Bond Energy

Key Stage 4 Higher

Meaning

Bond Energy is the energy needed to break a chemical bond between two atoms.

About Bond Energy

When chemical bonds are formed energy is released to the surroundings increasing the temperature. An exothermic process]].

To break chemical bonds energy is needed to separate the atoms, decreasing the temperature of the surroundings. An endothermic process]].

Examples

Some common bond energies are given in the table below.

Bond	Energy in kJ/mol
H-H	436
O=O	498
N≡N	941
C-C	347
C=C	614
C≡C	839
C-H	413
O-H	464
C=O	799
Cl-Cl	243
H-Cl	432
N-H	391

These can be used to calculate the energy released per mole in a chemical reaction.

Example 1

In the reaction between Methane and Oxygen the chemical bonds in the reactants must be broken first before the bonds in the products are formed.

There are 4 C-H bonds and 2 O=O bonds.

4 x 413 + 2 x 498 = 2648kJ

Therefore 2648kJ/mol are needed to break the bonds in the reactants.

There are 2 C=O bonds and 4 O-H bonds.

2 x 799 + 4 x 464 = 3454kJ

Therefore 3454kJ/mol is released when the bonds in the products form.

Once the reaction is complete 806kJ will be released.

Example 2

In the reaction between Hydrogen and Nitrogen the chemical bonds in the reactants must be broken first before the bonds in the products are formed.

There are 3 H-H bonds and 1 N≡N bonds.

3 x 436 + 1 x 941 = 2249kJ

Therefore 2249kJ/mol are needed to break the bonds in the reactants.

There are 6 N-H bonds.

6 x 391 = 2346kJ

Therefore 2346kJ/mol is released when the bonds in the products form.

Once the reaction is complete 97kJ will be released.

References

AQA

Bond energies, page 63, GCSE Chemistry; The Revision Guide, CGP, AQA

Bond energies, pages 133-5, GCSE Chemistry, Hodder, AQA

Bond energies, pages 158, 159, GCSE Combined Science Trilogy; Chemistry, CGP, AQA

Bond energies, pages 183, 184, GCSE Chemistry, CGP, AQA

Bond energies, pages 235-7, GCSE Combined Science Trilogy 1, Hodder, AQA

Bond energy, pages 117-119, GCSE Chemistry; Third Edition, Oxford University Press, AQA

Edexcel

Bond energies, page 136, GCSE Combined Science; The Revision Guide, CGP, Edexcel

Bond energies, page 85, GCSE Chemistry; The Revision Guide, CGP, Edexcel

Bond energies, pages 248-250, GCSE Chemistry, CGP, Edexcel

Bond energy, page 147, GCSE Chemistry, Pearson, Edexcel

Bond energy, page 261, GCSE Combined Science, Pearson Edexcel

Bonds; energy calculations, page 147, GCSE Chemistry, Pearson, Edexcel

Bonds; energy calculations, page 261, GCSE Combined Science, Pearson Edexcel

OCR

Bond energies, page 42, GCSE Chemistry; The Revision Guide, CGP, OCR Gateway

Bond energies, pages 106-107, Gateway GCSE Chemistry, Oxford, OCR