Bond Energy Calculations (Edexcel GCSE Chemistry)
Revision Note
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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:
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:
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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