When a small amount of iodine is added to a mixture of...

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 $\Delta H^{\circ}$ 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 $211 \mathrm{~kJ} / \mathrm{mol}$ or $50 \mathrm{kcal} / \mathrm{mol}$.)

Key Concepts

Reaction Enthalpy Calculations

This concept involves calculating the net change in enthalpy (?H) for reactions by comparing the energies of bonds broken and formed. By using bond energies such as the I–Cl bond-dissociation enthalpy, one can predict whether the reactions of iodine with radicals are energetically favorable or not, and thereby rationalize the effectiveness of iodine as an inhibitor in free-radical processes.

Radical Inhibitors

Radical inhibitors are substances that react with radical intermediates to form less reactive, stable species. By doing so, they disrupt the chain propagation steps in free-radical reactions. The added iodine acts as a radical scavenger, reacting with active species (such as chlorine radicals), thus terminating the chain reaction and preventing the overall reaction from proceeding.

Bond Dissociation Enthalpy

Bond dissociation enthalpy refers to the energy required to break a particular bond in a molecule, forming radicals. It is essential in understanding the energetics of free-radical reactions because it helps determine which bonds are easier to break, thereby influencing the feasibility and rate of radical formation and reaction.

Free-Radical Chain Reactions

This concept involves reactions that proceed via a series of radical intermediates through steps including initiation, propagation, and termination. In many halogenation reactions such as methane chlorination, radicals are produced which then react in chain sequences, and any interruption in these sequences can significantly alter the overall reaction rate.

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