Question

Which step is the chain propagation step in the following mechanism?
1. $\text{C}{{\text{l}}_{\text{2}}}\xrightarrow{\text{hv}}\text{C}{{\text{l}}^{\text{}}}\text{+C}{{\text{l}}^{\text{}}}$
2. $\text{C}{{\text{l}}^{\text{}}}\text{+C}{{\text{H}}_{\text{4}}}\to {}^{\bullet }\text{C}{{\text{H}}_{3}}+\text{HCl}$
3. $\text{C}{{\text{l}}^{\text{}}}\text{+C}{{\text{l}}^{\text{}}}\to \text{C}{{\text{l}}_{\text{2}}}$
4. ${}^{\centerdot }\text{C}{{\text{H}}_{\text{3}}}\text{+C}{{\text{l}}^{\text{}}}\to \text{C}{{\text{H}}_{\text{3}}}\text{Cl}$

Answer 1

In the chain propagation step no. of present free radical species as the reactant is always equal to the no. of formed free radical species as a product. Chain propagation step will never be the first step in any chemical reaction.

Complete step by step solution:
As there are 4 steps which are given in the above question, among all of them step 1st is the initiation step because:
-In the presence of light chlorine molecules absorbs energy and dissociates into two chlorine free radical species which helps for further proceeding of reaction.
$\text{C}{{\text{l}}_{\text{2}}}\xrightarrow{\text{hv}}\text{C}{{\text{l}}^{\text{}}}\text{+C}{{\text{l}}^{\text{}}}$
-Now the formed chlorine free radical in the first step reacts with the methane molecule and forms methane free radical and ,
$\text{C}{{\text{l}}^{\text{}}}\text{+C}{{\text{H}}_{\text{4}}}\to {}^{\bullet }\text{C}{{\text{H}}_{3}}+\text{HCl}$
-Again the formed methane free radical reacts with the chlorine molecule and regenerates chlorine free radical, which further reacts with another methane molecule and this process goes on continuously,
${}^{\text{}}\text{C}{{\text{H}}_{\text{3}}}\text{+C}{{\text{l}}_{\text{2}}}\to \text{C}{{\text{H}}_{\text{3}}}\text{Cl+C}{{\text{l}}^{\text{}}}$
From the above mechanism it is clear that step 2nd is the chain propagation step because in this step regeneration of species takes place.
So, 3rd & 4th step is the termination step because after these steps no species are available for further reaction.
-As two chlorine free radical reacts together to form chlorine molecule.
$\text{C}{{\text{l}}^{\text{}}}\text{+C}{{\text{l}}^{\text{}}}\to \text{C}{{\text{l}}_{\text{2}}}$
-And one chlorine free radical & one methane free radical reacts together to form chloro methane.
${}^{\centerdot }\text{C}{{\text{H}}_{\text{3}}}\text{+C}{{\text{l}}^{\text{}}}\to \text{C}{{\text{H}}_{\text{3}}}\text{Cl}$
From the above discussion it is clear that step 2nd is the chain propagation step.

Note: In these type of questions some of you may get confused for finding the chain propagation step according to the concept of regeneration which is given in the explanation part, so to get rid of that type of confusion you should use the concept which is mention in the hint part.