Exothermic Reactions

Energy changes are at the heart of chemistry. Every chemical reaction involves a transfer of energy, and understanding whether energy is released or absorbed is a fundamental part of the Edexcel GCSE Chemistry (1CH0) specification. In this lesson you will learn what exothermic reactions are, explore key examples, understand reaction profile diagrams for exothermic reactions, and carry out a core practical measuring the temperature change during a neutralisation reaction.

---

What Is an Exothermic Reaction?

An exothermic reaction is a chemical reaction that transfers energy to the surroundings. The surroundings get hotter and there is a measurable temperature increase.

In an exothermic reaction:

• The energy released by forming new bonds in the products is greater than the energy needed to break bonds in the reactants.
• The excess energy is transferred to the surroundings, usually as heat (thermal energy).
• The products have less energy than the reactants.
• The overall energy change (ΔH) is negative.

---

Examples of Exothermic Reactions

Reaction type	Example	Observation
Combustion	Burning methane: CH₄ + 2O₂ → CO₂ + 2H₂O	Flame produced, surroundings get very hot
Neutralisation	HCl + NaOH → NaCl + H₂O	Solution gets warmer
Oxidation	Rusting: 4Fe + 3O₂ → 2Fe₂O₃ (slow)	Gradual heat release over time
Displacement	Mg + CuSO₄ → MgSO₄ + Cu	Solution gets noticeably hot
Respiration	C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O	Energy released for life processes

Everyday Uses of Exothermic Reactions

• Hand warmers — contain iron powder that oxidises slowly when exposed to air, releasing heat over several hours. The reaction is: 4Fe + 3O₂ → 2Fe₂O₃.
• Self-heating cans — contain calcium oxide and water in separate compartments. When mixed, the exothermic reaction CaO + H₂O → Ca(OH)₂ heats the contents.
• Combustion in engines — petrol and diesel burn to release energy that powers vehicles.
• Camping stoves — combustion of propane or butane releases energy for cooking.

Exam tip: Neutralisation reactions are always exothermic. This is a favourite exam question — if you see an acid reacting with an alkali, the temperature will always rise.

---

Reaction Profile Diagram for Exothermic Reactions

A reaction profile (also called an energy profile diagram) shows how the energy of the reactants and products changes during a reaction.

For an exothermic reaction, the reaction profile looks like this:

Description of the Diagram

1. The reactants are shown at a higher energy level on the left side of the diagram.
2. The energy rises to a peak — this is the activation energy (Eₐ), the minimum energy needed to start the reaction.
3. The energy then falls to a lower level on the right — this is the energy of the products.
4. The difference between the energy of the reactants and the energy of the products is the overall energy change (ΔH).
5. Because the products are at a lower energy than the reactants, energy has been released to the surroundings.
6. ΔH is negative for exothermic reactions.

Key Labels on the Diagram

Label	What it shows
Reactants energy level	Starting energy of the reaction
Products energy level	Final energy of the reaction (lower than reactants for exothermic)
Activation energy (Eₐ)	The energy barrier that must be overcome — measured from reactants to the peak
Overall energy change (ΔH)	The difference between reactants and products — negative for exothermic

Exam tip: When drawing or labelling a reaction profile, always include: the labels "Reactants" and "Products," an arrow showing the activation energy (Eₐ) from the reactants to the peak, and an arrow showing ΔH from the reactants to the products. Make sure ΔH points downward for exothermic reactions.

---

Core Practical: Measuring the Temperature Change During Neutralisation

This practical is required by the Edexcel specification and is commonly examined.

Aim

To investigate the temperature change that occurs when an acid reacts with an alkali (neutralisation), and to confirm that neutralisation is exothermic.

Equipment

• Polystyrene cup (acts as an insulating container to reduce heat loss)
• Lid with a hole for a thermometer
• Thermometer (or temperature probe)
• Measuring cylinder
• Dilute hydrochloric acid (e.g. 1.0 mol/dm³)
• Dilute sodium hydroxide solution (e.g. 1.0 mol/dm³)

Method

1. Measure 25 cm³ of dilute sodium hydroxide solution using a measuring cylinder and pour it into a polystyrene cup.
2. Record the initial temperature of the sodium hydroxide solution using a thermometer.
3. Measure 25 cm³ of dilute hydrochloric acid using a separate measuring cylinder.
4. Record the initial temperature of the acid. (Both solutions should be at approximately the same starting temperature.)
5. Add the acid to the alkali in the polystyrene cup.
6. Stir gently with the thermometer.
7. Record the maximum temperature reached.
8. Calculate the temperature change: ΔT = maximum temperature − initial temperature.

Expected Results

Measurement	Value
Initial temperature of NaOH	20.0 °C
Initial temperature of HCl	20.0 °C
Maximum temperature after mixing	26.5 °C
Temperature change (ΔT)	+6.5 °C

The temperature increases, confirming that neutralisation is an exothermic reaction.

Variables

Variable	Details
Independent variable	Whether or not a reaction takes place (or you could vary concentration/volume)
Dependent variable	Temperature change
Control variables	Volume of acid and alkali, concentration, starting temperature, type of cup

Sources of Error and Improvements

Source of Error	Improvement
Heat is lost to the surroundings through the cup walls	Use better insulation (e.g. a lid, or a vacuum flask)
Heat is lost to the air	Put a lid on the cup with a hole for the thermometer
The thermometer may not be precise enough	Use a digital temperature probe for more accurate readings
Not all the acid may have been added before recording	Pour quickly and start recording immediately