Reaction Profiles

Reaction profiles are energy diagrams that show the relative energies of reactants and products during a chemical reaction. They are a key part of the AQA GCSE Chemistry specification and help you visualise whether a reaction is exothermic or endothermic, as well as understanding the role of activation energy.

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What Is a Reaction Profile?

A reaction profile (also called an energy level diagram) is a graph that shows the energy levels of the reactants and products during a reaction, plotted against the progress of the reaction.

• The y-axis shows energy.
• The x-axis shows the progress of the reaction (from reactants on the left to products on the right).

The difference in height between the reactants and products on the diagram tells you the overall energy change of the reaction.

Feature on Diagram	Meaning
Reactants level	Starting energy of the reactant molecules
Products level	Final energy of the product molecules
Activation energy (Ea)	Minimum energy needed to start the reaction — shown as the height of the energy barrier
Overall energy change	Difference between energy of reactants and energy of products

Exam Tip: When drawing reaction profiles, always label: (1) the reactants, (2) the products, (3) the activation energy arrow, and (4) the overall energy change arrow. Missing labels lose marks.

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Reaction Profile for an Exothermic Reaction

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

On the diagram:

• The products are drawn at a lower energy level than the reactants.
• The overall energy change arrow points downwards.
• The activation energy is shown as the difference between the reactants and the top of the energy barrier.

graph TD
    subgraph "Exothermic Reaction Profile"
        A["Reactants (higher energy)"] --> B["Energy Barrier (peak)"]
        B --> C["Products (lower energy)"]
    end
    D["Activation Energy (Ea) = Reactants to Peak"]
    E["Overall Energy Change = Reactants to Products (negative / energy released)"]

Key features:

• Products are lower than reactants.
• The overall energy change is negative (energy is released).
• The reaction gives out energy to the surroundings.

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Reaction Profile for an Endothermic Reaction

In an endothermic reaction, the products have more energy than the reactants. Energy has been absorbed from the surroundings.

On the diagram:

• The products are drawn at a higher energy level than the reactants.
• The overall energy change arrow points upwards.
• The activation energy is still shown as the difference between the reactants and the top of the energy barrier.

Key features:

• Products are higher than reactants.
• The overall energy change is positive (energy is absorbed).
• The reaction takes in energy from the surroundings.

Reaction Type	Position of Products	Overall Energy Change	Temperature Effect
Exothermic	Lower than reactants	Negative (energy released)	Surroundings heat up
Endothermic	Higher than reactants	Positive (energy absorbed)	Surroundings cool down

Exam Tip: In the exam, you may be given a reaction profile and asked whether the reaction is exothermic or endothermic. Simply check whether the products are LOWER (exothermic) or HIGHER (endothermic) than the reactants.

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Activation Energy

Activation energy (Ea) is the minimum amount of energy that colliding particles must have in order to react. It is the energy needed to break the bonds in the reactants and start the reaction.

Even in an exothermic reaction, some energy must be supplied initially to get the reaction going. This is why you need a spark or a match to ignite a fuel — even though the combustion reaction releases a large amount of energy overall.

Activation Energy on a Reaction Profile

On both exothermic and endothermic reaction profiles, the activation energy is shown as the difference between:

• The energy level of the reactants
• The peak of the energy barrier (the highest point on the curve)

Factor	Effect on Activation Energy
Type of reaction	Some reactions have high Ea (slow to start), others have low Ea (start easily)
Catalyst	Lowers the activation energy, making the reaction easier to start
Temperature	Does not change Ea, but gives more particles enough energy to exceed it

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The Effect of a Catalyst on a Reaction Profile

A catalyst is a substance that speeds up a chemical reaction without being used up. It works by providing an alternative reaction pathway with a lower activation energy.

On a reaction profile:

• A second curve is drawn with a lower peak than the original.
• The reactants and products remain at the same energy levels.
• The overall energy change is unchanged.
• Only the activation energy is reduced.

graph LR
    subgraph "Effect of Catalyst"
        A["Reactants"] --> B["High Ea (without catalyst)"]
        A --> C["Lower Ea (with catalyst)"]
        B --> D["Products"]
        C --> D
    end

Exam Tip: A catalyst does NOT change the overall energy change of a reaction. It only lowers the activation energy. If you are asked to draw the effect of a catalyst on a reaction profile, draw a second curve with a lower peak but the same start and end points.

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Drawing Reaction Profiles — Step by Step

For an Exothermic Reaction

1. Draw axes: y-axis labelled "Energy," x-axis labelled "Progress of reaction."
2. Draw a horizontal line on the left for the reactants and label it.
3. Draw a horizontal line on the right, lower than the reactants, for the products and label it.
4. Draw a smooth curve from the reactants that rises to a peak and then falls down to the products.
5. Label the activation energy (Ea) as an arrow from the reactants level to the peak.
6. Label the overall energy change as a downward arrow from reactants to products.

For an Endothermic Reaction