Question

Irradiation of a mixture of ethylene and mercury vapor with light of wavelength 253.7nm brings about the dissociation of ethylene. The reaction is an example of:

• A. Chemiluminescence
• B. Bioluminescence
• C. Photosensitized reaction
• D. Internal conversion reaction

Hint:

In a photosensitized reaction, think of the sensitizer (mercury) as a "middleman." It absorbs the light energy and then "hands it off" to the reactant (ethylene) to make the reaction happen.

Answer 1

The Correct Option is C

Step 1: Analyze the process. In this reaction, light is used to cause a chemical change (dissociation of ethylene). However, the light (253.7 nm) is absorbed by the mercury vapor, not directly by the ethylene. The excited mercury atoms then collide with ethylene molecules, transferring their energy and causing the ethylene to dissociate.

Step 2: Define the reaction types.
(1) Chemiluminescence: A chemical reaction that produces light. This is the opposite of the process described.
(2) Bioluminescence: Chemiluminescence occurring in a living organism.
(3) Photosensitized reaction: A chemical reaction initiated by a substance (the photosensitizer, in this case, mercury) that has been electronically excited by absorbing light. This excited substance then transfers its energy to the target molecule (ethylene). This is a perfect match.
(4) Internal conversion reaction: A non-radiative process where a molecule transitions to a lower electronic state without emitting light.
Conclusion: This is a classic example of a photosensitized reaction.