Question

Consider the following statements about reaction mechanisms?

Statement (I): A rate law can be written from the molecularity of the slowest elementary step

Statement (II): The overall rate law is the sum of the rate laws for the elementary steps.

• A. I and II both are correct

Answer 1

Statement I is correct, Statement II is incorrect.

Answer 2

Statement (I): A rate law can be written from the molecularity of the slowest elementary step.

• An elementary step is a reaction that occurs in a single step. For an elementary step, its rate law can be directly determined from its stoichiometry, which is related to its molecularity. For example, if an elementary step is $A + B \rightarrow P$, its molecularity is 2, and its rate law is Rate = $k[A][B]$.
• In a multi-step reaction mechanism, the slowest elementary step is called the rate-determining step (RDS). The rate of the overall reaction is governed by the rate of this slowest step.
• The rate law for the RDS itself is written directly from its molecularity. If the RDS involves intermediates, their concentrations are then expressed in terms of reactant concentrations using preceding fast equilibrium steps. Ultimately, the overall rate law is derived from the rate law of the slowest elementary step, which is based on its molecularity.
• Therefore, Statement (I) is correct.

Statement (II): The overall rate law is the sum of the rate laws for the elementary steps.

• This statement is incorrect. The overall rate law of a multi-step reaction is determined by the rate of the slowest step (rate-determining step), not by summing the rate laws of all elementary steps. The concept of a rate-determining step implies a bottleneck, not an additive contribution from all steps.
• Therefore, Statement (II) is incorrect.

Conclusion: Statement (I) is correct, and Statement (II) is incorrect. Option (A) states "I and II both are correct". Since Statement (II) is incorrect, option (A) is false.