It is true that: JEE Main 2020 (03 Sep Shift 1)
A. A second order reaction is always a multistep reaction
B. A zero order reaction is a multi-step reaction
C. A first order reaction is always a single step reaction
D. A zero order reaction is a single step reaction
The Correct Answer and Explanation is:
To determine the correct answer to the question regarding the nature of different orders of reactions, we will analyze each option:
A. A second order reaction is always a multistep reaction.
This statement is not true. While many second-order reactions can occur in multiple steps, a second-order reaction can also occur in a single elementary step. For example, the reaction ( A + B \rightarrow \text{Products} ) is second order with respect to ( A ) and ( B ) and can be a single-step reaction.
B. A zero order reaction is a multi-step reaction.
This statement is also not true. A zero-order reaction means that the rate of reaction is constant and does not depend on the concentration of the reactants. This can occur in a single elementary step or through a multi-step process. However, it is not a requirement that zero-order reactions must involve multiple steps. For instance, a simple reaction where the product is formed at a constant rate regardless of the concentration of the reactant could be zero-order and occur in a single step.
C. A first order reaction is always a single step reaction.
This statement is not true. Although first-order reactions are often derived from single elementary steps, they can also arise from a sequence of steps that overall have a first-order rate law. A classic example is a reaction mechanism involving intermediates, where the rate-determining step (the slowest step) is first order, even if the overall process is not a single step.
D. A zero order reaction is a single step reaction.
This statement is generally true but not exclusively. While many zero-order reactions are single-step processes, they can also occur in a multi-step mechanism where the overall rate remains zero-order. For instance, in enzyme kinetics, the saturation of an enzyme can lead to a zero-order behavior at high substrate concentrations, despite involving multiple steps in the mechanism.
Conclusion
The most accurate answer, considering generalizations and examples from kinetics, is D: A zero-order reaction is a single-step reaction, but it’s important to recognize that zero-order can also arise in more complex mechanisms.
In summary, the nature of reaction orders is nuanced, and while certain orders often suggest specific mechanisms, they do not strictly determine the complexity or the number of steps involved in the reaction. Understanding these relationships helps chemists predict and control reaction behaviors effectively.