottom of the graph.
  3. Always label the axes: Energy on the y-axis and Progress of reaction on the x-axis.
  4. Use a curved reaction pathway, not straight diagonal lines between reactants and products.
Self review

Check yourself

  • In a reaction profile, where should the activation energy arrow start and finish?
  • How can you tell from a profile whether a reaction is exothermic?
  • Why can particles collide but still fail to react?

Exothermic and endothermic reactions

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