Chemical changes · GCSE Chemistry
Exothermic and endothermic reactions
GCSE Chemistry revision on exothermic and endothermic reactions: energy transfer to or from the surroundings, temperature change, and everyday examples including combustion and sports packs.
Exothermic: energy to the surroundings, temperature of the surroundings rises, ΔH negative. Endothermic: energy from the surroundings, temperature falls, ΔH positive. Measure the surroundings, not the chemicals’ “feelings”.
The important bits
What you need to know
- 1
An exothermic reaction transfers energy to the surroundings. The temperature of the surroundings increases. Combustion, respiration, many neutralisations and hand-warmers are exothermic.
- 2
An endothermic reaction takes in energy from the surroundings. The temperature of the surroundings decreases. Thermal decomposition, photosynthesis and some sports injury packs are endothermic.
- 3
ΔH (enthalpy change) is negative for exothermic reactions and positive for endothermic reactions. Units are kJ/mol when you work per mole.
- 4
In a polystyrene-cup practical, mix known volumes, record start and maximum or minimum temperature, and calculate the change. A lid and stirring reduce heat loss (or gain).
- 5
If a reaction needs continuous heating to keep going, it is often endothermic (thermal decomposition of carbonates: CaCO₃ → CaO + CO₂). If it heats itself once started, think exothermic (combustion).
- 6
Neutralisation H⁺(aq) + OH⁻(aq) → H₂O(l) is exothermic: the cup warms. Dissolving can go either way: NaOH(s) dissolving is exothermic; some ammonium salts dissolving are endothermic.
- 7
Bond breaking takes energy in (endothermic). Bond making releases energy (exothermic). The overall type depends on which is larger — that is the next page, bond energies.
- 8
Simple cells and hydrogen fuel cells (Higher / Triple on some specifications) transfer chemical energy electrically. The fuel-cell reaction is 2H₂ + O₂ → 2H₂O, still an exothermic oxidation, but useful work is electrical rather than only heat.
Quotations worth analysing
Short evidence. Real method.
“Exothermic: energy transferred to the surroundings (temperature rises). Endothermic: energy taken in from the surroundings (temperature falls).”
Always name the surroundings. “The reaction gets cold” is sloppy: the reaction mixture and cup are the surroundings you measured with the thermometer.
“CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(l) ΔH negative”
Fuels are useful because combustion is strongly exothermic. Incomplete combustion is still exothermic, but you get less energy and CO or C as well.
“CaCO₃(s) → CaO(s) + CO₂(g) thermal decomposition, endothermic”
You must keep supplying energy. That is the opposite of a fuel. State symbols show a gas leaving; in an open system mass appears to fall.
Go deeper
The thermometer is in the surroundings
Energy is conserved. In an exothermic change the chemical stores of the reactants fall and the thermal store of the water, cup and air rises — so the reading goes up. In an endothermic change the chemical stores rise and the thermal store of the surroundings falls — so the reading goes down. Students say a reaction “gives off cold”. Energy is not a fluid of cold; it is transferred. If the cup is a good insulator (polystyrene), more of the temperature change stays in the mixture and your measured ΔT is closer to the true value. Heat loss to the bench makes an exothermic ΔT look too small, which is the usual accuracy evaluation.
Go deeper
Everyday uses are just labelled energy transfers
Hand-warmers: oxidation of iron (or crystallisation of a supersaturated solution) is exothermic, so the pack warms your pocket. Instant cold packs: dissolving ammonium nitrate is endothermic, so energy is taken from the injury. Self-heating cans use an exothermic reaction in a separate compartment. None of these break conservation of energy. A six-mark “explain with an example” needs the direction of transfer, the temperature change, and one named reaction. Combustion in a bunsen or a camp stove is still the same idea: useful thermal store increase in the surroundings. A thermometer in the pack, not a slogan, is the evidence.
Go deeper
Dissolving is not automatically exothermic
NaOH(s) → NaOH(aq) warms the water; ammonium chloride or ammonium nitrate often cools it. Both are physical processes of dissolving, not combustion, but GCSE still classifies the observed temperature change as exo- or endothermic. Neutralisation is a chemical change and is reliably exothermic for strong acid plus strong alkali. If a question mixes dissolving NaOH with then adding acid, you may see two temperature jumps. Read whether they ask for the dissolving, the reaction, or the whole experiment. Repeat measurements and use a lid; one run is not a conclusion.
See the idea in action
25 cm³ of 1.0 mol/dm³ HCl and 25 cm³ of 1.0 mol/dm³ NaOH start at 21.0 °C. After mixing, the maximum is 27.5 °C. ΔT = +6.5 °C, so the reaction is exothermic: energy has been transferred to the surroundings (the solution). Ionic equation: H⁺(aq) + OH⁻(aq) → H₂O(l). If the same volumes of water and ammonium nitrate crystals fell from 21.0 °C to 16.0 °C, that dissolving is endothermic. Polystyrene cups and a lid would make both ΔT values more accurate by reducing heat transfer to the air.
Exam technique
Turn knowledge into marks
State the direction of energy transfer and what happens to the temperature of the surroundings. Give a named example. For practicals, mention insulation, stirring, and repeats. Never write that energy is destroyed or that a reaction “gives out cold”.
Common mistakes
Do not give these marks away
- 01
Describing an endothermic reaction as one that “gives out cold”.
- 02
Mixing up the signs of ΔH, or saying combustion is endothermic.
- 03
Forgetting that heat loss makes experimental exothermic temperature rises too small.
The temperature of a solution falls when a solid dissolves. What type of change is this?
AExothermic, because energy is transferred to the surroundings
BEndothermic, because energy is taken in from the surroundings
CNeither, because dissolving is not a reaction
DExothermic, because bonds are made
Show the answer
Endothermic, because energy is taken in from the surroundings. A fall in temperature of the surroundings means energy has been taken in. GCSE classifies that observed change as endothermic, whether the process is chemical or dissolving.
Quick questions
If this is the bit you searched
What is the difference between exothermic and endothermic GCSE Chemistry?
Exothermic reactions transfer energy to the surroundings and the temperature rises. Endothermic reactions take in energy from the surroundings and the temperature falls.
Is combustion exothermic or endothermic?
Exothermic. Fuels such as methane transfer energy to the surroundings when they burn: CH₄ + 2O₂ → CO₂ + 2H₂O, ΔH negative.
What does a negative ΔH mean?
The reaction is exothermic: products are lower in energy than reactants and energy is transferred to the surroundings.
Give an example of an endothermic reaction.
Thermal decomposition of calcium carbonate, photosynthesis, or the dissolving of ammonium nitrate in a cold pack.