Explain the mechanism of the photochemical reaction occurring between hydrogen and chlorine gas.

Explanation

The probable mechanism of photochemical combination between `H_2` and `Cl_2` is as follows:
(i) Chain initiating step. In the first step called chain initiating step, `Cl_2` molecule dissociates into atoms (called free radicals) by absorbing energy from Sunlight
`Cl : Cl underset(hv) overset( "sunlight")to overset(* )(Cl ) + overset(* )(Cl)`
(ii) Chain prepagating steps. In the second step, called chain propagating step, the chlorine atom (or free radical) attacks a hydrogen molecule to form HCl and hydrogen atoms.
` overset(* )(Cl) +H_2 to HCl + overset(* )H`
The highly reactive `overset(* )H` further attacks a chlorine molecule to form HCl and `overet(* )(Cl)`
` overset(* )(H) +Cl_2 to HCl + overset(* )(Cl)`
Since in the above steps one reactive atom is consumed and another is generated, the above steps are called chain propagating steps.
(iii) Chain terminating steps. The third step known as chain terminating step, normally takes place on the walls of the container. The reaction stops when free radicals combine among themselves, e.g.
` overset(* )(Cl) + overset(* )(Cl) to Cl_2`
` overset(* )(H) + overset(* )(H) to H_2`
`overset(* )(H) + overset(* )(Cl) to HCl`