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 Combined Science Trilogy specification (8464) and help you visualise whether a reaction is exothermic or endothermic, and understand 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 progress of reaction (from reactants on the left to products on the right).

The difference in height between the reactants and products tells you the overall energy change ($\Delta H$ΔH) 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 ($E_a$Ea​)	Minimum energy needed to start the reaction — shown as the height of the energy barrier
Overall energy change ($\Delta H$ΔH)	Difference between energy of reactants and energy of products

Exam Tip: When drawing reaction profiles, always label: (1) reactants, (2) products, (3) the activation energy arrow ($E_a$Ea​), and (4) the overall energy change arrow ($\Delta H$ΔH). 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:

• Products are drawn at a lower energy level.
• The overall energy change ($\Delta H$ΔH) is negative (arrow points downwards).
• Activation energy is shown from the reactant level to the peak.

graph TD
    subgraph "Exothermic Reaction Profile"
        A["Reactants \n(higher energy)"] --> B["Energy Barrier \n(peak / transition state)"]
        B --> C["Products \n(lower energy)"]
    end
    D["Ea = Reactants → Peak \n(activation energy)"]
    E["ΔH = Reactants → Products \n(negative — energy released)"]

Key features:

• Products are lower than reactants on the energy axis.
• $\Delta H$ΔH is negative — energy is released.
• The reaction gives out energy to the surroundings (temperature rises).

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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:

• Products are drawn at a higher energy level.
• The overall energy change ($\Delta H$ΔH) is positive (arrow points upwards).
• Activation energy is still shown from the reactant level to the peak.

graph TD
    subgraph "Endothermic Reaction Profile"
        A["Reactants \n(lower energy)"] --> B["Energy Barrier \n(peak / transition state)"]
        B --> C["Products \n(higher energy)"]
    end
    D["Ea = Reactants → Peak \n(activation energy)"]
    E["ΔH = Reactants → Products \n(positive — energy absorbed)"]

Key features:

• Products are higher than reactants on the energy axis.
• $\Delta H$ΔH is positive — energy is absorbed.
• The reaction takes in energy from the surroundings (temperature falls).

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Comparing Exothermic and Endothermic Reaction Profiles

Feature	Exothermic	Endothermic
Products vs Reactants	Products lower	Products higher
$\Delta H$ΔH sign	Negative (energy released)	Positive (energy absorbed)
Temperature effect	Surroundings heat up	Surroundings cool down
Overall energy change arrow	Points downwards	Points upwards

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

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Activation Energy ($E_a$Ea​)

Activation energy ($E_a$Ea​) is the minimum amount of energy that colliding particles must have in order to react. It is the energy needed to start breaking bonds in the reactants.

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

Activation Energy on a Reaction Profile

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

$E_a = \text{Energy at peak} - \text{Energy of reactants}$Ea​=Energy at peak−Energy of reactants

Factor	Effect on Activation Energy
Nature of reactants	Different reactions have different $E_a$Ea​ values
Catalyst	Lowers the $E_a$Ea​ (provides an alternative pathway)

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Worked Example: Reading a Reaction Profile

A reaction profile shows:

• Reactants at 200 kJ
• Peak at 350 kJ
• Products at 100 kJ

Step 1: Calculate the activation energy:

$E_a = 350 - 200 = 150 \text{ kJ}$Ea​=350−200=150 kJ

Step 2: Calculate the overall energy change:

$\Delta H = \text{Products} - \text{Reactants} = 100 - 200 = -100 \text{ kJ}$ΔH=Products−Reactants=100−200=−100 kJ

Step 3: Determine the reaction type:

• $\Delta H$ΔH is negative, so the reaction is exothermic.
• Products are lower than reactants — confirms energy is released.

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Drawing a Reaction Profile — Step by Step

graph LR
    A["Step 1: Draw axes \n(Energy vs Progress)"] --> B["Step 2: Mark reactant \nenergy level"]
    B --> C["Step 3: Mark product \nenergy level"]
    C --> D["Step 4: Draw the curve \nwith a peak (Ea)"]
    D --> E["Step 5: Label Ea \nand ΔH arrows"]

1. Draw the y-axis (Energy) and x-axis (Progress of reaction).
2. Mark the reactant energy level on the left.
3. Mark the product energy level — lower for exothermic, higher for endothermic.
4. Draw a smooth curve from reactants over a peak (energy barrier) down to products.
5. Label the activation energy ($E_a$Ea​) arrow from reactants to peak.
6. Label the overall energy change ($\Delta H$ΔH) arrow from reactants to products.

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Common Mistakes

Mistake	How to Avoid It
Drawing products higher for exothermic	Exothermic = products LOWER
Labelling $E_a$Ea​ from products to peak	$E_a$Ea​ is ALWAYS from reactants to peak
Confusing $E_a$Ea​ and $\Delta H$ΔH	$E_a$Ea​ = reactants to peak; $\Delta H$ΔH = reactants to products
Forgetting to draw a smooth curve	The profile should be a smooth curve, not straight lines
Not labelling the diagram	Always label reactants, products, $E_a$Ea​, and $\Delta H$ΔH

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Summary