Which of the following statements about reaction mechanisms is false?

• A. Intermediates are species that are produced and consumed during the course of a reaction mechanism.
• B. The rate-determining step in a reaction mechanism is always the fastest step.
• C. A reaction mechanism describes the step-by-step sequence of elementary reactions by which overall chemical change occurs.
• D. The molecularity of an elementary step refers to the number of reactant particles involved in that step.

Answer: (B)

Verified step by step guidance

1. Step 1: Understand what a reaction mechanism is. It is a detailed description of the sequence of elementary steps that lead from reactants to products in a chemical reaction.
2. Step 2: Define intermediates. Intermediates are species that are formed in one elementary step and consumed in a subsequent step, so they do not appear in the overall balanced equation.
3. Step 3: Explain the rate-determining step. This is the slowest step in the reaction mechanism, which controls the overall reaction rate because it acts as a bottleneck.
4. Step 4: Clarify molecularity. The molecularity of an elementary step is the number of reactant particles (atoms, ions, or molecules) that collide simultaneously to bring about that step.
5. Step 5: Identify the false statement by comparing each option to these definitions. The false statement is the one that incorrectly describes the rate-determining step as the fastest step, when in fact it is the slowest.