Question

How to estimate (Delta H) for the bromination of ethylene from bond dissociation energies ?

Answer 1

`ΔH_"rxn " = "-93 kJ"`

This problem is an application of Hess's Law.

We break all the bonds to form atoms, and then we reassemble the atoms to form new bonds.

`ΔH_"rxn" = ΣΔH_"bonds broken" - ΣΔH_"bonds formed" = ΣBE_"reactants" – ΣBE_"products"`

The equation is

H₂C=CH₂ + Br-Br → Br-CH₂-CH₂-Br

`ΔH = (4BE_"C-H" + 1BE_"C=C" + 1BE_"Br-Br") – (4BE_"C-H" + 1BE_"C-C" + 2BE_"C-Br")`

We can ignore the C-H bonds, because they are just being broken and re-formed.

`ΔH = (1BE_"C=C" + 1BE_"Br-Br") – (1BE_"C-C" + 2BE_"C-Br")`

We need a table of bond energies like the one below.

`ΔH = "(614 kJ + 193 kJ) – (348 kJ + 2×276 kJ)" = "-93 kJ"`