Energy changes: exothermic, endothermic and bond energies — AQA GCSE Chemistry revision

Exothermic and endothermic reactions

• An exothermic reaction transfers energy to the surroundings, so the temperature of the surroundings increases.
• An endothermic reaction takes in energy from the surroundings, so the temperature of the surroundings decreases.
• Combustion, neutralisation and oxidation reactions are usually exothermic.
• Thermal decomposition and the reaction between citric acid and sodium hydrogencarbonate are typical endothermic reactions.

Reaction profiles

• A reaction profile shows the energy of reactants and products, plus the activation energy needed to start the reaction.
• In an exothermic reaction, the products have less energy than the reactants, so the profile line drops overall.
• In an endothermic reaction, the products have more energy than the reactants, so the profile line rises overall.
• Activation energy is the minimum energy needed for particles to react when they collide.

Key terms

Exothermic

A reaction that transfers energy to the surroundings, increasing temperature.

Endothermic

A reaction that takes in energy from the surroundings, decreasing temperature.

Activation energy

The minimum energy needed for a reaction to occur.

Reaction profile

A diagram showing the energy changes during a reaction, including activation energy.

Bond breaking

An endothermic process requiring energy input.

Bond forming

An exothermic process that releases energy.

Common exam traps

• An exothermic reaction releasing energy does not mean it has no activation energy — energy is still needed to start it.
• Don't mix up which line goes up or down in a reaction profile — exothermic drops overall, endothermic rises overall.
• Breaking bonds absorbs energy (endothermic), it does not release energy — a very common mix-up.

Practice questions with answers

1. 1. Which type of reaction releases energy to the surroundings?

   • • Endothermic
   • • Exothermic
   • • Neutral
   • • Catalytic

   Answer: Exothermic

   Exothermic reactions transfer energy out to the surroundings.

2. 2. What happens to the temperature of the surroundings in an endothermic reaction?

   • • Increases
   • • Decreases
   • • Stays the same
   • • Doubles

   Answer: Decreases

   Endothermic reactions absorb energy from the surroundings, cooling them.

3. 3. Give one example of an everyday exothermic reaction.

   Answer: Combustion (burning fuel), e.g. a bonfire

   Combustion releases large amounts of energy as heat and light.

4. 4. What is activation energy?

   • • The total energy released in a reaction
   • • The minimum energy needed for a reaction to start
   • • The energy of the products only
   • • The energy absorbed by the surroundings

   Answer: The minimum energy needed for a reaction to start

   Activation energy is the energy barrier particles must overcome to react.

5. 5. Is breaking bonds exothermic or endothermic?

   Answer: Endothermic

   Breaking bonds requires an input of energy.

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