Bond energies can be used to calculate the energy change in a chemical reaction.
Energy change calculations are important to determine whether a reaction is exothermic or endothermic.
Example 1: Hydrogen and Oxygen Reaction
Reaction: 2 H₂ + O₂ → 2 H₂O
Bonds to Break:
Hydrogen-Hydrogen (H-H) bond energy = 436 kJ/mol
Total for H-H bonds = 2 x 436 = 872 kJ
Oxygen-Oxygen (O=O) bond energy = 495 kJ/mol
Total energy input = 872 kJ (H-H) + 495 kJ (O=O) = 1367 kJ
Bonds to Make:
Oxygen-Hydrogen (O-H) bond energy = -463 kJ/mol (negative because forming bonds releases energy)
Total for O-H bonds = 4 x -463 = -1852 kJ
Overall Energy Change Calculation:
Total energy = Energy input - Energy release = 1367 - 1852 = -485 kJ
Conclusion: Reaction is exothermic.
Example 2: Complete Combustion of Methane
Reaction: CH₄ + 2 O₂ → CO₂ + 2 H₂O
Bonds to Break:
Carbon-Hydrogen (C-H) bond energy = 413 kJ/mol
Total for C-H bonds = 4 x 413 = 1652 kJ
Oxygen-Oxygen (O=O) bond energy = 495 kJ/mol
Total for O=O bonds = 2 x 495 = 990 kJ
Total energy input = 1652 + 990 = 2642 kJ
Bonds to Make:
Carbon-Oxygen (C=O) bond energy = -799 kJ/mol
Total for C=O bonds = 2 x 2 x -799 = -1598 kJ
Oxygen-Hydrogen (O-H) bond energy = -463 kJ/mol
Total for O-H bonds = 4 x -463 = -1852 kJ
Overall Energy Change Calculation:
Total energy = Energy input - Energy release = 2642 - 3450 = -808 kJ
Conclusion: Reaction is exothermic.
Conclusion
Calculating energy changes using bond energies involves breaking and forming bonds, with the respective energies being positive for breaking and negative for forming.
Both example reactions are exothermic, meaning they release energy.
Additional Resources
More examples and exercises can be found in the provided workbook.