CBSE Class 12 · 2003 · Delhi · Chemical KineticsShort Answer

What are photochemical reactions? Explain the mechanism of the photochemical reaction occurring between hydrogen and chlorine gas.

Solution

Photochemical reactions are reactions that occur by the absorption of light (photons), e.g. H

_{2}

+ Cl

_{2} h ν

2HCl.
Mechanism (chain reaction):
Initiation:

C l_{2} h ν 2C l^{∙}

Propagation:

C l^{∙} + H_{2} ⟶ HCl + H^{∙}

H^{∙} + C l_{2} ⟶ HCl + C l^{∙}

Termination: combination of radicals, e.g.

C l^{∙} + C l^{∙} \to C l_{2}

.
A single photon can thus produce a very large number of HCl molecules.

CBSE Class 12 Board · 2003 · Delhi · Subjective