MP Assistant Professor Exam 2022
High quantum yields of photochemical reactions are due to:
Correct Answer: (C) accompanying side reaction.
Explanation:
Quantum yield (Φ) measures the efficiency of photon usage. While Einstein's law suggests Φ = 1, experimental values can reach 106.
1. Primary Process: Absorption of one photon leads to the excitation or dissociation of one molecule. (Φ ≤ 1)
2. Secondary Process: The reactive atoms or radicals produced initiate a series of accompanying reactions (chain reactions).
2. Secondary Process: The reactive atoms or radicals produced initiate a series of accompanying reactions (chain reactions).
Why (C) is the correct: In photochemical kinetics, accompanying side reactions refers to these secondary chain steps. Although free radicals (D) are often the carriers, the yield increases only because these radicals go on to participate in a sequence of further reactions.
Other Options:
- (A) & (B): These affect the rate of reaction but do not explain why one single photon causes thousands of molecules to react.
- (D): If radicals are formed but immediately undergo recombination, the quantum yield remains very low. Thus, the chain reactions (accompanying reactions) are the functional cause of high yield.