How do we calculate the overall energy change of a reaction from bond energies?
Bond energy calculations: breaking bonds is endothermic and making bonds is exothermic, and calculating the overall energy change of a reaction from bond energies.
A focused answer to Edexcel GCSE Chemistry topic 7, covering why bond breaking is endothermic and bond making is exothermic, and how to calculate the overall energy change of a reaction from bond energies as the energy to break bonds minus the energy released making bonds, including identifying exothermic and endothermic reactions from the result.
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What this dot point is asking
Edexcel wants you to explain that breaking bonds is endothermic and making bonds is exothermic, and to calculate the overall energy change of a reaction from bond energies as the energy needed to break the reactant bonds minus the energy released making the product bonds. This is Higher and separate-style content, and the calculation is a reliable high-value mark.
Bond breaking and bond making
Energy is involved whenever bonds change:
The bond energy is the energy needed to break (or released when making) one mole of a particular bond, measured in kJ/mol. The same value applies in both directions: the energy taken in to break a bond equals the energy released when that same bond forms. Bond energies are average values, because the exact strength of a bond depends slightly on the rest of the molecule, but they are accurate enough to predict whether a reaction is exothermic or endothermic and to estimate the size of the energy change.
During a reaction, energy is first taken in to break the bonds in the reactants, and then released as new bonds form in the products. Whether the reaction is exothermic or endothermic overall depends only on which of these two totals is larger, not on the order in which they happen.
Calculating the overall energy change
In a reaction, the reactant bonds break (taking in energy) and the product bonds form (releasing energy). The overall energy change is:
The method:
- Add up the bond energies of all the bonds in the reactants (the energy needed to break them).
- Add up the bond energies of all the bonds in the products (the energy released when they form).
- Subtract: breaking total minus making total.
A negative answer means the reaction is exothermic (more energy out than in); a positive answer means it is endothermic (more energy in than out).
Worked calculation
Try this
Q1. State whether bond breaking is exothermic or endothermic. [1 mark]
- Cue. Endothermic (energy must be supplied).
Q2. A reaction needs kJ/mol to break bonds and releases kJ/mol making bonds. Calculate the overall energy change and state the type of reaction. [2 marks]
- Cue. kJ/mol; exothermic.
Q3. Explain why a reaction is endothermic if the bond-energy calculation gives a positive value. [2 marks]
- Cue. A positive value means more energy is taken in breaking bonds than is released making them, so the reaction absorbs energy overall.
Exam-style practice questions
Practice questions written in the style of Pearson Edexcel exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Edexcel 20204 marksHydrogen reacts with chlorine: . Use the bond energies to calculate the overall energy change. Bond energies in kJ/mol: H-H = 436, Cl-Cl = 242, H-Cl = 431.Show worked answer →
A 4-mark bond-energy calculation, a standard Edexcel Higher question.
Energy to break bonds (reactants) (H-H) (Cl-Cl) kJ/mol (1 mark). Energy released making bonds (products) (H-Cl) kJ/mol (1 mark). Overall energy change energy in to break energy out to make kJ/mol (1 mark). The negative sign shows the reaction is exothermic (1 mark).
Markers reward breaking minus making, the correct multiplication of the two H-Cl bonds, and identifying the negative value as exothermic.
Edexcel 20223 marksExplain why breaking bonds is an endothermic process and making bonds is an exothermic process, and state what this means for a reaction in which more energy is released making bonds than is used breaking them.Show worked answer →
A 3-mark bond-energy reasoning question.
Breaking bonds requires energy to be supplied, so bond breaking is endothermic (1 mark). Making bonds releases energy, so bond making is exothermic (1 mark). If more energy is released making bonds than is needed to break them, the reaction releases energy overall, so it is exothermic (1 mark).
Markers reward "breaking takes in energy", "making releases energy", and the conclusion that a net release means an exothermic reaction.
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Sources & how we know this
- Pearson Edexcel GCSE (9-1) Chemistry (1CH0) specification — Pearson Edexcel (2016)