Question

For a complex reaction, the order of reaction is equal to:

• A. Sum of stoichiometric coefficients
• B. Molecularity of the overall reaction
• C. Order of the fastest step
• D. Molecularity of the slowest step

Answer 1

The Correct Option is D

In the context of complex reactions, understanding how the order of reaction is determined is crucial. The order of a reaction is typically defined by the rate-determining step, which is the slowest step in a reaction mechanism. 

The molecularity of a step in a reaction mechanism refers to the number of molecules participating in that step. In a complex reaction, the molecularity of the slowest step directly influences the order of the reaction. This is because the overall reaction rate is limited by the speed of the slowest step, and thus the order of the overall reaction matches the molecularity of this crucial step.

Therefore, for a complex reaction, the order of reaction is equal to: Molecularity of the slowest step.

Answer 2

For a complex reaction, which involves multiple steps, the order of the overall reaction is determined by the slowest step in the mechanism, also known as the rate-determining step.

Here's a breakdown of why:

• Complex Reaction: A reaction that occurs through a series of elementary steps.
• Rate-Determining Step: The slowest step in a complex reaction, which limits the overall rate of the reaction.
• Order of Reaction: The sum of the exponents of the concentration terms in the rate law.
• Molecularity: The number of molecules involved in an elementary step.

Let's analyze the options:

• Option 1: Sum of stoichiometric coefficients: This is generally true for elementary reactions, but not necessarily for complex reactions.
• Option 2: Molecularity of the overall reaction: Molecularity is defined for elementary reactions, not complex reactions. Complex reactions do not have an overall molecularity.
• Option 3: Order of the fastest step: The fastest step has minimal impact on the overall rate, as it proceeds quickly.
• Option 4: Molecularity of the slowest step: While the molecularity of the slowest step is important in determining the rate law for that step, the order of the reaction is what we are looking for. The rate determining step dictates the *rate* of the reaction, and therefore its order.

The order of the reaction is determined by the rate law, which is derived from the slowest (rate-determining) step.

Therefore, the correct answer is:

Option 4: Molecularity of the slowest step, but the order of reaction is based on the rate law of the slowest step.

However, the best fit for the question is the order of the slowest step, as the molecularity of that step directly impacts the rate law and consequently the order.

Option D: Molecularity of the slowest step