AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
The energy change of reactions (HT only): During a chemical reaction:
This statement introduces the higher-tier treatment of energy changes during a chemical reaction.
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
This statement introduces the higher-tier treatment of energy changes during a chemical reaction. Energy is transferred when bonds in reactants are broken and new bonds in products are formed. Breaking bonds takes in energy from the surroundings, while forming bonds releases energy. The overall energy change is the difference between energy needed to break bonds and energy released when new bonds form. If more energy is released than taken in, the reaction is exothermic and the surroundings warm up. If more energy is taken in, it is endothermic and the surroundings cool.
For example: Burning methane is exothermic because the bonds formed release more energy than was needed to break the bonds in methane and oxygen.
What earns the marks
- Bond breaking is endothermic because it takes in energy from the surroundings.
- Bond forming is exothermic because it releases energy to the surroundings.
- The overall energy change is calculated from the energy required to break bonds in reactants minus the energy released when bonds form in products.
- An exothermic reaction releases more energy than it takes in, so the surroundings warm up.
- An endothermic reaction takes in more energy than it releases, so the surroundings cool down.
- Energy is conserved overall; the energy transferred is not created or destroyed but moved between the reaction and its surroundings.
Common mistakes and how to put them right
Saying that breaking bonds releases energy.
Put it right: Breaking bonds always takes in energy, while forming bonds releases energy.
Reversing the subtraction: overall energy change is energy in for bond breaking minus energy out for bond forming, not the other way round.
Assuming that an exothermic reaction needs no activation energy.
Put it right: It still needs an initial energy input to break bonds before new bonds form.
Exam tips
- Write the bond energy calculation clearly, showing energy taken in and energy released before subtracting.
- Use the sign and direction of temperature change to justify whether a reaction is exothermic or endothermic.
- Check that every bond broken and every bond formed has been counted, including coefficients in the balanced equation.
- Remember that calculating overall energy changes using bond energies is assessed at Higher Tier only.
Try the lesson
The start of the The energy change of reactions (HT only): During a chemical reaction: lesson, made from checked teaching notes for this specification point.
Have a go first
During a chemical reaction, how does the energy associated with bond breaking and bond making determine if a reaction is exothermic?
Listen
This statement introduces the higher-tier treatment of energy changes during a chemical reaction. Energy is transferred when bonds in reactants are broken and new bonds in products are formed. Breaking bonds takes in energy from the surroundings, while forming bonds releases energy. The overall energy change is the difference between energy needed to break bonds and energy released when new bonds form. If more energy is released than taken in, the reaction is exothermic and the surroundings warm up. If more energy is taken in, it is endothermic and the surroundings cool. For example, burning methane is exothermic because the bonds formed release more energy than was needed to break the bonds in methane and oxygen. A strong answer earns marks for these points. Bond breaking is endothermic because it takes in energy from the surroundings. Bond forming is exothermic because it releases energy to the surroundings. The overall energy change is calculated from the energy required to break bonds in reactants minus the energy released when bonds form in products. An exothermic reaction releases more energy than it takes in, so the surroundings warm up. Exam tip: Write the bond energy calculation clearly, showing energy taken in and energy released before subtracting.
See it
Key words
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Bond breaking
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Bond forming
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Exothermic reaction
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Endothermic reaction
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See every definition
- Bond breaking
- Takes in energy from the surroundings, so it is endothermic.
- Bond forming
- Releases energy to the surroundings, so it is exothermic.
- Exothermic reaction
- Releases more energy than it takes in, so the surroundings warm up.
- Endothermic reaction
- Takes in more energy than it releases, so the surroundings cool down.
Label the diagram
Some boxes have lost their names. Pick the right name for each number.
1Box 1
2Box 2
3Box 3
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