Chemical Kinetics

Step 1: Recall first-order kinetics
For a first-order reaction: where: - = time - = rate constant - = initial concentration - = reacted concentration
Step 2: Calculate time for 90% completion
For 90% completion ( ):
Step 3: Calculate time for 99% completion
For 99% completion ( ):
Step 4: Compute the ratio

Step 5: Verify with options
The ratio equals 2, corresponding to option (b).

Molecularity is the number of molecules or ions colliding in an elementary step of a reaction. It is always a positive integer (1, 2, 3, etc.) for elementary reactions.
Step 1: Analyze options
- (A) Zero: Molecularity cannot be zero, as at least one species must participate in an elementary step.
- (B) : Molecularity cannot be negative, as it represents a count of colliding species.
- (C) 1: Unimolecular reactions (e.g., radioactive decay, ) have molecularity 1.
- (D) 0.5: Molecularity cannot be fractional, as it is a whole number.
Step 2: Conclusion
Molecularity can be 1 (unimolecular), matching option (C).

Step 1: Recall first-order kinetics
For a first-order reaction: where is the rate constant and is the concentration.
Step 2: Derive half-life expression
The integrated rate law gives: At half-life ( ), :
Step 3: Verify options
Only option (a) matches the derived relationship:
Step 4: Characteristics

• Half-life is independent of initial concentration
• is (natural log of 2)
• Units: in , in seconds