Question

why sunlight is needed for the reaction of hydrogen and chlorine​

Answer 1

The energy supplied by the Sun by way of ultraviolet radiation gives the gases enough "activation energy" to react.The molecules of Hydrogen (H2) and Chlorine (Cl2) exist as relatively stable gases, and in a sealed container mixed together would not react.

Answer 2

HCL gas can also be formed by reaction of moist hydrogen gas with chlorine in the presence of the diffused sunlight .

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