Which of the presented mechanisms would be the most energetically favorable and thus the most likely mechanism to actually occur for the following free radical chain reaction?
(bond dissociation energies - H-H = 104 kcal/mol; Cl-Cl = 58 kcal/mol; H-Cl = 103 kcal/mol)
H₂+ Cl₂2 HCl

Question

Which of the presented mechanisms would be the most energetically favorable and thus the most likely mechanism to actually occur for the following free radical chain reaction?
(bond dissociation energies - H-H = 104 kcal/mol; Cl-Cl = 58 kcal/mol; H-Cl = 103 kcal/mol)
H₂+ Cl₂

2 HCl

Quizplus · Accepted Answer

The initiation step involves the homolytic cleavage of the H-Cl bond, which requires 103 kcal/mol. This yields a hydrogen radical (H∙) and a chlorine radical (Cl∙).

In option A, the propagation steps involve two exothermic reactions that release a total of 7 kcal/mol of energy. However, the third step involves the recombination of two radicals, which is endothermic and requires 104 kcal/mol of energy, making this mechanism unlikely.

Option C shares the same issue as option A. The last step, involving the recombination of H∙ and Cl∙ to form HCl, is endothermic.

Option D involves a highly endothermic initiation step because it requires breaking the Cl-Cl bond, which has a bond dissociation energy of 58 kcal/mol. The propagation steps release 8 kcal/mol of energy, but the final step requires 104 kcal/mol of energy, making this mechanism energetically unlikely as well.

Option B involves the exothermic formation of chlorine radicals via the homolytic cleavage of the Cl-Cl bond, which releases 58 kcal/mol of energy. The propagation steps involve two exothermic reactions that release a total of 207 kcal/mol of energy. The overall reaction is highly exothermic and yields the desired product. Therefore, option B is the best choice.

Which of the Presented Mechanisms Would Be the Most Energetically