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How do we calculate the overall energy change of a reaction from bond energies?

Bond energy calculations; energy in to break bonds and out to make bonds; calculating the overall energy change; and linking the sign to exothermic or endothermic.

A focused answer to AQA GCSE Chemistry 4.5.1, covering how breaking bonds takes in energy and making bonds releases energy, calculating the overall energy change from bond energies, and linking the result to exothermic and endothermic reactions.

Generated by Claude Opus 4.89 min answerUpdated 2026-06-02

Reviewed by: AI editorial process; not yet individually human-reviewed

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What this dot point is asking
Breaking and making bonds
The calculation
Method
Try this

What this dot point is asking

AQA wants you to explain that breaking bonds takes in energy and making bonds releases energy, calculate the overall energy change of a reaction from given bond energies, and use the sign of the result to say whether the reaction is exothermic or endothermic. Bond energies let you predict the energy change of a reaction from first principles, before any experiment, and they explain at the molecular level why a reaction releases or absorbs heat. This is Higher tier.

Breaking and making bonds

Every chemical reaction can be thought of in two stages: first all the bonds in the reactant molecules break (taking in energy), then new bonds form in the product molecules (releasing energy). A bond energy is the energy needed to break one mole of a particular bond, measured in kilojoules per mole; the same value is released when that bond forms. Bond energies are average values because the exact strength of, say, a $C-H$ bond varies slightly between molecules.

The calculation

A negative result means more energy was released forming bonds than was taken in breaking them, so the reaction is exothermic. A positive result means more energy was taken in than released, so the reaction is endothermic. The size of the number is the magnitude of the energy change per mole of reaction as written.

Method

Draw out or list the bonds in the reactants and add their energies (energy in to break).
List the bonds in the products and add their energies (energy out to make).
Subtract: bonds broken minus bonds made.
State the sign and whether the reaction is exothermic or endothermic.

The single most important habit is counting bonds correctly from the balanced equation: a coefficient of 2 doubles every bond in that molecule. Drawing the displayed (structural) formula helps you see, for example, that each water molecule has two $O-H$ bonds.

Energy change for the formation of ammonia

Calculate the overall energy change for $N_2 + 3H_2 \rightarrow 2NH_3$ . Bond energies in kJ per mole: N $\equiv$ N = 945, H-H = 436, N-H = 391.

step 1 Count and total the bonds broken

Reactants: one $N\equiv N$ bond and three $H-H$ bonds. Energy in $= 945 + (3 \times 436) = 945 + 1308 = 2253$ kJ.

step 2 Count and total the bonds made

Products: each $NH_3$ has three $N-H$ bonds, and there are two ammonia molecules, so $6$ $N-H$ bonds. Energy out $= 6 \times 391 = 2346$ kJ.

step 3 Subtract bonds broken minus bonds made

Energy change $= 2253 - 2346 = -93$ kJ per mole.

step 4 Interpret the sign

The value is negative, so more energy is released making bonds than is taken in breaking them. The reaction is exothermic.

Try this

Q1. State whether breaking bonds is exothermic or endothermic. [1 mark]

Cue. Endothermic, because energy must be supplied.

Q2. A reaction breaks bonds requiring $1500$ kJ and makes bonds releasing $1650$ kJ. Calculate the overall energy change and state the type of reaction. [2 marks]

Cue. $1500 - 1650 = -150$ kJ; exothermic.

Q3. Explain why a reaction is endothermic in terms of bond energies. [2 marks]

Cue. More energy is taken in breaking the reactant bonds than is released making the product bonds, so the energy change is positive.

Exam-style practice questions

Practice questions written in the style of AQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

AQA 20194 marksHydrogen burns in chlorine to form hydrogen chloride:

H_2 + Cl_2 \rightarrow 2HCl

. Use the bond energies to calculate the overall energy change, and state whether the reaction is exothermic or endothermic. Bond energies in kJ per mole: H-H = 436, Cl-Cl = 242, H-Cl = 431.

Show worked answer →

A 4-mark Higher bond-energy calculation: marks for bonds broken, bonds made, the subtraction, and the conclusion.

Bonds broken (reactants): $H-H + Cl-Cl = 436 + 242 = 678$ kJ (1 mark). Bonds made (products): two $H-Cl$ bonds $= 2 \times 431 = 862$ kJ (1 mark). Energy change $= \text{broken} - \text{made} = 678 - 862 = -184$ kJ per mole (1 mark). The value is negative, so the reaction is exothermic (1 mark).

Markers reward counting two $H-Cl$ bonds (because of the $2HCl$ ) and doing the subtraction in the order broken minus made.

AQA 20224 marksMethane burns completely in oxygen:

CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O

. Calculate the overall energy change using the bond energies, and explain what the sign tells you. Bond energies in kJ per mole: C-H = 413, O=O = 498, C=O = 805, O-H = 464.

Show worked answer →

A 4-mark Higher calculation involving several bonds.

Bonds broken: four $C-H$ plus two $O=O$ $= (4 \times 413) + (2 \times 498) = 1652 + 996 = 2648$ kJ (1 mark). Bonds made: two $C=O$ in $CO_2$ plus four $O-H$ in two waters $= (2 \times 805) + (4 \times 464) = 1610 + 1856 = 3466$ kJ (1 mark). Energy change $= 2648 - 3466 = -818$ kJ per mole (1 mark). The negative sign means the reaction is exothermic: more energy is released making bonds than is taken in breaking them (1 mark).

The most common error is miscounting bonds, especially forgetting that two water molecules contain four $O-H$ bonds.

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Sources & how we know this

AQA GCSE Chemistry (8462) specification — AQA (2016)