5 min readConcept sheetPart I (1 mark), Part II (2 marks: define with examples) and Part III (4 marks: distinguish with an activity)

Exothermic and endothermic reactions

Exothermic reactions give out heat, like burning and respiration. Endothermic reactions take in heat, like decomposing limestone and photosynthesis.

ExothermicReleases heat to the surroundings: CaO + H₂O → Ca(OH)₂ + heat

EndothermicAbsorbs heat from the surroundings: CaCO₃ + heat → CaO + CO₂

RespirationC₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy (exothermic)

Photosynthesis6CO₂ + 6H₂O + light energy → C₆H₁₂O₆ + 6O₂ (endothermic)

Exothermic vs endothermic

Exothermic (exo = out)Heat is released. The surroundings get warmer. Write '+ heat' on the product side.

Endothermic (endo = in)Heat is absorbed. The surroundings get cooler, or heat must be supplied all the time. Write '+ heat' on the reactant side.

Why: breaking bonds in reactants takes energy; making new bonds releases energy. Whichever is larger decides the type.

Examples to quote

Reaction	Equation	Type
Burning of methane	CH₄ + 2O₂ → CO₂ + 2H₂O + heat	Exothermic
Respiration	C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy	Exothermic
Slaking of quicklime	CaO + H₂O → Ca(OH)₂ + heat	Exothermic
Neutralisation	NaOH + HCl → NaCl + H₂O + heat	Exothermic
Decomposition of limestone	CaCO₃ + heat → CaO + CO₂	Endothermic
Photosynthesis	6CO₂ + 6H₂O + light energy → C₆H₁₂O₆ + 6O₂	Endothermic
Nitrogen and oxygen in lightning	N₂ + O₂ + heat → 2NO	Endothermic

Activity: a hot tube and a cold tube

1
Take water in two test tubes, A and B. Note the temperature of each with a thermometer.
2
To A, add a small spoon of quicklime (calcium oxide) carefully. Stir with a glass rod.
3★
Observe: the temperature rises and the tube feels hot. The reaction is exothermic.
4
To B, add a spoon of ammonium chloride and stir.
5★
Observe: the temperature falls and the tube feels cold. Dissolving ammonium chloride absorbs heat (an endothermic process).

★ key step

In real life

Respiration, an exothermic reaction, keeps your body warm. Instant cold packs for sports sprains work because the salt inside takes in heat as it dissolves. Plants run photosynthesis, an endothermic reaction, on sunlight.

Exam angle

1 marks: “Photosynthesis is an ______ reaction.” Endothermic, because it absorbs light energy.

2 marks: “What are exothermic reactions? Give two examples.” Reactions that release heat. Examples: burning of methane, CH₄ + 2O₂ → CO₂ + 2H₂O + heat; slaking of lime, CaO + H₂O → Ca(OH)₂ + heat.

4 marks: “Distinguish between exothermic and endothermic reactions with examples. Describe an activity to show each.” Heat released vs absorbed, where '+ heat' is written, two examples each. Activity: quicklime in water gets hot; ammonium chloride in water gets cold.

Remember it

EXothermic: heat EXits. ENdothermic: heat ENters.