Exothermic & Endothermic Reactions

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CognitoExothermic and endothermic reactions

Key facts & methods

Exothermic reactions

Exothermic: energy is transferred TO the surroundings (temperature increases).
Products have less energy than reactants — energy is released.
Examples: combustion, respiration, neutralisation, many oxidation reactions, hand warmers.
Reaction profile: products are at lower energy than reactants. Energy released = difference.
Temperature of surroundings increases during reaction.

Endothermic reactions

Endothermic: energy is absorbed FROM the surroundings (temperature decreases).
Products have more energy than reactants — energy is taken in.
Examples: thermal decomposition, dissolving ammonium nitrate in water, photosynthesis, sports injury packs.
Reaction profile: products are at higher energy than reactants.
Temperature of surroundings decreases during reaction.

Reaction profiles and activation energy

Reaction profiles show the energy changes during a reaction.
Activation energy (Eₐ): energy needed to start the reaction (shown as the "hill" on the profile).
Exothermic profile: reactants higher → peak (transition state) → products lower.
Endothermic profile: reactants lower → peak (transition state) → products higher.
A catalyst lowers the activation energy — the "hill" is smaller on the profile.

Bond breaking and forming

Energy is always required to break bonds (endothermic).
Energy is always released when bonds form (exothermic).
If energy needed to break bonds > energy released forming bonds → endothermic reaction overall.
If energy released forming bonds > energy needed to break bonds → exothermic reaction overall.
Overall energy change = energy released (bond forming) − energy required (bond breaking).

Exam questions — 4 questions · 11 marks · AQA 8462 style

1 markDefinitionAQA 8462 style

Which statement correctly describes an exothermic reaction?

A Energy is transferred from the surroundings to the reaction

B The temperature of the surroundings decreases

C Energy is transferred from the reaction to the surroundings, causing a temperature increase

D The products have more energy than the reactants

3 marksClassify reactionsAQA 8462 P1 style

Classify each of the following as exothermic or endothermic, and give a reason: (a) combustion of methane (b) thermal decomposition of calcium carbonate (c) neutralisation of sodium hydroxide with hydrochloric acid. (3 marks)

Hint: Clue: anything that burns = exothermic. Thermal decomposition requires heat input = endothermic. Neutralisation = always exothermic.

+30 XP

1 markReaction profileAQA 8462 style

In a reaction profile diagram, the activation energy is shown as:

A The difference in energy between reactants and products

B The energy of the products above zero

C The energy difference between the reactants and the peak (transition state)

D The total energy released during the reaction

4 marksCatalyst on reaction profileAQA 8462 Higher style

Sketch-describe a reaction profile for a catalysed and uncatalysed exothermic reaction on the same axes. Explain how the catalyst affects the activation energy and the overall energy change. (4 marks)

Hint: Both: same start and end energy (same overall ΔH). Difference: the peak is LOWER for the catalysed route. State explicitly that overall energy change is unchanged.

+40 XP

Exothermic & Endothermic Reactions

Exothermic reactions

Endothermic reactions

Reaction profiles and activation energy

Bond breaking and forming

Module complete! 🎉