Why is breaking bonds endothermic and making bonds exothermic, and how do we calculate the energy change?
Explain bond breaking and bond making in terms of energy and calculate the energy change of a reaction from bond energies.
A focused answer to WJEC GCSE Chemistry topic 2.4, explaining why bond breaking is endothermic and bond making is exothermic, and showing how to calculate the overall energy change of a reaction from given bond energies.
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What this topic is asking
WJEC topic 2.4 (Higher) wants you to explain why breaking bonds is endothermic and making bonds is exothermic, and to calculate the overall energy change of a reaction from given bond energies. This is a key Higher-tier calculation.
Bond breaking and bond making
In a reaction the bonds in the reactants must break and new bonds in the products must form. These two steps have opposite energy effects:
If more energy is released making bonds than was taken in breaking them, the reaction is exothermic. If more energy is needed to break bonds than is released making them, the reaction is endothermic.
Bond energies
A bond energy is the energy needed to break one mole of a particular bond (and, equally, the energy released when that bond forms). Bond energies are given in per mole. For example, the C-H bond energy is about , and the O=O bond energy is about .
Calculating the overall energy change
The steps are:
- List the bonds in the reactants and add up their bond energies (this is the energy taken in).
- List the bonds in the products and add up their bond energies (this is the energy released).
- Subtract: bonds broken minus bonds made.
- Read the sign: negative is exothermic, positive is endothermic.
Linking back to the reaction profile
The result matches the reaction profile. An exothermic reaction (negative energy change) releases energy, so the products sit lower than the reactants. An endothermic reaction (positive energy change) absorbs energy, so the products sit higher. Bond energy calculations give you the number that the profile shows as a gap.
Working carefully with the equation
Most mistakes in these calculations come from miscounting bonds, so it pays to be methodical. First write out the balanced equation and, if it helps, draw the molecules showing every bond. Then count how many of each type of bond there are on each side, remembering to multiply by any big numbers in front of a formula. For example, in there are four O-H bonds in total (two per molecule). Double bonds such as have their own bond energy and count as one bond, not two. Only once you have the full list of bonds broken and bonds made do you add them up and subtract. Keeping the "in" energy (bonds broken) and "out" energy (bonds made) in separate columns makes the final subtraction much less error-prone.
Exam-style practice questions
Practice questions written in the style of WJEC exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
WJEC sample2 marksExplain why breaking chemical bonds is endothermic but making chemical bonds is exothermic.Show worked answer →
A Unit 2.4 explanation question. Reward: breaking bonds requires energy to be put in (to pull the bonded atoms apart), so it is endothermic. Making bonds releases energy (as new bonds form and the atoms become more stable), so it is exothermic. Markers credit energy taken in to break bonds (endothermic) and energy given out when bonds form (exothermic). A common slip is to get the two the wrong way round.
WJEC sample4 marksUsing bond energies, the energy to break the bonds in a reaction is 2750 kJ and the energy released making new bonds is 3000 kJ. Calculate the overall energy change and state whether the reaction is exothermic or endothermic.Show worked answer →
A Unit 2.4 calculation. Reward: overall energy change = energy to break bonds minus energy released making bonds = . The value is negative, so more energy is released than taken in, and the reaction is exothermic. Markers credit the method (bonds broken minus bonds made), the answer , and the conclusion exothermic. A common error is to subtract the wrong way round, giving and the wrong conclusion.
Related dot points
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A focused answer to WJEC GCSE Chemistry topic 1.1, covering word and balanced symbol equations, the law of conservation of mass, state symbols, and identifying exothermic and endothermic reactions from energy transfer.
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Sources & how we know this
- WJEC GCSE Chemistry specification (from 2016) — WJEC (2016)