GCSE Chemistry revision

Reaction profiles

Exothermic and endothermic reactions

AQA 4.5.1.2 Foundation & Higher

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A reaction profile is a diagram where the y-axis shows the total energy of the molecules and the x-axis shows the progress of the reaction.
Reactants are placed on the left and products on the right of the reaction profile.
For exothermic reactions, the products are drawn lower than the reactants, and the energy difference is labelled as energy released.
For endothermic reactions, the products are drawn higher than the reactants, and the energy difference is labelled as energy absorbed.

1. Reaction Profiles: The Basics AQA

A reaction profile is a diagram where the y-axis shows the total energy of the molecules and the x-axis shows the progress of the reaction.
Reactants are placed on the left and products on the right of the reaction profile.
For exothermic reactions, the products are drawn lower than the reactants, and the energy difference is labelled as energy released.
For endothermic reactions, the products are drawn higher than the reactants, and the energy difference is labelled as energy absorbed.

A reaction profile has total energy on the y-axis and progress of the reaction on the x-axis, with reactants on the left and products on the right.
For an exothermic reaction, the products are drawn lower than the reactants on the profile, and the energy difference is labelled as energy released.
The combustion of methane (CH₄ + 2O₂ → CO₂ + 2H₂O) is a classic example of an exothermic reaction shown on a profile.

2. Reaction Profiles for Exothermic Reactions AQA

A reaction profile has total energy on the y-axis and progress of the reaction on the x-axis, with reactants on the left and products on the right.
For an exothermic reaction, the products are drawn lower than the reactants on the profile, and the energy difference is labelled as energy released.
The combustion of methane (CH₄ + 2O₂ → CO₂ + 2H₂O) is a classic example of an exothermic reaction shown on a profile.

For an endothermic reaction profile, the products are drawn higher than the reactants, and the energy difference is labelled as energy absorbed.
The thermal decomposition of calcium carbonate is a key example of an endothermic reaction that can be represented on a reaction profile.

3. Reaction Profiles for Endothermic Reactions AQA

For an endothermic reaction profile, the products are drawn higher than the reactants, and the energy difference is labelled as energy absorbed.
The thermal decomposition of calcium carbonate is a key example of an endothermic reaction that can be represented on a reaction profile.

Activation energy is the minimum amount of energy that reactant particles need in order to collide successfully and react.
On a reaction profile, activation energy is shown as the difference in energy between the reactants' energy level and the highest point of the curve.
The greater the activation energy, the more energy is required to start the reaction.
Even in exothermic reactions, some energy input is still needed to get the reaction started before energy is released overall.

4. Activation Energy AQA

Activation energy is the minimum amount of energy that reactant particles need in order to collide successfully and react.
On a reaction profile, activation energy is shown as the difference in energy between the reactants' energy level and the highest point of the curve.
The greater the activation energy, the more energy is required to start the reaction.
Even in exothermic reactions, some energy input is still needed to get the reaction started before energy is released overall.

Even in exothermic reactions, some energy must be supplied initially to overcome the activation energy barrier before the reaction can proceed.
A higher curve on a reaction profile indicates a higher activation energy, whilst a lower curve indicates a lower activation energy.
The activation energy is represented the same way on both exothermic and endothermic reaction profiles — from the reactants' level to the peak of the curve.

5. Activation Energy on Reaction Profiles AQA

Even in exothermic reactions, some energy must be supplied initially to overcome the activation energy barrier before the reaction can proceed.
A higher curve on a reaction profile indicates a higher activation energy, whilst a lower curve indicates a lower activation energy.
The activation energy is represented the same way on both exothermic and endothermic reaction profiles — from the reactants' level to the peak of the curve.

A curve is drawn from the reactants, rising to a peak (representing the activation energy), then falling to the products' energy level.
A higher peak on the curve represents a greater activation energy, whilst a lower peak represents a smaller activation energy.
The shape of the curve applies to both exothermic and endothermic reaction profiles.

6. Drawing the Curve on Reaction Profiles AQA

A curve is drawn from the reactants, rising to a peak (representing the activation energy), then falling to the products' energy level.
A higher peak on the curve represents a greater activation energy, whilst a lower peak represents a smaller activation energy.
The shape of the curve applies to both exothermic and endothermic reaction profiles.

When drawing a reaction profile, a smooth curve should be drawn from the reactants, rising to a peak (representing activation energy), then falling to the products' energy level.
For specific reactions, the actual chemical names or formulae should be written on the reactants and products lines rather than just the words 'reactants' and 'products'.
Key labels to include on a reaction profile are: reactants, products, activation energy, and either energy released (exothermic) or energy absorbed (endothermic).

7. Drawing Reaction Profiles in Exams AQA

When drawing a reaction profile, a smooth curve should be drawn from the reactants, rising to a peak (representing activation energy), then falling to the products' energy level.
For specific reactions, the actual chemical names or formulae should be written on the reactants and products lines rather than just the words 'reactants' and 'products'.
Key labels to include on a reaction profile are: reactants, products, activation energy, and either energy released (exothermic) or energy absorbed (endothermic).