Answer
See explanation
Work Step by Step
It is preferable to produce chlorethane by the reaction of HCl(g) with ethene than by the reaction of $Cl_{2}$ with ethane. The reasons are:
1. The first reaction is an addition reaction. Addition reaction of alkene happens in normal condition like at room temperature, normal atmospheric pressure etc. where as substitution reaction of alkane happens in presence of sunlight. It means that for the reaction to happen heat or sunlight is necessary which is an extreme condition than execute the addition reaction.
2. Another reason for addition reaction to be preferable than substitution reaction is that in addition reaction the yield of the product is maximum. When ethene reacts with $Cl_{2}$ it produces chloroethane exclusively. No other products or side products are produced along with chloroethane.
On the other hand, when ethane reacts with $Cl_{2}$ it produces chloroethane along with HCl as a side product. So, the yield of the desired product will be lower than yield from addition reaction. Also, we have to filter chloroethane from the mixture of chloroethane & HCl.
Look at the two reactions in the figure.
