Use bond-dissociation enthalpies (Table 4-2, p. 143) to calculate values of 
Solution 40SP:
Here, we are going to calculate the bond enthalpy change for each of the reaction.
Step 1:
The overall enthalpy change for a reaction is the sum of the dissociation enthalpies
of the bonds broken minus the sum of the dissociation enthalpies of the bonds formed.
o = [Bond dissociation energy for bonds broken] -[Bond dissociation energy for bonds formed]
Step 2: Calculation of the overall enthalpy change for each reaction.
CH3-CH3 + I2 → CH3-CH3-I +H-IBond dissociation energy for bonds broken
CH3-CH2 - H = 410 kJ/mol (98 kcal/mol)
I-I = 151 kJ/mol (36 kcal/mol)
Total Bond dissociation energy for bonds broken = 561 kJ/mol(134 kcal/mol)
Bond dissociation energy for bonds formed
CH3-CH3-I =222 kJ/mol (53 kcal/mol)
H-I = 297 kJ/mol (71 kcal/mol)
Total Bond dissociation energy for bonds formed = 519 kJ/mol(124 kcal/mol)
Thus,
o = [Bond dissociation energy for bonds broken] -[Bond dissociation energy for bonds formed]
= 561 kJ/mol (134 kcal/mol) -519 kJ/mol(124 kcal/mol)
= 42.0 kJ/mol (10 kcal/mol)
Step 3: Calculation of the overall enthalpy change for each reaction.
CH3-CH3-Cl +H-I → CH3-CH3-I +H-ClBond dissociation energy for bonds broken
CH3-CH2 - Cl = 339 kJ/mol (81 kcal/mol)
H-I = 297 kJ/mol (71 kcal/mol)
Total Bond dissociation energy for bonds broken = 636 kJ/mol(152 kcal/mol)
Bond dissociation energy for bonds formed
CH3-CH3-I =222 kJ/mol (53 kcal/mol)
H-Cl = 431 kJ/mol (103 kcal/mol)
Total Bond dissociation energy for bonds formed = 653 kJ/mol(156 kcal/mol)
Thus,
o = [Bond dissociation energy for bonds broken] -[Bond dissociation energy for bonds formed]
= 636 kJ/mol(152 kcal/mol) - 653 kJ/mol(156 kcal/mol)
= -17.0 kJ/mol (-4.0 kcal/mol)
