Bond energy/bond enthalpy
- Bond energy or bond enthalpy is the average amount of energy required to separate an atom from a covalent molecule.
- The stronger the bond strength, the larger the value of bond energy. This means, it takes a lot more energy to separate the individual atoms from their bonds if it has a strong bond strength.
- The bond energies of different bonds are usually provided in examination questions, so there is no need to memorise any of them.
Bond breaking and bond forming during chemical reactions
- Breaking of bonds is always an endothermic process, as the substance will take in energy from its surroundings.
- On the other hand, forming bonds is an exothermic process, as it releases heat to the surroundings.
- Usually, during a chemical reaction, the total energy absorbed during bond breaking is not equal to the energy released during bond.
  - If the energy absorbed is greater than the energy released, the overall chemical reaction is endothermic.
  - If the energy absorbed is less than the energy released, the overall chemical reaction is exothermic.

Sample question: Calculation of enthalpy change of combustion of 1 mol of propane gas (C₃H₈).
- 1 molecule of propane reacts with 5 oxygen molecules to form 3 molecules of carbon dioxide and 4 molecules of water.
- Now, we need to calculate the energy absorbed to break the bonds of the reactants and the energy released to form the bonds of the products using the provided values of bond energy given.
- In the reactants side, there are:
  - 2 C–C single bonds → 2 × 347 = 694 kJ/mol
  - 8 C–H single bonds → 8 × 413 = 3304 kJ/mol
  - 5 O=O double bonds → 5 × 498 = 2490 kJ/mol
  - Total energy used to break bonds = 6488 kJ/mol
- In the products side, there are:
  - 6 C=O double bonds → 6 × -799 = -4794 kJ/mol
  - 8 O–H single bonds → 8 × -467 = -3736 kJ/mol
  - Total energy released to form bonds = -8530 kJ/mol
  - Do take note that bond forming is exothermic, so the energies used are negative.
- Next, we calculate the total enthalpy change ΔH by summing up the energy absorbed during bond-breaking with the energy released during bond forming:
  ΔH = 6488 + (-8530)
  = -2042 kJ/mol
  - (tips) Do be very careful with the negative sign of the bond forming energy!

5.03 - Bond Enthalpy

Learn Step-By-Step Video Explanation For This Topic