A mixture of hydrogen gas and chlorine gas remains unreacted...

A mixture of hydrogen gas and chlorine gas remains unreacted until it is exposed to ultraviolet light from a burning magnesium strip. Then the following reaction occurs very rapidly:
$$\mathrm{H}_{2}(g)+\mathrm{Cl}_{2}(g) \longrightarrow 2 \mathrm{HCl}(g)$$
Explain.

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Key Concepts

Activation Energy

Activation energy is the minimum energy required for a chemical reaction to occur. In a mixture of hydrogen and chlorine gases, the molecules do not react at room temperature because their kinetic energy is insufficient to overcome this barrier. Ultraviolet light supplies the additional energy needed, allowing the reaction to proceed once the activation energy threshold is surpassed.

Photochemical Activation

This concept refers to the process by which light energy, especially from ultraviolet light, is absorbed by molecules, providing the energy needed to break chemical bonds. In the context of the reaction between hydrogen and chlorine gases, ultraviolet light initiates the reaction by activating the chlorine molecules, splitting them into reactive radicals that can then react with hydrogen.

Free Radical Chain Reaction

A free radical chain reaction involves the generation of highly reactive radical species that propagate a reaction sequence. In the reaction of hydrogen and chlorine, the UV light causes the homolytic cleavage of Cl2, creating chlorine radicals. These radicals then react with hydrogen to form HCl and another radical, perpetuating the reaction chain until the available reactants are consumed.

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