AQA · GCSE · Chemistry
Course version: 8462 · Version from Sept 2018
Students should be able to calculate the energy transferred in chemical…
Full specification wording
The energy change of reactions (HT only): Students should be able to calculate the energy transferred in chemical reactions using bond energies supplied.
Bond breaking is endothermic because energy must be supplied to overcome attractions, while bond making is exothermic because energy is released when new bonds form.
Checked against the AQA 8462 specification (opens in a new tab) on .
- Point 6 of 6
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Bond breaking is endothermic because energy must be supplied to overcome attractions, while bond making is exothermic because energy is released when new bonds form. To find the overall energy transferred, identify every bond broken in the reactants and every bond made in the products, using the supplied bond energies in kJ mol⁻¹. Total the energy absorbed for bonds broken and the energy released for bonds made. The overall energy change is energy absorbed minus energy released. A negative result means the reaction is exothermic overall; a positive result means it is endothermic. For example, in the reaction H₂ + Cl₂ → 2HCl, breaking one H–H bond and one Cl–Cl bond absorbs energy, while forming two H–Cl bonds releases energy. The difference gives the overall energy change.
What earns the marks
- State that bond breaking is endothermic and bond making is exothermic.
- Identify all bonds broken in the reactants and all bonds made in the products from a displayed or supplied equation.
- Multiply each bond energy by the number of that bond present, using the supplied values in kJ mol⁻¹.
- Calculate total energy absorbed for bonds broken and total energy released for bonds made.
- Apply overall energy change = energy absorbed − energy released, and interpret the sign as exothermic or endothermic.
Common mistakes and how to put them right
Adding bond energies for bonds made instead of subtracting them.
Put it right by calculating energy absorbed minus energy released.
Forgetting to multiply a bond energy by the number of identical bonds, such as two H–Cl bonds in 2HCl.
Put it right by counting every bond in the equation.
Reversing the sign convention, so an exothermic reaction is reported as positive.
Put it right by checking that energy released greater than energy absorbed gives a negative overall value.
Exam tips
- Draw the displayed structures of reactants and products before counting bonds, so no bond is missed or double-counted.
- Write a clear two-column layout: bonds broken with total absorbed, then bonds made with total released.
- Check the final sign against the reaction context; combustion and neutralisation are normally exothermic overall.
Try the lesson
The start of the The energy change of reactions (HT only): Students should be able to calculate the energy transferred in chemical reactions using bond energies supplied. lesson, made from checked teaching notes for this specification point.
Have a go first
For the reaction H₂ + Cl₂ → 2HCl, the bond energies supplied are: H–H = 436 kJ/mol, Cl–Cl = 243 kJ/mol, H–Cl = 431 kJ/mol. Calculate the overall energy change for this reaction.
Listen
Bond breaking is endothermic because energy must be supplied to overcome attractions, while bond making is exothermic because energy is released when new bonds form. To find the overall energy transferred, identify every bond broken in the reactants and every bond made in the products, using the supplied bond energies in kJ mol⁻¹. Total the energy absorbed for bonds broken and the energy released for bonds made. The overall energy change is energy absorbed minus energy released. A negative result means the reaction is exothermic overall; a positive result means it is endothermic. For example, in the reaction H₂ + Cl₂ → 2HCl, breaking one H–H bond and one Cl–Cl bond absorbs energy, while forming two H–Cl bonds releases energy. The difference gives the overall energy change. A strong answer earns marks for these points. State that bond breaking is endothermic and bond making is exothermic. Identify all bonds broken in the reactants and all bonds made in the products from a displayed or supplied equation. Multiply each bond energy by the number of that bond present, using the supplied values in kJ mol⁻¹. Calculate total energy absorbed for bonds broken and total energy released for bonds made. A common slip: Adding bond energies for bonds made instead of subtracting them. Instead, calculating energy absorbed minus energy released. Exam tip: Draw the displayed structures of reactants and products before counting bonds, so no bond is missed or double-counted.
See it
Key words
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Bond breaking
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Bond making
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Overall energy change
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Bond energies
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See every definition
- Bond breaking
- Process that is endothermic because energy must be supplied to overcome attractions.
- Bond making
- Process that is exothermic because energy is released when new bonds form.
- Overall energy change
- Energy absorbed for bonds broken minus energy released for bonds made; a negative result means the reaction is exothermic overall and a positive result means it is endothermic.
- Bond energies
- Supplied values in kJ mol⁻¹ used to total the energy absorbed for bonds broken and the energy released for bonds made.
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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