Exothermic and Endothermic Reactions | Chemistry

📖 In-Depth Theory

Exothermic Reactions

An EXOTHERMIC reaction RELEASES energy to the surroundings — usually as HEAT, sometimes as light or sound.

The products have LESS energy than the reactants — energy is given out to the surroundings.

The TEMPERATURE of the surroundings INCREASES.

EXAMPLES of exothermic reactions:

COMBUSTION — burning fuels (methane, petrol, wood): releases heat and light.

OXIDATION of metals — iron rusting (slow, releases heat gradually).

NEUTRALISATION — acid + alkali → salt + water (temperature rises).

RESPIRATION — glucose + oxygen → CO₂ + water (releases energy in cells).

HANDWARMERS — iron oxidation or sodium acetate crystallisation.

SELF-HEATING CANS — reaction between calcium oxide and water.

Temperature change: ΔT is POSITIVE (temperature rises).

Energy change: ΔH is NEGATIVE for exothermic reactions (energy released).

Endothermic Reactions

An ENDOTHERMIC reaction ABSORBS energy from the surroundings — usually as HEAT.

The products have MORE energy than the reactants — energy is taken in from the surroundings.

The TEMPERATURE of the surroundings DECREASES (the surroundings get cooler).

EXAMPLES of endothermic reactions:

THERMAL DECOMPOSITION — heating calcium carbonate: CaCO₃ → CaO + CO₂.

DISSOLVING ammonium nitrate in water (temperature drops — used in instant cold packs).

PHOTOSYNTHESIS — CO₂ + H₂O → glucose + O₂ (absorbs light energy).

COOKING — baking bread, boiling eggs (energy absorbed to cook food).

INSTANT ICE PACKS — ammonium nitrate dissolving in water.

CITRIC ACID + SODIUM BICARBONATE reaction in baking powders.

Temperature change: ΔT is NEGATIVE (temperature falls).

Energy change: ΔH is POSITIVE for endothermic reactions (energy absorbed).

Measuring Energy Changes

CALORIMETRY is the method for measuring heat energy changes in reactions.

SIMPLE CALORIMETRY:

Reaction takes place in a polystyrene cup (good insulator).

Measure the temperature change (ΔT) of the water or solution.

Calculate energy change: Q = mcΔT

Where: m = mass of solution (g), c = specific heat capacity (4.18 J/g°C for water), ΔT = temperature change (°C).

Exothermic: temperature rises → ΔT is positive → Q is positive (heat released).

Endothermic: temperature falls → ΔT is negative → Q is negative (heat absorbed).

SOURCES OF ERROR in calorimetry:

HEAT LOSS to the surroundings and container → underestimates energy released.

INCOMPLETE combustion → less heat released than expected.

Using polystyrene cup reduces heat loss compared to a beaker.

Using these measurements, scientists can calculate the ENERGY PER MOLE for a reaction — the molar enthalpy change.

⚠️ Common Mistake

EXOTHERMIC reactions make the surroundings HOTTER (temperature rises). ENDOTHERMIC reactions make the surroundings COLDER (temperature falls). The reaction itself releases or absorbs energy — but the SURROUNDINGS show the opposite effect to what the reaction does. Also: ΔH is NEGATIVE for exothermic (releases energy) — students often think negative means the reaction is 'losing energy' in a bad way — it just means energy is released.

📐 Variables

QHeat energy transferred (Q) is measured in joules (J)

mMass of solution (m) is measured in grams (g)

cSpecific heat capacity (c) is measured in J/g°C (J/g°C)

ΔTTemperature change (ΔT) is measured in °C (°C)

📐 Key Equations

Q = m × c × ΔT

📌 Key Note

Exothermic: releases heat → temperature rises → examples: combustion, neutralisation, respiration. Endothermic: absorbs heat → temperature falls → examples: thermal decomposition, dissolving ammonium nitrate, photosynthesis. Q = mcΔT for calorimetry.

🎯 Matching Activity — Exothermic or Endothermic?

Sort each reaction into exothermic or endothermic. — drag the symbols on the right to match the component names on the left.

Exothermic

Drop here

Endothermic

Drop here

Exothermic

Drop here

Endothermic

Drop here

Exothermic

Drop here

Endothermic

Drop here

Photosynthesis — absorbs light energy to make glucose

Burning methane in air — releases heat and light

Thermal decomposition of calcium carbonate — absorbs heat energy

Respiration — glucose + oxygen releases energy in cells

Dissolving ammonium nitrate in water — temperature of solution falls

Neutralisation of NaOH with HCl — temperature of solution rises

⚽ FIFA Worked Examples

Calorimetry Calculation

50 g of water is heated by a reaction. The temperature rises from 20°C to 34°C. Calculate the energy released. (c = 4.18 J/g°C)

F

Q = m × c × ΔT

I

m = 50 g, c = 4.18 J/g°C, ΔT = 34 − 20 = 14°C

F

Q = 50 × 4.18 × 14 = 50 × 58.52

A

Q = 2926 J ≈ 2930 J (exothermic — temperature rose)

🧪 Required Practical

🔬 RP5 (Chemistry) — Investigate the variables that affect temperature changes in reactions: e.g. neutralisation of NaOH with HCl, or dissolving ammonium chloride in water.

Know the method, variables, equipment and how to analyse results.

🎯 Test Yourself

Question 1 of 2

1. A hand warmer gets hot when activated. What type of reaction is occurring inside?

2. 100 g of solution is heated in a calorimetry experiment. Temperature rises from 22°C to 30°C. Using c = 4.18 J/g°C, how much energy was released?

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🧪 Exothermic and Endothermic Reactions

Exothermic Reactions

Endothermic Reactions

Measuring Energy Changes

🔬 RP5 (Chemistry) — Investigate the variables that affect temperature changes in reactions: e.g. neutralisation of NaOH with HCl, or dissolving ammonium chloride in water.

Mr. Badmus AI