BackPredict the sign and magnitude of ∆Hoin 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?
•CH3 + HCl → CH4 + Cl•
b. What is the activation energy for this reverse reaction?
c. What is the heat of reaction (ΔH°) for this reverse reaction?
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.)
Calculate the ∆H° value for the following reaction:
Calculate ∆H° for the following equilibrium processes.
(b)
Iodination of alkanes using iodine (I2) is usually an unfavorable reaction. (See Problem 4-17 , for example.) Tetraiodomethane (CI4) can be used as the iodine source for iodination in the presence of a free-radical initiator such as hydrogen peroxide. Propose a mechanism (involving mildly exothermic propagation steps) for the following proposed reaction. Calculate the value of ΔH for each of the steps in your proposed mechanism.
The following bond-dissociation energies maybe helpful:
Give the approximate bond-dissociation energy for each indicated bond.
Use bond-dissociation enthalpies (Table 4-2, p. 167) to calculate values of ΔH° for the following reactions.
b. CH3CH2Cl + HI → CH3CH2I + HCl
In the following reactions, identify the bonds formed and the bonds broken.
(a)
To this point, hydrogenation has always been an exothermic process. Using the numbers from Figure 21.6, calculate ∆Hohydr for each step of the reduction of benzene.
∆H1 + ∆H2 + ∆H3 = ∆Htot = ―49.5 kcal /mol (―208 kJ/mol)
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)