Q.(a) Give an example of a pseudo first-order reaction.
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Order and Molecularity: The Two Faces of a Reaction
Imagine you are watching a chemical reaction happen. Two molecules of A collide, rearrange, and become B. That collision — the actual event where bonds break and form — is an elementary step. The number of molecules that must come together in that single step is its molecularity.
Now imagine a different reaction. You mix A and B, but the product appears at a rate that depends on the square of A and not at all on B. That dependence — how the speed of the overall reaction changes when you change concentrations — is its order.
These two ideas answer different questions. Molecularity asks: How many particles actually meet in one step? Order asks: What is the mathematical relationship between concentration and rate for the overall reaction?
Molecularity — The Microscope View
Molecularity is a property of an elementary reaction only. It is the number of reactant particles (atoms, ions, molecules) that collide in that step.
- Unimolecular (1): A single molecule decomposes or rearranges. Example: N2O5→NO2+NO3
- Bimolecular (2): Two particles collide. Example: NO2+CO→NO+CO2
- Termolecular (3): Three particles collide simultaneously — very rare because three-way collisions are improbable. Example: 2NO+O2→2NO2
Molecularity is never zero, never fractional, and never greater than 3. You cannot have a molecularity of 1.5 or 4 — collisions of four particles at once are astronomically unlikely.
Order — The Macroscope View
Order is an experimental quantity. You run the reaction, measure the initial rate at different concentrations, and deduce the rate law:
Rate=k[A]m[B]n
The order with respect to A is m, the order with respect to B is n, and the overall order is m+n.
Order can be:
- Integer (0, 1, 2, 3)
- Fractional (e.g., 0.5 for a reaction like H2+Br2→2HBr)
- Negative (if increasing a reactant slows the reaction)
- Zero (if concentration does not affect rate)
Order is determined by experiment, not by the balanced chemical equation. A reaction written as 2A+B→C does not automatically have order 3. The actual rate law could be Rate=k[A]2 (order 2) or Rate=k[A] (order 1) — only the lab can tell.
The Critical Difference
| Property | Molecularity | Order |
|---|---|---|
| Applies to | Elementary steps only | Overall reaction (or any step) |
| Determined by | Stoichiometry of the step | Experiment |
| Can be fractional? | No | Yes |
| Can be zero? | No | Yes |
| Can be negative? | No | Yes |
| Maximum value | 3 (rarely) | Any number |
Why the Confusion?
Many students think: "If the balanced equation says 2A + B, then molecularity is 3 and order is 3." This is wrong for two reasons.
First, most reactions are multi-step. The balanced equation shows only the net change, not the actual collision events. A reaction like 2NO+O2→2NO2 appears termolecular, but it actually happens in two bimolecular steps:
NO+NO→N2O2(fast)
N2O2+O2→2NO2(slow)
The molecularity of each step is 2 (bimolecular). The overall order, determined by the slow step, is 2+1=3 — but that is a coincidence, not a rule. …
A reaction that is truly higher order can appear first order when one reactant is present in such excess that its concentration barely changes. The other parts follow from how the rate law responds when a concentration is doubled or held effectively constant. …
(a) A reaction that is genuinely higher-order but behaves as first order because one reactant is in huge excess is pseudo first order. (b) Doubling [B] in a rate law second-order in B quadruples the rate; making [A] effectively constant leaves only [B]'s order visible.
(a) Example of a pseudo first-order reaction.
The acid-catalysed hydrolysis of an ester, e.g. ethyl acetate, in the presence of a large excess of water:
CH3COOC2H5 + H2O (large excess) --H+--> CH3COOH + C2H5OH
This reaction is truly second order overall (first order in ester, first order in water), but because water is present in such a large excess, its concentration stays effectively constant throughout the reaction. So the rate law simplifies to rate = k'[ester], and the reaction is experimentally observed to follow first-order kinetics - hence 'pseudo' first order.
(b) Rate = k[A][B]^2 (first order in A, second order in B).
(i) If [B] is doubled (keeping [A] fixed): new rate = kA^2 = k[A] x 4[B]^2 = 4 x (k[A][B]^2) = 4 x original rate. So the rate becomes 4 times as fast. …
Showing the 12 most recent of 65 on this concept.
- CBSE 2026Set 56/2/11 markMCQQ.Identify the correct statement : (A) Molecularity of a reaction is an experimental quantity. (B) For complex reactions molecularity has no meaning. (C) Molecularity of a reaction can be zero or even a fraction. (D) Molecularity more than three is very common in chemical reactions.
›Reveal solutionSolution
Molecularity is a theoretical concept tied to the elementary step, not an experimental quantity; it is always a positive integer (1, 2, or 3) and has no meaning for complex reactions. The correct statement is (B).
