$($a$)$ Give one reason why a steric factor may be needed to correct the rate constant for a bimolecular gas phase reaction when it is calculated using simple collision theory.

$($b$)$ Determine the value of the steric factor for the following gas phase reaction at a temperature of $600K$

$H_2+C_2{H}_4{C}_2{H}_6$

The experimentally determined Arrhenius pre$-$exponential factor for the reaction is $1\mathrm{.}2{10}^{6}mo{l}^{-1}d{m}^3{s}^{-1}.$ The radius of $H_2$ is $1\mathrm{.}45$ and the kinetic radius of $C_2{H}_4$ is $2\mathrm{.}08.$ The atomic mass of $H$ is $1\mathrm{.}008a$ and the atomic mass of $C$ is $12\mathrm{.}011a.$