Question

For a complex (multistep) reaction,

• A. the overall rate of the reaction is the rate of the slowest step.
• B. the overall molecularity has no significance.
• C. molecularity and order may or may not be same.
• D. the number of rate-determining steps may be more than one.

Answer 1

Answer: (A), (B), (C), (D)

The correct options are
A the overall rate of the reaction is the rate of the slowest step.
B the overall molecularity has no significance.
C molecularity and order may or may not be same.
D the number of rate-determining steps may be more than one.

For option (A) :

The overall reaction rate is determined by the rates of the steps up to (and including) the slowest elementary step. The slowest step in a reaction mechanism is called the rate-determining or rate-limiting step.

Another way to visualize this is by thinking about pouring water into a series of funnels with different diameters. The smallest diameter funnel controls the rate at which the bottle is filled, whether it is the first or the last in the series. Pouring liquid into the first funnel faster than it can drain through the smallest funnel will only result in an overflow! So, it is true that the overall rate of a reaction is the rate of the slowest step.

For option (B) :

It is true that the overall molecularity of a complex reaction has no significance. It is the only slowest step whose molecularity has significance for the overall reaction.

For option (C) :

The order of a reaction represents the sum of the exponents to which the concentration terms in the rate equation are raised, and the molecularity of a reaction is the number of reactant molecules that participate in the elementary reaction. So, it is true that the molecularity and the order of a reaction may not be the same.

For option (D) :

Since the experimental rate law does not match our predicted rate law, we know immediately that our reaction must involve more than one step. Reactions that involve more than one elementary step are called complex reactions.