It is true that
(a) a second order reaction is always a multistep reaction
(b) a zero order reaction

Q: Why can't a zero-order reaction be a single step?

In a single-step (elementary) reaction, the rate law is directly derived from the stoichiometry of the balanced equation. If a reaction were a single step, its rate would depend on the concentration of the colliding reactant molecules, meaning the order would be at least 1. A zero-order rate implies the rate is determined by an external factor (like catalyst surface or light) in a complex, multi-step mechanism.

Q: Are all fractional-order reactions multi-step?

Yes! Just like zero-order reactions, any reaction with a fractional order must be a complex, multi-step reaction. Elementary reactions can only have molecularities (and thus orders) of 1, 2, or rarely 3.

Q: Can a multi-step reaction have an overall order of 2?

Absolutely. The overall order of a complex reaction is determined by its slowest (rate-determining) step. If the slowest step involves the collision of two molecules, the overall reaction will typically be second order, even if there are many other fast steps.

Q: What is the difference between molecularity and order?

Molecularity is a theoretical concept representing the number of molecules colliding in a single elementary step (it must be a positive integer). Order is an experimental quantity representing how the rate depends on concentrations (it can be zero, fractional, or an integer).

Q: Why is $S_N1$ a first-order reaction if it has two steps?

In an $S_N1$ reaction, the first step (formation of the carbocation) is much slower than the second step. The rate of the overall reaction is limited by this slow step, which only involves one molecule breaking apart. Hence, the rate depends only on the concentration of that single substrate, making it first order.

Question

It is true that

Options:

(A) a second order reaction is always a multistep reaction

(B) a zero order reaction is a multistep reaction

(C) a first order reaction is always a single step reaction

(D) a zero order reaction is a single step reaction

Sigma Percentile · Accepted Answer

### The Mystery of Reaction Orders and Steps

When we dive into the world of chemical kinetics, one of the most fascinating puzzles is understanding how the **order of a reaction** relates to its **mechanism**—the actual steps the molecules take to transform from reactants to products. Let's break down the options provided in the question and see which one holds the truth.

### Analyzing Zero-Order Reactions

Imagine a reaction where the rate is completely independent of the concentration of the reactants. This is a **zero-order reaction**. But how can a reaction proceed without depending on how much reactant is present? This happens when the reaction is constrained by other factors, such as the availability of a catalyst's surface area or the intensity of light.

Because of these constraints, zero-order reactions are never simple, single-step processes. They are inherently **complex, multi-step reactions**. A classic example is a photochemical reaction involving free radicals. These reactions proceed through a sequence of steps: **chain-initiation**, **chain-propagation**, and **chain-termination**. Therefore, the statement that a zero-order reaction is a multistep reaction is absolutely true.

### What About First and Second-Order Reactions?

Let's look at the other claims. Is a second-order reaction always a multistep reaction? Not at all! Think back to organic chemistry and the famous **$S_N2$** (Substitution Nucleophilic Bimolecular) reaction. In an $S_N2$ mechanism, the nucleophile attacks and the leaving group departs simultaneously in a single, concerted step. Yet, its rate depends on two species, making it a second-order reaction. Thus, a second-order reaction can definitely be a single-step process.

Similarly, is a first-order reaction always a single-step reaction? Again, no. Consider the **$S_N1$** reaction. It happens in two distinct steps: first, the slow formation of a carbocation intermediate, followed by the fast attack of a nucleophile. The slow first step determines the rate, which depends only on the substrate, making it a first-order reaction despite being a multi-step process.

### The Final Verdict

By carefully analyzing the mechanisms, we can confidently conclude that zero-order reactions are always complex and involve multiple steps. The other statements make incorrect generalizations about first and second-order reactions. Therefore, the only true statement is that a zero-order reaction is a multistep reaction.

Animated Solution for Chemistry - Chemical Kinetics: It is true that

Select Answer:

Visualized Solution

$Zero Order Reactions$

$Second Order Reactions$

$First Order Reactions$

$Conclusion$

The Sigma Insight: Order and Molecularity

The Mystery of Reaction Orders and Steps

Analyzing Zero-Order Reactions

What About First and Second-Order Reactions?

The Final Verdict

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