Exothermic Reactions

This lesson covers exothermic reactions — reactions that release energy to the surroundings — including examples, temperature changes, everyday uses and energy profile diagrams as required by the Edexcel GCSE Combined Science specification (1SC0).

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What Is an Exothermic Reaction?

An exothermic reaction is one that transfers energy to the surroundings, usually as heat. The temperature of the surroundings (and the reaction mixture) increases.

Term	Meaning
Exothermic	"Exo" = out; "thermic" = heat. Energy is released
Temperature change	The temperature of the surroundings rises
Energy transfer	From the chemical system to the surroundings

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Examples of Exothermic Reactions

Reaction	Detail
Combustion	Burning fuels in oxygen — e.g. CH₄ + 2O₂ → CO₂ + 2H₂O. The main source of energy for heating and transport
Neutralisation	Acid + alkali → salt + water. Mixing HCl with NaOH causes the temperature to rise
Oxidation (metals)	Reactive metals combining with oxygen — e.g. respiration in cells
Respiration	C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O — energy is released for life processes
Displacement reactions	More reactive metal displacing less reactive metal — often releases heat (e.g. thermit reaction)

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Detecting Exothermic Reactions

You can detect an exothermic reaction in the lab by measuring the temperature change using a thermometer.

Observation	Interpretation
Temperature increases	The reaction is exothermic
The reaction mixture feels warm or hot	Energy is being transferred to the surroundings

Practical Example: Neutralisation

1. Measure 25 cm³ of dilute NaOH in a polystyrene cup.
2. Record the starting temperature.
3. Add 25 cm³ of dilute HCl.
4. Stir and record the maximum temperature reached.
5. The temperature rise shows the reaction is exothermic.

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Reaction Profile for an Exothermic Reaction

A reaction profile (energy level diagram) shows how the energy of the reactants and products compares.

graph TD
    subgraph "Exothermic Reaction Profile"
        R["Reactants<br/>(higher energy)"] -->|"Activation<br/>energy (Eₐ)"| T["Transition state<br/>(energy peak)"]
        T --> P["Products<br/>(lower energy)"]
    end

    style R fill:#e74c3c,color:#fff
    style T fill:#f39c12,color:#fff
    style P fill:#2980b9,color:#fff

Key features

Feature	Description
Reactants	At a higher energy level
Products	At a lower energy level
Activation energy (Eₐ)	The energy barrier from reactants to the top of the curve
Overall energy change (ΔH)	Negative — energy is released to the surroundings
Energy released	Equals the difference in energy between reactants and products

Exam Tip: In an exothermic reaction profile, the products are lower than the reactants. The arrow for ΔH points downwards. Always label the activation energy (Eₐ) as the difference from reactants to the peak, not to the products.

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Everyday Uses of Exothermic Reactions

Hand Warmers

Reusable hand warmers contain a supersaturated solution of sodium acetate. When a small metal disc is clicked, crystallisation begins — an exothermic process that releases heat.

Disposable hand warmers contain iron powder, salt, water, activated charcoal and vermiculite. The iron undergoes slow oxidation (rusting), releasing heat over several hours.

Type	How it works	Reusable?
Crystallisation hand warmer	Supersaturated sodium acetate crystallises, releasing heat	Yes — boil to re-dissolve
Iron powder hand warmer	Iron is oxidised (rusts) slowly, releasing heat	No — the iron is used up

Self-Heating Cans

Some self-heating cans (e.g. for coffee) contain calcium oxide in a separate compartment. When water is added, the reaction CaO + H₂O → Ca(OH)₂ is exothermic and heats the drink.

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Energy and Bond Breaking/Making

In an exothermic reaction, the energy released when new bonds form in the products is greater than the energy needed to break bonds in the reactants.

Process	Energy
Breaking bonds	Requires energy (endothermic)
Making bonds	Releases energy (exothermic)
Exothermic overall	Energy released making bonds > energy required breaking bonds

Exam Tip: Bond breaking is always endothermic, and bond making is always exothermic. In an exothermic reaction overall, more energy is released making new bonds than is needed to break old bonds.

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Summary

• Exothermic reactions release energy to the surroundings.
• The temperature of the surroundings increases.
• Examples: combustion, neutralisation, respiration, displacement reactions.
• On a reaction profile, the products are lower than the reactants and ΔH is negative.
• Energy released forming new bonds is greater than energy needed to break old bonds.
• Everyday uses include hand warmers and self-heating cans.

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Extended Worked Example 1: Bond Energy Calculation Confirming Exothermic

Question: Use bond energies to show that the combustion of hydrogen, 2H₂ + O₂ → 2H₂O, is exothermic. Bond energies (kJ/mol): H–H 436, O=O 498, O–H 464.

Bonds broken (energy in):

Bond	Number	Energy each	Total
H–H	2	436	872
O=O	1	498	498
		Total	1370 kJ

Bonds made (energy out):

Bond	Number	Energy each	Total
O–H	4	464	1856
		Total	1856 kJ

ΔH = 1370 − 1856 = −486 kJ/mol.

The bond energy released making O–H bonds exceeds the energy needed to break H–H and O=O bonds, so ΔH is negative → exothermic. This confirms the temperature rise observed when hydrogen burns.

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Extended Worked Example 2: Neutralisation Temperature Rise

A student mixes 50 cm³ of 1 mol/dm³ HCl with 50 cm³ of 1 mol/dm³ NaOH. The temperature rises from 21 °C to 28 °C.

Temperature rise: 28 − 21 = 7 °C.

Using q = mcΔT (with mass 100 g of solution and c = 4.2 J/g/°C):

$$q = 100 \times 4.2 \times 7 = 2940 \text{ J} = 2.94 \text{ kJ}$$q=100×4.2×7=2940 J=2.94 kJ

Energy released per mole of water formed ≈ 2.94 ÷ 0.050 = 58.8 kJ/mol — close to the textbook value of ~57 kJ/mol for strong acid + strong alkali neutralisation.

Exam Tip: For neutralisation and displacement experiments, always use a polystyrene cup (low thermal conductivity) to minimise heat loss. Put a lid on to reduce evaporation.

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Comparison Table: Exothermic Reactions Everyday vs Industrial