Energy transfer during exothermic and endothermic reactions: An exothermic reaction is one that transfers energy to the surroundings so the temperature of the surroundings increases.

The idea

An exothermic reaction transfers energy from the reacting system to the surroundings, causing the surroundings to warm up. The products have less chemical energy than the reactants, and the difference is released, often as heat. A familiar example is the combustion of fuels such as methane, where the surroundings become hot. Another is the reaction of an acid with an alkali, which warms the mixture. In contrast, endothermic reactions absorb energy from the surroundings, so the surroundings cool. To identify an exothermic reaction, measure the temperature change of the surroundings: an increase shows energy is being transferred out of the system. Energy level diagrams show products at a lower energy level than reactants, with the energy gap equal to the energy transferred.

What earns the marks

• Defines an exothermic reaction as one that transfers energy to the surroundings.
• States that the temperature of the surroundings increases during an exothermic reaction.
• Links the temperature increase to energy transfer from the reaction to the surroundings.
• Gives a named example of an exothermic reaction, such as combustion of methane or neutralisation of an acid with an alkali.
• Explains that in an exothermic reaction the products have less energy than the reactants, and the energy difference is transferred to the surroundings.
• Interprets an energy level diagram for an exothermic reaction, identifying products at a lower energy level than reactants.

Common mistakes and how to put them right

• Saying the temperature of the reaction mixture itself increases rather than the surroundings.

  Put it right: The surroundings increase in temperature because energy is transferred to them.

• Confusing exothermic with endothermic.

  Put it right: Exothermic transfers energy to the surroundings (temperature rises); endothermic takes in energy (temperature falls).

• Thinking energy is created in an exothermic reaction.

  Put it right: Energy is conserved; it is transferred from the reactants to the surroundings.

• Assuming all reactions that release gas are exothermic.

  Put it right: Exothermicity depends on energy transfer, not gas production.

Exam tips

• When describing an exothermic reaction, always mention that energy is transferred to the surroundings and that this causes a temperature increase.
• Use a named example, such as burning methane or neutralisation, to support your explanation.
• If given temperature data, state that a rise in temperature indicates an exothermic reaction because energy is transferred to the surroundings.
• Avoid saying 'heat is given out' without specifying that it is transferred to the surroundings; use precise language.