AQA · GCSE · Combined Science

Course version: 8464 · Version from Sept 2018

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

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

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

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

Every reaction involves breaking bonds in the reactants and forming bonds in the products.

4 key terms

Study notes

From the checked teaching notes for this specification point.

The idea

For example: In methane combustion (CH₄ + 2O₂ → CO₂ + 2H₂O), the bonds formed in CO₂ and H₂O release more energy than is needed to break the bonds in CH₄ and O₂.

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 exothermic reaction, energy released from bond formation is greater than energy needed for bond breaking.
The overall energy change is negative because more energy is released than absorbed.
Energy is transferred to the surroundings, so the temperature of the surroundings typically rises.
Higher Tier candidates must compare total energy released versus total energy needed, not just one bond.

Common mistakes and how to put them right

Thinking that exothermic reactions release energy because bonds break.
Put it right: Breaking bonds absorbs energy, and it is bond formation that releases energy.
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 exothermic reactions do not need any energy input.
Put it right: An initial energy input is needed to break bonds, but overall more energy is released than absorbed.

Exam tips

Use the phrase 'energy released from forming bonds is greater than energy needed to break bonds' to make the comparison explicit.
Link the bond-energy comparison to the sign of the overall energy change and to the observed temperature rise 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 exothermic reaction, the energy released from forming new bonds is greater than the energy needed to break existing bonds. lesson, made from checked teaching notes for this specification point.

Have a go first

A reaction releases energy to the surroundings. Which statement describes the bond energies in this reaction?

Tip: press 1 to 4 to choose

Listen

Every reaction involves breaking bonds in the reactants and forming bonds in the products. Breaking a bond requires an input of energy, while forming a bond releases energy. In an exothermic reaction, the total energy released when new bonds form is greater than the total energy needed to break the existing bonds. The overall energy change is therefore negative, transferring energy to the surroundings and warming them. For example, in methane combustion (CH₄ + 2O₂ → CO₂ + 2H₂O), the bonds formed in CO₂ and H₂O release more energy than is needed to break the bonds in CH₄ and O₂. 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 exothermic reaction, energy released from bond formation is greater than energy needed for bond breaking. The overall energy change is negative because more energy is released than absorbed. A common slip: Thinking that exothermic reactions release energy because bonds break. Instead, breaking bonds absorbs energy, and it is bond formation that releases energy. Exam tip: Use the phrase 'energy released from forming bonds is greater than energy needed to break bonds' to make the comparison explicit.

See it

A top-down flowchart. One branch starts at a node labelled 'Breaking bonds in reactants', which leads to 'Energy needed from the surroundings'. A second branch starts at 'Forming bonds in products', which leads to 'Energy released to the surroundings'. Both of these lead into a node labelled 'Compare total energy needed with total energy released'. That leads to 'Exothermic: energy released is greater than energy needed', then to 'Overall energy change is negative', and finally to 'Energy transferred to the surroundings, temperature rises'. — It shows that breaking bonds needs energy and forming bonds releases energy, and that in an exothermic reaction the energy released is greater, giving a negative overall energy change and a temperature rise.

Key words

Say what each word means before you turn the card.

Bond breaking and bond formation

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Exothermic and endothermic reactions

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Energy transfer to and from the surroundings

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Overall energy change of a reaction

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See every definition

Bond breaking and bond formation: Every reaction involves breaking bonds in the reactants and forming bonds in the products; breaking a bond requires an input of energy, while forming a bond releases energy.
Exothermic and endothermic reactions: In an exothermic reaction, the energy released from forming new bonds is greater than the energy needed to break existing bonds.
Energy transfer to and from the surroundings: Breaking bonds in reactants requires an input of energy from the surroundings, and forming bonds in products releases energy to the surroundings.
Overall energy change of a reaction: The difference between energy released and energy absorbed; in an exothermic reaction it is negative because more energy is released than absorbed.

Label the diagram

Some boxes have lost their names. Pick the right name for each number.

The diagram again, with 3 boxes numbered instead of named. A top-down flowchart. One branch starts at a node labelled 'Breaking bonds in reactants', which leads to 'Energy needed from the surroundings'. A second branch starts at 'Forming bonds in products', which leads to 'Energy released to the surroundings'. Both of these lead into a node labelled 'Compare total energy needed with total energy released'. That leads to 'Exothermic: energy released is greater than energy needed', then to 'Overall energy change is negative', and finally to 'Energy transferred to the surroundings, temperature rises'.

1Box 1
2Box 2
3Box 3

0 of 3 labelled

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Study notes

The idea

What earns the marks

Common mistakes and how to put them right

Exam tips

Try the lesson

Have a go first

Listen

See it

Key words

Label the diagram

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

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