Question

Given below are two statements: one is labelled as Assertion (A) and the other is labelled as Reason (R). Assertion (A): The quantum yield of a photochemical reaction can be greater than one.
Reason (R): A single photon can initiate chain of reactions, leading to the conversion of multiple reactant molecules.
In the light of the above statements, choose the most appropriate answer from the options given below :

• A. Both (A) and (R) are correct and (R) is the correct explanation of (A)
• B. Both (A) and (R) are correct but (R) is not the correct explanation of (A)
• C. (A) is correct but (R) is not correct
• D. (A) is not correct but (R) is correct

Hint:

Quantum yield > 1 occurs in chain reactions due to propagation steps.

Answer 1

The Correct Option is A

Concept: Quantum yield in photochemistry is defined as the number of molecules reacting per photon absorbed.

Step 1: Understanding Assertion (A).
Quantum yield ($\phi$) is given by: \[ \phi = \frac{\text{Number of molecules reacted}}{\text{Number of photons absorbed}} \] In some photochemical reactions, $\phi > 1$, meaning one photon leads to multiple reactions.
Thus, Assertion (A) is correct.

Step 2: Understanding Reason (R).
In chain reactions:
• One photon initiates the reaction
• Reactive intermediates (free radicals) propagate the reaction
• Multiple molecules react per photon absorbed Thus, Reason (R) is correct.

Step 3: Linking A and R.
The chain reaction mechanism directly explains why quantum yield can exceed one.

Step 4: Final evaluation.
Both statements are correct and (R) correctly explains (A). Final Conclusion:
Hence, the correct answer is option (1).