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# Use bond-dissociation enthalpies (Table 4-2, p. 143) to ISBN: 9780321768414 33

## Solution for problem 40SP Chapter 4

Organic Chemistry | 8th Edition

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Problem 40SP

Use bond-dissociation enthalpies (Table 4-2, p. 143) to calculate values of for the following reactions.  Step-by-Step Solution:

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-I

Bond 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-Cl

Bond 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)

Step 4 of 6

Step 5 of 6

##### ISBN: 9780321768414

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