Q.Which of the following statements is not correct about order of a reaction.

The key idea is that the order of a reaction is defined by the rate law, not by the balanced equation.

• Statement (i) is correct: order can be fractional (e.g., $0.5$ for some decomposition reactions).
• Statement (ii) is correct: order is found only from experimental data, not from theory. …

The concept of reaction order is one of the first places where physical chemistry forces you to think like an experimentalist, not a mathematician. Many students instinctively want to "read off" the order from the balanced equation — after all, stoichiometric coefficients look like exponents. But that's a trap. The rate law is a purely empirical relationship: it tells you how the rate actually depends on concentration, not how you'd expect it to depend from the reaction's stoichiometry.

Let's examine each statement carefully.

1. Statement (i): "The order of a reaction can be a fractional number."

   This is correct. Orders like $1/2$, $3/2$, or even $0$ are common. For example, the decomposition of acetaldehyde (CH$_3$CHO) has order $3/2$. The rate law is determined by the mechanism, not by integer stoichiometry. So (i) is true.

2. Statement (ii): "Order of a reaction is experimentally determined quantity."

   Also correct. You cannot predict the order from the balanced equation alone — you must measure how the initial rate changes when you vary the concentration of each reactant. That's the whole point of the method of initial rates. So (ii) is true.

3. Statement (iii): "The order of a reaction is always equal to the sum of the stoichiometric coefficients of reactants in the balanced chemical equation for a reaction."

   This is the false one. It would only be true for elementary reactions — reactions that occur in a single step. But most reactions are multi-step, and the rate law is determined by the slowest step (the rate-determining step), which may involve different species or different coefficients than the overall balanced equation. For example, the reaction

has a rate law $\text{Rate} = k[\text{NO}]^2[\text{O}_2]$, which happens to match the stoichiometric coefficients — but that's a coincidence. In contrast, the reaction

$\text{H}_2 + \text{I}_2 \rightarrow 2\text{HI}$ …

Method: Conceptual Elimination Method

This method uses your understanding of reaction order definitions to identify the incorrect statement by checking each option against known facts.

Steps:

1. Recall the definition of order of a reaction

   The order of a reaction is the sum of the exponents (powers) of the concentration terms in the experimentally determined rate law.

   Example: For rate $= k[A]^m[B]^n$, order $= m + n$.

2. Evaluate each statement one by one:

   • (i) "The order of a reaction can be a fractional number."

     → Correct. Many reactions have fractional orders (e.g., $0.5$, $1.5$).

     Example: Decomposition of acetaldehyde has order $1.5$.

   • (ii) "Order of a reaction is experimentally determined quantity."

     → Correct. Order cannot be predicted from the balanced equation; it must be found from experiments.

   • (iii) "The order of a reaction is always equal to the sum of the stoichiometric coefficients of reactants in the balanced chemical equation for a reaction."

     → Incorrect. This is only true for elementary reactions (single-step). For most reactions (complex/multi-step), order and stoichiometric coefficients are not equal. …

Here are the common mistakes students make on this concept, along with how to avoid each.

Mistake 1: Confusing "Order of Reaction" with "Molecularity"

• The Error: Students often think the order of a reaction must be a whole number (like 1, 2, or 3) because they confuse it with molecularity (which is always a whole number).
• Why It’s Wrong: The order is derived from the rate law, which is determined experimentally. It can be zero, fractional, or even negative.
• How to Avoid: Remember: Order is experimental, molecularity is theoretical. If a question asks about fractional numbers, think of the rate law $r = k[A]^{1/2}$ — that’s a valid half-order reaction.

Mistake 2: Assuming Order = Sum of Stoichiometric Coefficients

• The Error: Many students pick option (iii) as correct because they think the order is always the sum of the coefficients in the balanced equation.
• Why It’s Wrong: This is only true for elementary reactions (single-step). Most reactions are complex (multi-step), and the rate law depends on the slowest step, not the overall balanced equation.
• How to Avoid: Always check: Is the reaction elementary? If not, the order has no relation to stoichiometric coefficients. For example:

The rate law is $r = k[NO]^2[O_2]$, so order = 3, which matches the sum of coefficients (2+1=3) — but this is a coincidence for this specific elementary step. In general, do not assume this.

Mistake 3: Misreading "Not Correct" as "Correct"

• The Error: The question asks: Which statement is not correct? Students sometimes rush and mark the first correct-looking option.
• Why It’s Wrong: Option (iii) is the incorrect statement (order is not always equal to the sum of stoichiometric coefficients). But students may mark (i), (ii), or (iv) because they sound true.
• How to Avoid: Underline the word "not" in the question. Then, for each option, ask yourself: Is this always true? If yes, eliminate it. The one that is false is your answer.

Mistake 4: Forgetting That Order is Experimentally Determined

• The Error: Some students think the order can be predicted from the balanced equation alone. …