Calculate the overall heat of reaction.
Given reaction is a substitution; bromine does not add to methane but a bromine atom substitutes for one hydrogen atoms, which becomes part of the HBr byproduct.
Bromination of methane consists of three steps, which are initiation,propagation and termination.
The initiation step, which generates a reactive intermediate:
Light absorbed by bromine, split a bromine molecule into two bromine atoms.
Propagation steps, in which the reactive intermediates reacts with a stable molecule to form another reactive intermediate, allowing the chain to continue until the supply of reactants is exhausted or the reactive intermediate is destroyed.
First propagation step:
Breaking a -Br bond +435 kJ/mol (+104 kcal/mol)
Forming a H-Br bond - 368 kJ/mol (-88 kcal/mol)
+67 kJ/mol (+kcal/mol)
Second propagation step:
Breaking a Br-Br bond + 192 kJ/mol (+45 kcal/mol)
Forming a -Br bond -293 kJ/mol (-70 kcal/mol)
-101 kJ/mol (-24 kcal/mol)
The sum of the values of for the individual propagation steps gives the overall enthalpy
change for the reaction. The initiation step is not added to give the overall enthalpy change because it is not necessary for each molecule to form product.