Exothermic and Endothermic Reactions

Energy changes are a fundamental part of every chemical reaction. In this lesson you will learn the difference between exothermic and endothermic reactions, understand how energy is transferred to and from the surroundings, and explore everyday examples of both types. This topic is a core part of the Energy Changes chapter of the AQA GCSE Chemistry specification (Section 5).

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What Happens to Energy During a Chemical Reaction?

During a chemical reaction, existing bonds in the reactants are broken and new bonds are formed to make the products. Breaking bonds requires energy (it is an endothermic process), while forming bonds releases energy (it is an exothermic process).

The overall energy change of a reaction depends on the balance between the energy needed to break bonds and the energy released when new bonds form.

Process	Energy Change
Breaking bonds	Energy is taken in (endothermic)
Forming bonds	Energy is released (exothermic)

Exam Tip: A common mistake is to say that breaking bonds releases energy. Remember: breaking bonds ALWAYS requires energy. It is only when NEW bonds form that energy is released.

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Exothermic Reactions

An exothermic reaction is one that transfers energy to the surroundings. The surroundings usually get hotter, so there is a measurable temperature increase.

In an exothermic reaction, the energy released by forming new bonds in the products is greater than the energy needed to break bonds in the reactants.

Examples of Exothermic Reactions

Reaction	Example
Combustion	Burning methane in a Bunsen burner
Neutralisation	Hydrochloric acid + sodium hydroxide
Oxidation	Rusting of iron, respiration in cells
Many displacement reactions	Magnesium reacting with copper sulfate solution

Everyday Uses of Exothermic Reactions

• Self-heating cans — used for coffee or soup; a reaction between calcium oxide and water heats the contents.
• Hand warmers — iron powder oxidises slowly, releasing heat over several hours.
• Combustion in engines — petrol or diesel burns to release energy that powers vehicles.

graph LR
    A[Reactants] -->|Bonds broken energy taken in| B[Transition State]
    B -->|New bonds formed energy released| C[Products]
    C -->|MORE energy released than taken in| D[Surroundings get HOTTER]
    style D fill:#ff9999,stroke:#cc0000

Exam Tip: When a question asks you to identify an exothermic reaction, look for clues such as a temperature rise, flames, or the word "combustion." Neutralisation reactions are also always exothermic — this is a favourite exam question.

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Endothermic Reactions

An endothermic reaction is one that takes in energy from the surroundings. The surroundings usually get cooler, so there is a measurable temperature decrease.

In an endothermic reaction, the energy needed to break bonds in the reactants is greater than the energy released when new bonds form in the products.

Examples of Endothermic Reactions

Reaction	Example
Thermal decomposition	Heating calcium carbonate to produce calcium oxide and carbon dioxide
Citric acid + sodium hydrogencarbonate	A classic endothermic reaction in solution
Photosynthesis	Plants absorb light energy to convert CO2 and water into glucose and oxygen
Dissolving ammonium nitrate in water	The solution becomes noticeably cold

Everyday Uses of Endothermic Reactions

• Instant cold packs — used for sports injuries; ammonium nitrate dissolves in water inside the pack, absorbing heat and cooling the area.
• Cooking and baking — thermal decomposition of sodium hydrogencarbonate (baking soda) is an endothermic process that helps dough rise.

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Comparing Exothermic and Endothermic Reactions

Feature	Exothermic	Endothermic
Energy transfer	To the surroundings	From the surroundings
Temperature change	Surroundings get hotter (temperature rises)	Surroundings get cooler (temperature falls)
Bond energy balance	Energy released by forming bonds > energy needed to break bonds	Energy needed to break bonds > energy released by forming bonds
Examples	Combustion, neutralisation, oxidation	Thermal decomposition, photosynthesis, dissolving ammonium nitrate
Everyday use	Hand warmers, self-heating cans	Cold packs

Exam Tip: The terms "exo-" and "endo-" can help you remember. "Exo" means out (energy goes OUT to surroundings). "Endo" means in (energy goes IN from surroundings). Write these prefixes on your revision notes.

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Reversible Reactions and Energy Changes

Some reactions are reversible, meaning they can go in both directions. When a reversible reaction is exothermic in one direction, it is endothermic in the reverse direction, and the energy transferred is the same in both cases.

Example — hydrated and anhydrous copper sulfate:

• Hydrated copper sulfate (blue) heated gives anhydrous copper sulfate (white) + water — endothermic
• Anhydrous copper sulfate (white) + water gives hydrated copper sulfate (blue) — exothermic

graph LR
    A["Hydrated copper sulfate (blue)"] -->|"Heat (endothermic)"| B["Anhydrous copper sulfate (white) + Water"]
    B -->|"Add water (exothermic)"| A

This principle applies to all reversible reactions: the energy change in the forward direction is equal and opposite to the energy change in the reverse direction.

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Measuring Temperature Changes

You can detect whether a reaction is exothermic or endothermic by measuring the temperature change using a thermometer or a temperature probe in a polystyrene cup (calorimeter).

Observation	Conclusion
Temperature of solution rises	Reaction is exothermic
Temperature of solution falls	Reaction is endothermic
No temperature change	No reaction, or energy changes balance out

Key Practical Considerations

• Use a polystyrene cup as a calorimeter because it is a good insulator and reduces heat loss to the environment.
• Stir the mixture to ensure even temperature distribution.
• Record the maximum temperature reached (for exothermic) or the minimum temperature reached (for endothermic).
• Use a lid on the cup to reduce heat loss to the air.

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The Concept of Energy Conservation

In all chemical reactions, energy is conserved. This means that the total energy of the reactants equals the total energy of the products plus the energy transferred to or from the surroundings. Energy cannot be created or destroyed — it can only be transferred between stores.

Exam Tip: AQA may ask you to explain energy changes in terms of bond breaking and bond forming. Always structure your answer: (1) breaking bonds takes in energy, (2) forming bonds releases energy, (3) state which is greater to determine whether the reaction is exothermic or endothermic overall.

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Summary

• Exothermic reactions transfer energy to the surroundings, causing a temperature rise (e.g., combustion, neutralisation).
• Endothermic reactions take in energy from the surroundings, causing a temperature drop (e.g., thermal decomposition, dissolving ammonium nitrate).
• Breaking bonds is endothermic; forming bonds is exothermic.
• The overall energy change depends on whether more energy is released forming bonds or taken in breaking bonds.
• In reversible reactions, the energy change in the forward direction is equal and opposite to that in the reverse direction.
• Energy is always conserved in chemical reactions.

Exam Tip: A 6-mark question on exothermic vs endothermic reactions is very common. Make sure you can clearly define both terms, give examples of each, explain the role of bond breaking and forming, and link temperature changes to the type of reaction.