\(\mathrm{CH_4(g) + Cl_2(g) \rightarrow CH_3Cl_{(s)} + HCl_{(g)}}\)

The major factor that influence the rate of the reaction above is

Correct idea: The reaction rate is mainly controlled by light (UV).

Why? Chlorination of methane is a photochemical, free-radical substitution. It starts when light splits chlorine molecules:

\(\mathrm{Cl_2 \xrightarrow{h\nu} 2\,Cl\cdot}\)  (initiation)

The resulting chlorine radicals drive the fast chain steps that form \(\mathrm{CH_3Cl}\) and \(\mathrm{HCl}\). More light (especially UV) → more radicals → faster reaction.

• Catalyst: not required for this reaction.
• Temperature: has some effect, but the reaction hardly proceeds in the dark; light is the key driver.
• Concentration: affects collision frequency, but without radical initiation by light, the rate remains very low.

Exam tip: Pick the option that says light/UV radiation.