Bond Energy Calculations (Edexcel GCSE Chemistry)

Revision Note

Stewart Hird

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Bond Energy Calculations

  • Each chemical bond has a specific bond energy associated with it

  • This is the amount of energy required to break the bond or the amount of energy given out when the bond is formed

  • This energy can be used to calculate how much heat would be released or absorbed in a reaction

  • To do this it is necessary to know the bonds present in both the reactants and products

  • We can calculate the total change in energy for a reaction if we know the bond energies of all the species involved

  • Add together all the bond energies for all the bonds in the reactants – this is the ‘energy in’

  • Add together the bond energies for all the bonds in the products – this is the ‘energy out’

  • Calculate the energy change using the equation:

Energy change = Energy taken in - Energy given out

Worked Example

Example 1

Hydrogen and chlorine react to form hydrogen chloride gas:

H+ Cl2 ⟶ 2HCl

The table below shows the bond energies. Calculate the energy change for the reaction and deduce whether it is exothermic or endothermic.

Calculating energy Changes WE Table 1, downloadable IGCSE & GCSE Chemistry revision notes

Answer:

Calculating energy Changes WE1 Answer Table, downloadable IGCSE & GCSE Chemistry revision notes

Worked Example

Example 2

Hydrogen bromide decomposes to form hydrogen and bromine:

2HBr  ⟶ H2  + Br2

The table below shows the bond energies. Calculate the energy change for the reaction and deduce whether it is exothermic or endothermic.

Calculating energy Changes WE Table 2, downloadable IGCSE & GCSE Chemistry revision notes

Answer:

Calculating energy Changes WE2 Answer Table, downloadable IGCSE & GCSE Chemistry revision notes

Examiner Tips and Tricks

For bond energy questions, it is helpful to write down a displayed formula equation for the reaction before identifying the type and number of bonds, to avoid making mistakes.

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Stewart Hird

Author: Stewart Hird

Expertise: Chemistry Lead

Stewart has been an enthusiastic GCSE, IGCSE, A Level and IB teacher for more than 30 years in the UK as well as overseas, and has also been an examiner for IB and A Level. As a long-standing Head of Science, Stewart brings a wealth of experience to creating Topic Questions and revision materials for Save My Exams. Stewart specialises in Chemistry, but has also taught Physics and Environmental Systems and Societies.