4.16 use average bond energies to calculate the enthalpy change during a simple chemical reaction.

Bond Energies

The energy required to break a bond is called bond energy. It's measured in kJ/mol (of bonds).

Breaking bonds takes in energy i.e. is endothermic

Making bonds gives out energy i.e. is exothermic

To calculate enthalpy change using bond energies:

1. Calculate the sum of energies for the bonds broken, Σ (bonds broken)
2. Calculate the sum of energy for bonds made, Σ (bonds made)
3. Calculate H by using the formulaH = Σ (bonds broken) - Σ (bonds made)

e.g. 

H₂ + O₂ à 2H₂O

Σ (bonds broken) = (H-H) + (O-O) = 436 + 496 = 932

Σ (bonds made) = 4(O-H) = (4 x 463) = 1852

Σ (bonds broken) – Σ (bonds made) = 932 – 1852

H = 932 – 1852 = -920 kJ/mol