Bond Energy Calculations
Recap Bond Energy Calculations for GCSE Chemistry with this free worksheet and full mark scheme — Higher-tier exam-style questions with worked answers for AQA, Edexcel and OCR. A Higher topic: overall energy change = energy to break bonds − energy released making bonds; a negative value means the reaction is exothermic.
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These worksheets and mark schemes are original, written for Virtus Academy and checked against the current AQA, Edexcel and OCR specifications. Every worksheet comes with a full mark scheme.
This is a Higher tier only topic, so there's no Foundation paper — only the Higher worksheet and mark scheme below.
Topic overview
Bond energy calculations work out the overall energy change of a reaction from the energies needed to break and make bonds. This is a Higher-only topic.
Breaking bonds is endothermic — energy must be supplied. Making bonds is exothermic — energy is released. The overall energy change is the energy needed to break bonds minus the energy released making bonds.
A negative answer means the reaction is exothermic, because more energy was released than absorbed. A positive answer means endothermic. Getting the sign right, and stating what it means, is where most of the marks lie.
Revision notes
The principle
Breaking bonds requires energy, so it is endothermic.
Making bonds releases energy, so it is exothermic. Energy change = energy to break bonds − energy released making bonds.
Carrying out the calculation
Add together the bond energies of all bonds broken in the reactants.
Add together the bond energies of all bonds made in the products. Subtract the second total from the first. Count every bond carefully, including all bonds in each molecule.
Interpreting the sign
A negative energy change means the reaction is exothermic — more energy was released than was needed.
A positive energy change means endothermic. Always state which it is, not just the number.
Key points
- Breaking bonds is endothermic.
- Making bonds is exothermic.
- Energy change = bonds broken − bonds made.
- A negative value means exothermic.
- A positive value means endothermic.
- Count every bond in every molecule.
Worked examples
Example 1
Bonds broken require 1500 kJ and bonds made release 1800 kJ. Calculate the overall energy change and state the type of reaction. [3 marks]
Model answer
Energy change = 1500 − 1800 (1 mark) = −300 kJ (1 mark). The value is negative, so the reaction is exothermic (1 mark).
Example 2
Explain why breaking bonds is an endothermic process. [2 marks]
Model answer
Energy must be supplied to overcome the forces holding the atoms together (1 mark), so energy is taken in from the surroundings (1 mark).
Example 3
Bonds broken require 800 kJ and bonds made release 650 kJ. State the type of reaction and explain. [2 marks]
Model answer
Energy change = 800 − 650 = +150 kJ (1 mark). The value is positive, meaning more energy was absorbed than released, so the reaction is endothermic (1 mark).
Common mistakes
Reversing the subtraction.
It is bonds broken minus bonds made, in that order.
Saying breaking bonds releases energy.
Breaking requires energy; making releases it.
Giving the number without stating the type.
Always say exothermic or endothermic and link it to the sign.
Missing bonds in a molecule.
Count every bond, including all the C–H bonds in a hydrocarbon.
Exam tips
- Write the subtraction the right way round every time.
- Count all the bonds carefully before adding.
- State exothermic or endothermic alongside the value.
- Remember this is a Higher-only topic.
Key terms
- Bond energy
- The energy needed to break one mole of a particular bond.
- Endothermic
- Taking in energy, as when bonds break.
- Exothermic
- Releasing energy, as when bonds form.
- Energy change
- The overall difference between energy absorbed and released.
Related topics
Written and reviewed against the current AQA, Edexcel and OCR specifications. Spotted an error? Let us know.