6. Thermodynamics and Kinetics

Predict the sign and magnitude of ∆H^o in kj/mol for the following reaction. Identify the reaction as either exothermic or endothermic.

The combustion of alkanes is exothermic ( ∆H° < 0) . Would you expect the combustion of butane or cyclobutane to be more exothermic?

Reactions (a) and (b) are disfavored overall (∆G° > 0) , yet they are favored based on ∆H°. Identify the bonds formed and broken for (a) and (b).

(b) <IMAGE>

For each of the following processes, indicate whether you expect ∆S° to be greater than, less than, or equal to 0. Explain your answer.

(c) <IMAGE>

Calculate ∆H° for the following reactions.

(b) <IMAGE>

Calculate ∆H° for the following reactions.

(c) <IMAGE>

Calculate ∆H° for the following reactions.

(a) <IMAGE>

Using bond-dissociation energies, identify the most stable radical. Justify the difference in stability based on the structure.

(a) <IMAGE>

Using bond-dissociation energies, identify the most stable radical. Justify the difference in stability based on the structure.

(c) <IMAGE>

Using bond-dissociation energies, identify the most stable radical. Justify the difference in stability based on the structure.

(d) I• vs •OH

In the following reactions, identify the bonds formed and the bonds broken.

(a) <IMAGE>

In the following reactions, identify the bonds formed and the bonds broken.

(c) <IMAGE>

Teflon-coated frying pans routinely endure temperatures that would cause polyethylene or polypropylene to oxidize and decompose. Decomposition of polyethylene is initiated by ­free-radical abstraction of a hydrogen atom by O2. 

Bond-dissociation energies of CH bonds are about 400 kJ/mol, and CF bonds are about 460 kJ/mol. The BDE of the HOO bond is about 192 kJ/mol, and the FOO bond is about 63 kJ/mol. 

Show why Teflon [FIGURE 7-5] <IMAGE> is much more resistant to oxidation than polyethylene is. 

Propylene (monomer) <IMAGE>  polymerize—> polypropylene (polymer) <IMAGE> 

Vinyl chloride <IMAGE>  polymerize—> poly(vinyl chloride) PVC, “vinyl” <IMAGE> 

tetrafluoroethylene <IMAGE>.  Polymerize—> poly(tetrafluoroethylene) PTFE, Teflon <IMAGE> 

Use the bond-dissociation enthalpies in [TABLE 4-2] <IMAGE> (page 167) to calculate the heats of reaction for the two possible first propagation steps in the chlorination of isobutane. 

Use this information to draw a reaction-energy diagram like [FIGURE 4-8] <IMAGE> , comparing the activation energies for formation of the two radicals.

isobutane 2-methylpropane <IMAGE> isopentane 2-methylbutane <IMAGE> 

Isooctane 2,2,4-trimethylpentane <IMAGE> 

When a small amount of iodine is added to a mixture of chlorine and methane, it prevents chlorination from occurring. 

Therefore, iodine is a free-radical inhibitor for this reaction. 

Calculate ΔH° values for the possible reactions of iodine with species present in the chlorination of methane, 

and use these values to explain why iodine inhibits the reaction. 

(The I―Cl bond-dissociation enthalpy is 211 kJ/mol or 50 kcal/mol.)

Tributyltin hydride (Bu3SnH) is used synthetically to reduce alkyl halides, replacing a halogen atom with hydrogen. 

­Free-radical initiators promote this reaction, and free-radical inhibitors are known to slow or stop it. 

Your job is to develop a mechanism, using the following reaction as the example.

<IMAGE> + BU3SnH trace Br2 hv —> <IMAGE> + Bu3SnBr 

The following bond-dissociation enthalpies may be helpful: 

<IMAGE> 413 kJ/mol

<IMAGE> 309 kJ/mol 

Br—Br 190 kJ/mol

H—Br 366 kJ/mol

Bu3Sn—H 310 kJ/mol

Bu3Sn—Br 552 kJ/mol 

b. Calculate values of 

ΔH for your proposed steps to show that they are energetically feasible. 

(Hint: A trace of Br2 and light suggests it’s there only as an initiator, to create Br⋅ radicals. Then decide which atom can be abstracted most favorably from the starting materials by the Br⋅radical. That should complete the initiation. Finally, decide what energetically favored propagation steps will accomplish the reaction.)

Which of the following would be expected to give a hotter flame during combustion? Explain. <IMAGE>

Calculate ∆H° for the following equilibrium processes.

(a) <IMAGE>

Calculate ∆H° for the following equilibrium processes.

(d) <IMAGE>

a. Calculate the ∆H° value for the following reaction:

<IMAGE>