AQA · GCSE · Combined Science

Course version: 8464 · Version from Sept 2018

The energy change of reactions (HT only): In an endothermic reaction, the energy needed to break existing bonds is greater than the energy released from forming new bonds.

Q: How do I revise The energy change of reactions (HT only): In an endothermic reaction, the energy needed to break existing bonds is greater than the energy released from forming new bonds. for GCSE?

Read the notes on The energy change of reactions (HT only): In an endothermic reaction, the energy needed to break existing bonds is greater than the energy released from forming new bonds., then test yourself without looking. Answering questions and checking what you missed builds recall faster than re-reading.

Q: Is this The energy change of reactions (HT only): In an endothermic reaction, the energy needed to break existing bonds is greater than the energy released from forming new bonds. guide aligned to the AQA specification?

Yes. This guide is linked to the The energy change of reactions (HT only): In an endothermic reaction, the energy needed to break existing bonds is greater than the energy released from forming new bonds. topic in the AQA GCSE specification.

In any reaction, energy must be supplied to break bonds in the reactants, and energy is released when new bonds form in the products.

Study notes

From the checked teaching notes for this specification point.

The idea

For example: In the thermal decomposition of calcium carbonate (CaCO₃ → CaO + CO₂), the energy needed to break bonds in CaCO₃ is greater than the energy released when bonds form in CaO and CO₂.

What earns the marks

Breaking bonds in reactants requires an input of energy from the surroundings.
Forming bonds in products releases energy to the surroundings.
In an endothermic reaction, energy needed to break existing bonds is greater than energy released from forming new bonds.
The overall energy change is positive because more energy is absorbed than released.
Energy is taken in from the surroundings, so the temperature of the surroundings typically falls.
Higher Tier candidates must compare total energy needed versus total energy released, not just one bond.

Common mistakes and how to put them right

Thinking that endothermic reactions release energy because new bonds form.
Put it right: Bond formation releases energy, but in endothermic reactions the energy needed to break bonds is greater overall.
Comparing only one bond broken with one bond formed.
Put it right: Compare the total energy needed to break all reactant bonds with the total energy released forming all product bonds.
Stating that endothermic reactions absorb no energy at all.
Put it right: They absorb more energy than they release, giving a positive overall energy change.

Exam tips

Use the phrase 'energy needed to break bonds is greater than energy released from forming bonds' to make the comparison explicit.
Link the bond-energy comparison to the sign of the overall energy change and to the observed temperature fall of the surroundings.
As a Higher Tier student, when given bond energies, calculate total energy absorbed and total energy released separately before comparing them.

Try the lesson

The start of the The energy change of reactions (HT only): In an endothermic reaction, the energy needed to break existing bonds is greater than the energy released from forming new bonds. lesson, made from checked teaching notes for this specification point.

Have a go first

A reaction is endothermic. Which statement describes the bond energies in this reaction?

Tip: press 1 to 4 to choose

Listen

In any reaction, energy must be supplied to break bonds in the reactants, and energy is released when new bonds form in the products. In an endothermic reaction, the total energy needed to break the existing bonds is greater than the total energy released when the new bonds form. The overall energy change is therefore positive, taking in energy from the surroundings and cooling them. For example, in the thermal decomposition of calcium carbonate (CaCO₃ → CaO + CO₂), the energy needed to break bonds in CaCO₃ is greater than the energy released when bonds form in CaO and CO₂. A strong answer earns marks for these points. Breaking bonds in reactants requires an input of energy from the surroundings. Forming bonds in products releases energy to the surroundings. In an endothermic reaction, energy needed to break existing bonds is greater than energy released from forming new bonds. The overall energy change is positive because more energy is absorbed than released. A common slip: Thinking that endothermic reactions release energy because new bonds form. Instead, bond formation releases energy, but in endothermic reactions the energy needed to break bonds is greater overall. Exam tip: Use the phrase 'energy needed to break bonds is greater than energy released from forming bonds' to make the comparison explicit.

See it

It shows how the energy needed to break bonds and the energy released forming bonds are compared to give a positive overall energy change and a temperature fall in an endothermic reaction.

Label the diagram

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3Box 3

0 of 3 labelled

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