Bond Energy Calculations (Edexcel GCSE Chemistry)
Revision Note
Author
StewartExpertise
Chemistry Lead
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:
H2 + Cl2 ⟶ 2HCl
The table below shows the bond energies. Calculate the energy change for the reaction and deduce whether it is exothermic or endothermic.
Answer:
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.
Answer:
Exam Tip
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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