Advices For Studying Organic Chemistry

10.2 HOMOLYTIC BOND DISSOCIATION ENERGIES

1. Bond formation is an exothermic process:
   H• + H• H–H ∆H° = – 435 kJ mol–1
   Cl• + Cl• Cl–Cl ∆H° = – 243 kJ mol–1


2. Bond breaking is an endothermic process:
   H–H H• + H• ∆H° = + 435 kJ mol–1
   Cl–Cl Cl• + Cl• ∆H° = + 243 kJ mol–1


3. The hemolytic bond dissociation energies, ∆H°, of hydrogen and chlorine:
   H–H Cl–Cl
   (∆H° = 435 kJ mol–1) (∆H° = 243 kJ mol–1)

10.2A HOMOLYTIC BOND DISSOCIATION ENERGIES AND HEATS OF

REACTION:

1. Bond dissociation energies can be used to calculate the enthylpy change (∆H°)
   for a reaction.

1. For bond breaking ∆H° is positive and for bond formation ∆H° is negative.

HH + Cl Cl 2 H Cl
∆H° = 435 kJ mol–1 ∆H° = 243 kJ mol–1 (∆H° = 431 kJ mol–1) × 2
+678 kJ mol–1 is required – 862 kJ mol–1 is evolved
for bond cleavage. in bond formation.

i) The overall reaction is exothermic:

∆H° = (– 862 kJ mol–1 + 678 kJ mol–1) = – 184 kJ mol–1

ii) The following pathway is assumed in the calculation:

H–H 2 H•

Cl–Cl 2 Cl•