Question

Assertion (A) : For complex reaction, order of reaction is given by the slowest step.
Reason (R) : Order of a reaction is an experimental quantity.

• A. Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of the Assertion (A).
• B. Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of the Assertion (A).
• C. Assertion (A) is true, but Reason (R) is false.
• D. Assertion (A) is false, but Reason (R) is true.

Hint:

Molecularity applies only to elementary steps. Order applies to the whole reaction.
If a reaction is elementary, its order and molecularity are usually the same.

Answer 1

The Correct Option is B

Step 1: Check the Assertion.
A complex reaction moves through several elementary steps, and the overall rate can never outrun its slowest step, so the rate law, and therefore the order, of the whole reaction is dictated by that rate determining step. This makes the Assertion true.
Step 2: Check the Reason.
Order cannot simply be read off the balanced equation the way molecularity can, it has to be measured in the lab by watching how the rate actually changes as concentrations are varied. So the Reason is also a true statement on its own.
Step 3: Test whether the Reason explains the Assertion.
The Assertion is true because of the kinetic bottleneck created by the slowest step, not because order happens to be measured experimentally. These are two separate, unconnected facts about reaction kinetics, so the Reason does not explain why the slow step controls the order.
Step 4: Choose the matching option.
Both statements are true, but the Reason is not the correct explanation of the Assertion. \[ \boxed{\text{Option (B)}} \]