Question 1Warm-upHigher
. Bond energies: H–H = 436, Cl–Cl = 243, H–Cl = 432 kJ/mol. Calculate the energy change.
Broken: H–H + Cl–Cl. Made: 2 × H–Cl.
Worked solution
- Broken: kJ
- Made: kJ
- Change kJ/mol
Answer: kJ/mol (exothermic)
GCSE & IGCSE chemistry practice
Bond energies questions and answers
Breaking bonds takes in energy (endothermic); making bonds gives out energy (exothermic). The overall energy change = energy to break bonds in the reactants − energy released making bonds in the products. A negative answer means the reaction is exothermic.
8 questions · Higher tier · Every answer is checked as you type it, with a full worked solution. Updated .
Where bond energies comes up
| GCSE (AQA) | Chemistry and Combined Science, Higher only: calculate the energy transferred in a reaction from bond energies supplied. |
|---|---|
| Cambridge IGCSE (0620) | Supplement (Extended): bond breaking is endothermic and bond making is exothermic; calculate the enthalpy change of a reaction using bond energies. |
The method
- Draw or list every bond in the reactants and the products (use the numbers in the equation).
- Add up the energy to break all the reactant bonds.
- Add up the energy released forming all the product bonds.
- Energy change = broken − made. Negative: exothermic; positive: endothermic.
Bond energies questions
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Warm-up
Question 2Warm-upHigher
Breaking the bonds in the reactants needs 2000 kJ. Making the bonds in the products releases 2400 kJ. What is the energy change, and is the reaction exothermic or endothermic?
Broken − made.
Worked solution
- kJ
- Negative, so exothermic.
Answer: kJ: exothermic
Exam-style
Question 3Exam-styleHigher
. Bond energies: H–H = 436, Br–Br = 193, H–Br = 366 kJ/mol. Calculate the energy change.
Broken − made.
Worked solution
- Broken:
- Made:
- kJ/mol
Answer: kJ/mol
Question 4Exam-styleHigher
. Bond energies: H–H = 436, O=O = 498, O–H = 464 kJ/mol. Calculate the energy change.
Each water molecule has 2 O–H bonds, and there are 2 waters.
Worked solution
- Broken:
- Made:
- kJ/mol
Answer: kJ/mol
Question 5Exam-styleHigher
. Bond energies: C–H = 412, O=O = 498, C=O = 805, O–H = 464 kJ/mol. Calculate the energy change.
Broken: 4 C–H and 2 O=O. Made: 2 C=O and 4 O–H.
Worked solution
- Broken:
- Made:
- kJ/mol (exothermic)
Answer: kJ/mol
Question 6Exam-styleHigher
. Bond energies: N≡N = 945, H–H = 436, N–H = 391 kJ/mol. Calculate the energy change.
Each NH₃ has 3 N–H bonds.
Worked solution
- Broken:
- Made:
- kJ/mol
Answer: kJ/mol
Challenge
Question 7ChallengeHigher
. Bond energies: H–I = 298, H–H = 436, I–I = 151 kJ/mol. Calculate the energy change and say whether it's exothermic or endothermic.
Broken: 2 H–I.
Worked solution
- Broken:
- Made:
- kJ/mol, so endothermic
Answer: kJ/mol: endothermic
Question 8ChallengeHigher
has an energy change of −185 kJ/mol. H–H = 436 and H–Cl = 432 kJ/mol. Calculate the Cl–Cl bond energy.
Broken − made = −185.
Worked solution
- , so kJ/mol
Answer: kJ/mol
Common mistakes
- Forgetting a double bond (O=O, C=O) is a different value.
- Missing the numbers in front of formulae.
- Subtracting the wrong way round.
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Sources
The questions are our own. The “where it comes up” notes are from each board's published specification, checked on 27 October 2026:
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