AQA · GCSE · Chemistry
Course version: 8462 · Version from Sept 2018
The difference between the sum of the energy needed to break bonds in the…
Full specification wording
The energy change of reactions (HT only): The difference between the sum of the energy needed to break bonds in the reactants and the sum of the energy released when bonds in the products are formed is the overall energy change of the reaction.
The overall energy change of a reaction is found by subtracting the energy released when new bonds form from the energy needed to break existing bonds.
Checked against the AQA 8462 specification (opens in a new tab) on .
- Point 3 of 6
- 3 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
The overall energy change of a reaction is found by subtracting the energy released when new bonds form from the energy needed to break existing bonds. In symbols, ΔH = energy needed to break bonds − energy released when bonds form. For H₂ + Cl₂ → 2HCl, breaking bonds needs 436 + 243 = 679 kJ/mol and forming bonds releases 2 × 431 = 862 kJ/mol, so ΔH = 679 − 862 = −183 kJ/mol. A negative result means the reaction is exothermic overall; a positive result means it is endothermic overall. The calculation must use the balanced equation so that the number of each bond is correct, and the sign of the answer must be interpreted, not just calculated.
What earns the marks
- Calculate the total energy needed to break bonds in the reactants by adding the relevant bond energies.
- Calculate the total energy released when bonds form in the products by adding the relevant bond energies.
- Subtract the energy released when bonds form from the energy needed to break bonds to obtain the overall energy change.
- Interpret a negative overall energy change as exothermic and a positive overall energy change as endothermic.
- Include the correct sign and units, usually kJ/mol, with the final answer.
- Use the balanced equation to ensure the number of each bond broken and formed is correct.
Common mistakes and how to put them right
Subtracting in the wrong order, for example energy to break bonds minus energy released when bonds form reversed.
Put it right by using energy needed to break bonds minus energy released when bonds form.
Omitting the sign of the final answer.
Put it right by stating whether the value is positive or negative and linking that sign to endothermic or exothermic behaviour.
Forgetting to multiply bond energies by coefficients in the balanced equation.
Put it right by counting bonds from displayed formulae that match the balanced equation.
Exam tips
- Set out the calculation in two labelled stages: energy to break bonds, then energy released when bonds form, before subtracting.
- State the final answer with its sign and units, and add a short sentence interpreting the sign as exothermic or endothermic.
- If the question supplies bond energies in a table, copy each value accurately and show which bond it applies to.
Try the lesson
The start of the The energy change of reactions (HT only): The difference between the sum of the energy needed to break bonds in the reactants and the sum of the energy released when bonds in the products are formed is the overall energy change of the reaction. lesson, made from checked teaching notes for this specification point.
Have a go first
A reaction needs 600 kJ/mol to break bonds in the reactants and releases 850 kJ/mol when bonds form in the products. What is the overall energy change of the reaction?
Listen
The overall energy change of a reaction is found by subtracting the energy released when new bonds form from the energy needed to break existing bonds. In symbols, ΔH = energy needed to break bonds − energy released when bonds form. For H₂ + Cl₂ → 2HCl, breaking bonds needs 436 + 243 = 679 kJ/mol and forming bonds releases 2 × 431 = 862 kJ/mol, so ΔH = 679 − 862 = −183 kJ/mol. A negative result means the reaction is exothermic overall; a positive result means it is endothermic overall. The calculation must use the balanced equation so that the number of each bond is correct, and the sign of the answer must be interpreted, not just calculated. A strong answer earns marks for these points. Calculate the total energy needed to break bonds in the reactants by adding the relevant bond energies. Calculate the total energy released when bonds form in the products by adding the relevant bond energies. Subtract the energy released when bonds form from the energy needed to break bonds to obtain the overall energy change. Interpret a negative overall energy change as exothermic and a positive overall energy change as endothermic. A common slip: Subtracting in the wrong order, for example energy to break bonds minus energy released when bonds form reversed. Instead, using energy needed to break bonds minus energy released when bonds form. Exam tip: Set out the calculation in two labelled stages: energy to break bonds, then energy released when bonds form, before subtracting.
See it
Key words
Say what each word means before you turn the card.
Overall energy change
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Exothermic
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Endothermic
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See every definition
- Overall energy change
- The difference between the sum of the energy needed to break bonds in the reactants and the sum of the energy released when bonds in the products are formed.
- Exothermic
- Described by a negative overall energy change, meaning the reaction is exothermic overall.
- Endothermic
- Described by a positive overall energy change, meaning the reaction is endothermic overall.
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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