The Concept: What Molecularity Really Means
Molecularity is one of those terms that sounds like it should be complicated, but it’s actually very simple — provided you remember one crucial thing: molecularity only applies to an elementary step, never to the overall reaction.
An elementary step is a single collision or a single molecular event. If two molecules collide and react in one step, that step is bimolecular (molecularity = 2). If one molecule breaks apart on its own, it’s unimolecular (molecularity = 1). That’s it. There is no such thing as a termolecular step involving four or five molecules simultaneously — the probability of that many particles colliding at the exact same instant is astronomically small. So molecularity is always a small positive integer: 1, 2, or rarely 3.
Now contrast this with order of reaction, which is an experimental quantity. Order can be zero, fractional, or even negative — it comes from the rate law you determine in the lab. Molecularity is a theoretical label for a step, not something you measure.
Let’s examine each option carefully.
Step-by-Step Analysis
1. Option (A): “Molecularity of a reaction is an experimental quantity.”
This is false. You cannot measure molecularity by doing an experiment. You can measure the rate and find the order, but molecularity is deduced from the mechanism — it’s a theoretical count of how many molecules participate in an elementary step. For example, if you propose that NO2+CO→NO+CO2 happens in one step, you assign it a molecularity of 2; you don’t “find” it experimentally.
2. Option (B): “For complex reactions molecularity has no meaning.”
This is true. A complex reaction proceeds through multiple elementary steps. The overall reaction has no single molecularity — each step has its own. Asking “what is the molecularity of the overall reaction?” is like asking “what is the age of a family?” — it doesn’t apply. Molecularity is defined only for an elementary step. …
- CBSE 2026Set 56/2/11 markMCQQ.For question number 13 to 16, two statements are given – one labelled as Assertion (A) and the other labelled as Reason (R). Select the correct answer to these questions from the codes (A), (B), (C) and (D) as given below : (A) Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of the Assertion (A). (B) Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of the Assertion (A). (C) Assertion (A) is true, but Reason (R) is false. (D) Assertion (A) is false, but Reason (R) is true. Assertion (A) : The molecularity of the given reaction is 2 2HI→H2+I2 Reason (R) : Two molecules of the reactants are involved in simultaneous collision between them.
›Reveal solutionSolution
The assertion is true (the reaction is bimolecular), and the reason correctly explains why — two HI molecules must collide simultaneously. The correct choice is (A).
Concept and Intuition
Molecularity of a reaction is a fundamental concept in chemical kinetics. It refers to the number of reactant particles (atoms, ions, or molecules) that must collide simultaneously in the rate-determining step (the slowest elementary step) for the reaction to occur. It is always a whole number (1, 2, or rarely 3) and is defined only for elementary reactions — not for overall balanced equations that may represent a sequence of steps.
The given reaction is:
2HI→H2+I2
This is an elementary reaction — it occurs in a single step. Two molecules of HI must come together and collide with sufficient energy and proper orientation to break the H–I bonds and form H–H and I–I bonds. Since two reactant molecules are involved in this single collision event, the molecularity is 2 (bimolecular).
The reason statement says exactly this: "Two molecules of the reactants are involved in simultaneous collision between them." That is the very definition of molecularity = 2 for this reaction.
Watch outA common mistake is to confuse molecularity with order of reaction. Molecularity is a theoretical concept based on the reaction mechanism (always an integer), while order is experimental (can be fractional, zero, or negative). For this elementary reaction, the order also happens to be 2, but that is not what the assertion is about.
Step-by-step reasoning
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Identify the nature of the reaction.
The reaction 2HI→H2+I2 is known to be an elementary reaction — it proceeds in a single step without any intermediate species. This is a standard fact from chemical kinetics.
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Define molecularity.
Molecularity is the number of reactant particles that come together in the rate-determining step of an elementary reaction. For a single-step reaction, it equals the number of molecules on the reactant side of the balanced elementary equation.
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Count the reactant molecules in the collision. …
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- CBSE 2026Set V11 markMCQQ.Which of the following statement/s is/are incorrect about order and molecularity of a chemical reaction? I. The probability that more than three molecules can collide and react simultaneously is very small. II. There can be only three values for the order of the reaction, that is zero, first and second. III. The order depends on rate equation while molecularity does not. IV. There can be infinite number of values for order.(a) I and IV(b) III only(c) II and IV(d) II only
›Reveal solutionSolution
Only statement II is wrong — the order of a reaction is not restricted to just zero, first and second.
Evaluate each statement:
- I. Correct. The chance of more than three molecules colliding and reacting simultaneously (a termolecular-plus event) is indeed very small; this is why high molecularities are not observed.
