Exothermic and Endothermic Reactions

Every 0620 series contains at least one question giving a temperature reading before and after a reaction, then asking whether it is exothermic or endothermic “and explain your answer” (2 to 3 marks). Candidates who know the definitions still drop the explanation mark by saying the reaction gets hot rather than the surroundings. The direction of energy transfer is the whole topic; get the wording right and the marks follow.

The two definitions (Core)

	Exothermic	Endothermic
Energy transfer	Thermal energy transferred to the surroundings	Thermal energy taken in from the surroundings
Temperature of surroundings	Increases	Decreases
ΔH sign (S)	Negative	Positive
Stock examples	Combustion, neutralisation, respiration, hand warmers	Thermal decomposition, photosynthesis, sports injury cold packs

“Surroundings” means everything that is not the reacting chemicals: the solvent, the beaker, the thermometer, the air. The thermometer sits in the surroundings. That is why an exothermic reaction makes the reading rise.

Reading the question’s data

A typical data line: “the temperature of the mixture rose from 21 °C to 35 °C.” The reasoning chain worth both marks: the temperature increased (1), so thermal energy was transferred to the surroundings and the reaction is exothermic (1). The chain runs the same way in reverse: temperature fell, energy taken in from the surroundings, endothermic.

Dissolving also appears here. Dissolving ammonium nitrate in water cools the solution (endothermic); dissolving anhydrous copper(II) sulfate warms it (exothermic). The syllabus treats these as fair game even though dissolving is a physical change: the energy logic is identical, and the physical/chemical distinction itself is tested under Physical and Chemical Change.

ΔH: the enthalpy change (Supplement)

Extended candidates attach a symbol and a sign. The enthalpy change, ΔH, is the thermal energy transferred during a reaction:

• Exothermic: ΔH is negative because the products hold less chemical energy than the reactants, and the difference left as heat.
• Endothermic: ΔH is positive because the products hold more chemical energy, and the difference came from the surroundings.

A useful self-check: the sign tracks the system, not the thermometer. Energy leaving the chemicals (negative ΔH) is energy arriving in the surroundings (temperature up). Students who anchor the sign to the thermometer reading reverse it under pressure.

The sign convention becomes visual on energy level diagrams, where ΔH is drawn as an arrow from reactants down to products (exothermic) or up to products (endothermic). The bond-level explanation (breaking bonds takes energy in, making bonds gives energy out) belongs to Bond Energy Calculations and decides exo vs endo by which side is bigger.

Where this appears beyond Section 5

Energetics language leaks into half the syllabus. Combustion of fuels: exothermic. Neutralisation of acid by alkali: exothermic, and a favourite Paper 5/6 experiment with a polystyrene cup. Thermal decomposition of calcium carbonate in the lime kiln: endothermic, which is why the kiln needs constant heating. Each appearance tests the same two sentences, so the definitions repay their learning time more than almost any other pair in the course. The full topic structure is on the Chemical Energetics pillar.

Worked exam question

A student adds zinc powder to copper(II) sulfate solution in a polystyrene cup. The temperature rises from 22.0 °C to 31.5 °C. (a) State whether the reaction is exothermic or endothermic. Explain your answer. [2] (b) (Extended) State the sign of ΔH for this reaction. [1] (c) Suggest why a polystyrene cup is used rather than a copper can. [1]

Model answer: (a) Exothermic (1); the temperature of the surroundings increased, showing thermal energy was transferred to the surroundings (1). (b) Negative (1). (c) Polystyrene is an insulator, so less thermal energy is lost to the air, making the temperature change more accurate (1).

Mark-by-mark: (a) splits cleanly: the classification is one mark, the energy-transfer sentence the other. “Because it got hotter” alone is usually allowed for the second mark only if tied to energy released; the safe version names the transfer to the surroundings. (b) is a single recall link: exothermic ↔ negative. (c) rewards the word “insulator”; “keeps the heat in” without it is borderline.

The mistakes that cost marks

1. “The reaction takes in heat so it gets hot.” Backwards. Taking in heat is endothermic and the surroundings cool down.
2. Wrong ΔH sign. Exothermic = negative, always. Anchor it to energy exiting the system.
3. “The reaction is exothermic because it is combustion.” Circular in a data question: the evidence is the temperature rise, so quote it.
4. Confusing the system with the surroundings. The solution in the cup counts as surroundings; the reacting particles are the system.

How examiners want it phrased

Student wording	Mark-scheme wording
”It gives off heat"	"Thermal energy is transferred to the surroundings"
"It gets cold so it’s endothermic"	"The temperature of the surroundings decreases because thermal energy is taken in from the surroundings"
"Energy is lost"	"Energy is transferred to the surroundings; ΔH is negative"
"It needs heat to keep going"	"The reaction is endothermic, so thermal energy must be continuously supplied”

One template covers every variant: direction of temperature change → direction of energy transfer → classification (→ ΔH sign). Students who write that chain never lose these marks, and building automatic chains like it is the core of what we do in a free 1-hour trial lesson with a Chemistry specialist.

Test yourself

Write your three answers first; click to check each one against the mark scheme.

Q1 (2 marks). A student mixes citric acid solution with sodium hydrogencarbonate. The temperature falls from 24 °C to 18 °C. State whether the reaction is exothermic or endothermic and explain your answer.

Show answer

• endothermic [1] • thermal energy is taken in from the surroundings, so the temperature of the surroundings decreases [1]

Q2 (2 marks). Define the term exothermic reaction and give one example of an exothermic reaction.

Show answer

• a reaction that transfers thermal energy to the surroundings, so the temperature of the surroundings increases [1] • any one of: combustion, neutralisation, respiration [1]

Q3 (2 marks). (Extended) The decomposition of calcium carbonate has ΔH = +178 kJ/mol. State whether the reaction is exothermic or endothermic, and state what must be done to keep the reaction going in a lime kiln.

Show answer

• ΔH is positive, so the reaction is endothermic [1] • thermal energy must be continuously supplied / the kiln must be kept heated [1]