GCSE Combined Science revision · Chemistry: reactions and energy

Higher: bond energy calculations

Higher only

The idea

Breaking bonds takes in energy (endothermic); making bonds releases energy (exothermic). The overall energy change = energy needed to break bonds in the reactants − energy released making bonds in the products. A negative answer means the reaction is exothermic.

Draw or list every bond in the reactants and products.
Add up the energy to break and the energy released.
Overall = broken − made.

Watch out Count every bond, using the numbers in front of each formula.

See it worked

Worked example

H₂ + Cl₂ → 2HCl. Bond energies: H–H 436, Cl–Cl 243, H–Cl 432 kJ/mol. Calculate the energy change.

Broken: 436 + 243 = 679 kJ.
Made: 2 × 432 = 864 kJ.
Change = 679 − 864 = −185 kJ/mol (exothermic).

Answer −185 kJ/mol

Your turn

Question 1Calculator allowed2 marks

H₂ + Br₂ → 2HBr. Bond energies: H–H 436, Br–Br 193, H–Br 366 kJ/mol. Calculate the energy change in kJ/mol.

Question 2Calculator allowed1 mark

If more energy is released making bonds than is taken in breaking them, the reaction is:

Question 3Calculator allowed1 mark

Breaking chemical bonds is:

In past papers

18 official questions in 18 papers since 2018. The latest:

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