Endothermic and Exothermic Reactions: KS3 Chemistry

Chemical reactions always involve a transfer of energy with the surroundings. Exothermic reactions release energy as heat, warming the surroundings; endothermic reactions absorb energy from the surroundings, making them feel cold. The difference comes down to whether bonds store or release more energy overall.

What is an exothermic reaction?

An exothermic reaction transfers energy to the surroundings, usually as heat (and sometimes as light or sound). The temperature of the surroundings rises during an exothermic reaction.

The word comes from the Greek exo (outside) + therm (heat) — heat goes out.

Everyday examples of exothermic reactions:

Burning fuels (wood, gas, petrol) — combustion

Respiration in your cells (glucose + oxygen → carbon dioxide + water)

Neutralisation (acid + alkali → salt + water, e.g. hydrochloric acid + sodium hydroxide)

Rusting of iron (slow oxidation)

Hand warmers (oxidation of iron powder inside the sachet)

In an energy profile diagram for an exothermic reaction, the products sit lower than the reactants. The difference in height is the energy released (the exothermic energy change, often shown as a negative value in GCSE notation).

What is an endothermic reaction?

An endothermic reaction absorbs energy from the surroundings. The temperature of the surroundings falls — they feel cold — because the reaction is taking heat in.

The word comes from endo (inside) + therm (heat) — heat goes in.

Everyday examples of endothermic reactions:

Photosynthesis in plants (absorbs light energy from the Sun)

Dissolving ammonium nitrate in water (used in instant cold packs)

Thermal decomposition — heating calcium carbonate to make calcium oxide (lime kilns)

Citric acid dissolving in water with sodium bicarbonate (the fizzing you feel cold during)

In an energy profile diagram for an endothermic reaction, the products sit higher than the reactants — energy is stored in the products.

How does bond breaking and bond making explain the energy change?

All chemical reactions involve breaking bonds in the reactants and forming new bonds in the products. This bond-energy model explains why reactions are exothermic or endothermic:

Breaking bonds requires energy (energy is put in).

Making bonds releases energy (energy is given out).

The overall energy change depends on which dominates:

Outcome | What it means | Overall reaction
Energy to break bonds < energy from making bonds | More energy released than absorbed | Exothermic (temperature rises)
Energy to break bonds > energy from making bonds | More energy absorbed than released | Endothermic (temperature falls)
Energy to break bonds = energy from making bonds | No net energy change | Neither (no temperature change)

For example, in the combustion of methane (natural gas), breaking C–H and O=O bonds requires energy, but the C=O and O–H bonds formed in carbon dioxide and water release considerably more — so combustion is strongly exothermic.

How do you draw and read an energy profile diagram?

An energy profile diagram (also called a reaction profile) shows how the energy of the system changes from reactants to products:

Draw two horizontal lines: reactants on the left, products on the right.

Add a peak (the activation energy hump) between them — this is the minimum energy needed to start the reaction.

If the products line is lower than the reactants line → exothermic.

If the products line is higher than the reactants line → endothermic.

The activation energy is the energy that must be supplied to get the reaction going, even for exothermic reactions. This is why you need a spark to light a gas hob — without sufficient activation energy, no combustion occurs.

How are endothermic and exothermic reactions used in real life?

Application | Type | How it works
Hand warmers | Exothermic | Oxidation of iron or crystallisation of sodium acetate releases heat
Instant cold packs | Endothermic | Ammonium nitrate dissolves, absorbing heat from the surroundings
Combustion engines | Exothermic | Burning fuel releases energy to drive pistons
Photosynthesis | Endothermic | Plants absorb light energy to build glucose from CO₂ and H₂O
Lime kilns | Endothermic | Heating limestone (CaCO₃) at ~900 °C drives off CO₂, forming lime (CaO)
Self-heating cans | Exothermic | Calcium oxide + water reaction heats a beverage without an external flame

What is the difference between temperature and energy?

A common misconception: temperature and energy (heat) are not the same thing.

Temperature measures how hot something is (the average kinetic energy of its particles). It is measured in °C or K.

Energy (heat) is the total thermal energy transferred. A large bath of warm water contains more total thermal energy than a small cup of boiling water, even though the cup is hotter.

When we say an exothermic reaction "releases energy as heat", we mean energy is transferred from the chemical system to the surroundings, raising the surroundings' temperature. The amount of energy transferred depends on how much reacting substance there is.

Frequently asked questions

What is the difference between exothermic and endothermic reactions?

In an exothermic reaction, energy is released to the surroundings, so the temperature of the surroundings rises. In an endothermic reaction, energy is absorbed from the surroundings, so the temperature of the surroundings falls. Burning fuels and respiration are exothermic; photosynthesis and dissolving ammonium nitrate are endothermic.

Why does breaking bonds require energy?

Bonds hold atoms together by electrostatic forces. To break a bond, you must pull the atoms apart against these attractive forces — that takes an input of energy, just as stretching a spring takes effort. Bond breaking always absorbs energy; bond making always releases it.

What does an energy profile diagram show?

An energy profile diagram shows the energy of the reactants and products and the activation energy barrier between them. If the products line sits below the reactants line, the reaction is exothermic. If it sits higher, the reaction is endothermic. The height of the peak above the reactants shows the activation energy needed to start the reaction.

Is respiration exothermic or endothermic?

Respiration is exothermic. During aerobic respiration, glucose reacts with oxygen to produce carbon dioxide and water. The energy stored in the chemical bonds of glucose is released — some as useful energy for cellular processes, and some as heat, which is why exercise makes you warm.

Why do hand warmers get hot but cold packs get cold?

Hand warmers use exothermic reactions — the oxidation of iron or the crystallisation of sodium acetate releases energy as heat into your hand. Cold packs use endothermic reactions — ammonium nitrate dissolves in water, absorbing heat energy from the surroundings (your injury), which is why the pack feels cold.

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