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: Process Analysis. This reaction utilizes light to induce a chemical change (ethylene dissociation). However, the 253.7 nm light is absorbed by mercury vapor, not directly by ethylene. Excited mercury atoms then collide with ethylene, transferring energy and causing dissociation.

Step 2: Reaction Type Definitions.
(1) Chemiluminescence: Light production from a chemical reaction. This is the reverse of the described process.
(2) Bioluminescence: Chemiluminescence within a living organism.
(3) Photosensitized reaction: A chemical reaction triggered by a light-excited substance (photosensitizer, mercury in this case). This substance transfers energy to the target molecule (ethylene). This is the correct description.
(4) Internal conversion reaction: A non-radiative transition to a lower electronic state.
Conclusion: This exemplifies a photosensitized reaction.