Question

Standard heats reaction values are given for the first three primary mechanistic steps in the free-radical chlorination of methane. Explain why the -105kJ/mol overall standard heats of reaction value for the reaction does not include the standard heats of reaction value for the initiation step, but instead is the sum of the standard heats of reaction values for the two propagation steps.

A. Standard heats of reaction (delta H degree) values are given for the first three primary mechanistic steps in the free-radical chlorination of methane. Explain why the -105 kJ/mol overall delta H degree value for the reaction does not include the delta H degree value for the initiation step, but instead is the sum of the delta H degree values for the two propagation steps. initiation step: 242 kJ/mol propagation step #1: 4 kJ/mol propagation step #2: -109 kJ/mol

Answer 1

In the initiation step Cl-Cl bond breaks in presence of light converting the Cl₂ molecule into free radical chlorine.

Cl₂   $\overset{h\nu }{\rightarrow}$    2Cl^.

After the free radical is formed, the radical attacks on the methane molecule proceeding the reaction. Therefore the initiation step does not contribute to the reaction mechaism.

Thus, $\Delta$ H^o value for the reaction does not include the $\Delta$ H^o value for the initiation step, but instead is the sum of the $\Delta$ H^o values for the two propagation steps.