The reaction of hydrogen and chlorine to produce hydrogen chloride gas is explosive in

Analyze the question

The question asks about the conditions under which H₂ and Cl₂ react explosively to form HCl. This is a classic photochemical reaction.

Explain the concept

Hydrogen and chlorine do not react in the dark at room temperature. Light supplies the energy that starts a chain reaction:

• Initiation: Cl₂ + light (hν) → 2Cl· (the Cl-Cl bond breaks, making chlorine free radicals)
• Propagation: • Cl· + H₂ → HCl + H· • H· + Cl₂ → HCl + Cl·
• The chlorine radical is regenerated each time, so the cycle repeats very fast. Each photon can lead to thousands of HCl molecules.

Compare the conditions

• Dark: no reaction.
• Diffused light: the reaction is slow and steady, with no explosion.
• Bright sunlight (or burning magnesium/UV light): the high intensity of light produces many radicals at once, so the chain reaction runs out of control and the reaction is explosive.
• Infrared light: it carries heat but not enough energy per photon to break the Cl-Cl bond, so it does not start the reaction.
• Raman light: this is not a type of light that triggers reactions. Raman scattering is a spectroscopy phenomenon.

Answer

The reaction is explosive in bright sunlight, which is option B.

H₂(g) + Cl₂(g) → 2HCl(g) (explosive in sunlight)

Exam tip

Remember: "Dark = nothing, diffused light = slow, sunlight = BOOM." Whenever JAMB asks about H₂ + Cl₂, think "photochemical chain reaction" and pick the strongest light source in the options.