- II. Incorrect. The order of a reaction is not limited to only three values (0, 1, 2). Order is determined experimentally and can be 0, 1, 2, 3, a fraction (e.g. 1/2, 3/2) or even negative. This statement is false. …
- CBSE 2026Set ANNUAL1 markMCQQ.Rate = K [X]^3/2 [Y]^-1. The overall order of reaction for the above rate expression is(a) 1/2(b) 1(c) 3/2(d) 5/2
›Reveal solutionSolution
The overall order of a reaction is the sum of the powers (exponents) of the concentration terms in the experimentally determined rate law.
…
- CBSE 2026Set ANNUAL1 markQ.If the value of rate constant for any reaction is found to be 1.75 x 10^-5 L mol^-1 s^-1, then order of reaction will be ______ (fill in the blank).
›Reveal solutionSolution
The unit of the rate constant k reveals the overall order n of a reaction, since k has units of (concentration)^(1-n) time^-1.
- Zero order: mol L⁻1 s⁻1
- First order: s⁻1
- Second order: L mol⁻1 s⁻1 (mol⁻1 L s⁻1) …
- CBSE 2026Set ANNUAL1 markMCQQ.Calculate the overall order of a reaction which has the rate expression: Rate = K[A]^1/2 [B]^3/2(a) Second order(b) First order(c) Zero order(d) Third order
›Reveal solutionSolution
The overall order of a reaction is the sum of the powers (exponents) of the concentration terms in the experimentally determined rate law.
Given rate law: Rate = K[A]^(1/2)[B]^(3/2)
Order with respect to A = 1/2 …
- CBSE 2026Set ANNUAL1 markQ.Fill in the blank: For a chemical reaction ............... can never be a fraction.
›Reveal solutionSolution
Molecularity of an elementary reaction (the number of reacting species colliding simultaneously) is always a positive whole number — it can never be zero, negative, or a fraction. Order, by contrast, CAN be zero, negative, or fractional.
It is important not to confuse order and molecularity:
- Order of a reaction is an experimentally determined quantity (sum of the powers of concentration terms in the rate law). It can be zero, a whole number, or even a fraction (e.g., order 1.5 in some complex reactions), and is defined for the overall reaction (or steps of a mechanism). …
- CBSE 2026Set ANNUAL1 markMCQQ.For a reaction A+B+C→ Product, the rate law is expressed as, r=k[A]^(1/2)[B]^(1/3)[C]^(1/4), then the total order of reaction is-(a)(i) 13/11(b)(ii) 13/14(c)(iii) 12/13(d)(iv) 13/12
›Reveal solutionSolution
Overall order =21+31+41=1213. Correct option: (iv).
Concept. The order of a reaction is the sum of the powers (exponents) to which the concentration terms are raised in the experimentally determined rate law.
Steps.
Order=21+31+41
Take LCM =12: …
- CBSE 2026Set ANNUAL1 markQ.Write answer in one word/sentence: Write unit of rate constant for second order reaction.
›Reveal solutionSolution
The rate constant of a second-order reaction has units mol^-1 L s^-1.
For an n-th order reaction the unit of k is (concentration)^(1-n) time^-1, i.e. (mol L^-1)^(1-n) s^-1.
…
- CBSE 2026Set ANNUAL1 markQ.Why are reactions with three or higher molecularity very rare?
›Reveal solutionSolution
A trimolecular (or higher) step needs three or more particles to collide at the same instant with proper orientation and enough energy — a very low-probability event — so such reactions are rare.
Molecularity is the number of reacting species (atoms, ions or molecules) that come together in a single elementary reaction step.
- A bimolecular collision (two particles) is common.
- A termolecular step requires three particles to be at the same point in space at the same instant, all with the correct orientation and sufficient energy. The probability of such a simultaneous three-body collision is very small compared with a two-body collision. …
- CBSE 2026Set ANNUAL1 markQ.A reaction is found to be of ½ order. What is the unit of its rate constant?
›Reveal solutionSolution
Using rate =k[A]1/2 with rate in molL−1s−1 and concentration in molL−1, the unit of k is mol1/2L−1/2s−1.
The general rule for an nth-order reaction is that the unit of the rate constant is
k=mol(1−n)L(n−1)s−1
Derivation for n=21: the rate law is
rate=k[A]1/2
so …
- CBSE 2025Set ANNUAL1 markMCQQ.In reaction A→B, the rate of reaction is doubled on increasing concentration of reactant four times. The order of reaction is-(a) 1/2(b) 2(c) 4(d) Zero
›Reveal solutionSolution
Order found from rate = k[A]^n using the given fold-changes.
Rate law: rate =k[A]n. When [A] is increased 4 times, rate becomes 2 times.
rate2rate=k[A]nk(4[A])n=4n
…
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