Q.For the reaction A → B, the rate of reaction becomes three times when the concentration of A is increased by nine times. What is the order of 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. …
Using the rate law rate=k[A]n, if increasing [A] nine times makes the rate three times, then 3=9n=(32)n, so 2n=1. …
The order comes straight from the rate law: rate ×3 for concentration ×9 gives 9n=3, so n=21.
Concept. For A→B, the rate depends on [A] as rate=k[A]n, where n is the order with respect to A. This is a standard CBSE Class-12 Chemistry chemical-kinetics numerical.
Why. The order is the experimentally found exponent linking rate to concentration; we recover it by comparing two rates.
Steps. …
Showing the 12 most recent of 15 on this concept.
- KEAM 2026Set eng-2026-04174 marksMCQQ.The overall order of the following reaction is KClO3+6FeSO4+3H2SO4→KCl+3Fe2(SO4)3+3H2O (A) ten (B) six (C) two (D) three (E) one
›Reveal solutionSolution
[!TLDR]
Summing the stoichiometric coefficients of all reactants (1+6+3) gives an overall value of ten.
Concept
While true reaction order is strictly experimental, questions of this form take the "overall order/molecularity" as the total number of reactant species indicated by the balanced equation — i.e. the sum of the reactant coefficients.
Solution
The equation is
KClO3+6FeSO4+3H2SO4→KCl+3Fe2(SO4)3+3H2O.
The reactant coefficients are:
- KClO3: 1
- FeSO4: 6
- H2SO4: 3 …
- KEAM 2026Set eng-2026-04194 marksMCQQ.Which of the following reactions are complex reactions?(i) Oxidation of ethane(ii) Thermal decomposition of HI on gold surface(iii) Saponification of methyl acetate(iv) Nitration of phenol(v) Decomposition of NH3 on hot Pt surface. (A)(i) and(iii) (B)(ii) and(iv) (C)(i) and(iv) (D)(ii) and(v) (E)(i) and (v)
›Reveal solutionSolution
Complex reactions occur via a sequence of elementary steps. Oxidation of ethane and nitration of phenol are multi-step (complex); the others are simple/single-step.
Classify:
- (i) Oxidation of ethane — combustion, a chain of many elementary steps → complex.
- (ii) Thermal decomposition of HI on a gold surface — a simple (heterogeneously catalysed, zero-order) reaction.
- (iii) Saponification of methyl acetate — a simple bimolecular reaction. …
- KEAM 2026Set eng-2026-04224 marksMCQQ.Which of the following statement is incorrect? (A) All natural radioactive processes follow first order kinetics. (B) Thermal decomposition of HI on gold surface is a zero-order reaction. (C) Decomposition of hydrogen peroxide catalysed by iodide in an alkaline medium is a first order reaction. (D) The unit of second order rate constant is mol−1L s−1. (E) The experimental rate expression of the reaction, CHCl3+Cl2→CCl4+HCl is, Rate = k[CHCl3][Cl2].
›Reveal solutionSolution
Statements (A)–(D) are correct NCERT facts. The experimental rate law for CHCl3+Cl2→CCl4+HCl is Rate =k[CHCl3][Cl2]1/2 (order 3/2), so (E) is wrong. …
- KEAM 2026Set pha-2026-0418F4 marksMCQQ.Which of the following is a first order reaction? (A) Decomposition of ammonia on Pt surface at high temperature. (B) Hydrogenation of ethene to ethane. (C) Decomposition of HI on gold surface. (D) Hydrolysis of ethyl acetate in the presence of NaOH. (E) Oxidation of KI by peroxy disulphate.
›Reveal solutionSolution
C2H4+H2→C2H6, Rate =k[C2H4] — a first order reaction (NCERT).
Checking each:
- Decomposition of NH3 on Pt at high T: zero order.
- Hydrogenation of ethene to ethane: first order (Rate = k[C2H4]).
- Decomposition of HI on gold surface: zero order.
- Hydrolysis of ethyl acetate with NaOH (saponification): second order. …
- KEAM 2026Set pha-2026-0418F4 marksMCQQ.The units of rate constants of two reactions I and II are respectively mol−1 L s−1 and mol L−1s−1. Then, (A) reaction I is first order and reaction II is second order. (B) reaction I is second order and reaction II is first order. (C) reaction I is first order and reaction II is zero order. (D) reaction I is zero order and reaction II is first order. (E) reaction I is second order and reaction II is zero order.
›Reveal solutionSolution
Sulphuric acid (H2SO4) converts sodium chromate to sodium dichromate.
Reasoning
In the preparation of potassium dichromate, the yellow chromate is acidified to give the orange dichromate:
2Na2CrO4+H2SO4→Na2Cr2O7+Na2SO4+H2O. …
- KEAM 2026Set pha-2026-0419F4 marksMCQQ.The rate equation for the reaction xA+yB→mC+nD is rate = k[A]1/2[B]3/2. What is the total order of the reaction? (A) 2.0 (B) 1.5 (C) 1.0 (D) 0.5 (E) 2.5
›Reveal solutionSolution
Overall order is the sum of the exponents: 21+23=2.
For rate =k[A]1/2[B]3/2, the order in A is 1/2 and in B is 3/2. The total (ove …
- KEAM 2026Set pha-2026-0420F4 marksMCQQ.For the elementary reaction, M→N, the rate of disappearance of ‘M’ increases by a factor of 8 upon doubling the concentration of M. The order of the reaction with respect to M is (A) 1 (B) 2 (C) 3 (D) 4 (E) 5
›Reveal solutionSolution
2n=8⇒n=3; the reaction is third order in M.
For rate =k[M]n, doubling the concentration changes the rate by a factor 2n. Given that the rate increases 8-fold: …
- KEAM 2025Set eng-2025-04274 marksMCQQ.Ammonium ion (NH4+) reacts with nitrite ion (NO2−) according to the following equation: NH4++NO2−→N2(g)+2H2O(l). The following initial rates of reaction have been measured for the given reactant concentrations. Experiment I: [NH4+]0=0.010M, [NO2−]0=0.020M, Initial rate = 0.020 M/hour. Experiment II: [NH4+]0=0.015M, [NO2−]0=0.020M, Initial rate = 0.030 M/hour. Experiment III: [NH4+]0=0.010M, [NO2−]0=0.010M, Initial rate = 0.005 M/hour. Which of the following is the rate law (rate equation) for this reaction? (A) Rate = k[NH4+]1/2[NO2−] (B) Rate = k[NH4+][NO2−] (C) Rate = k[NH4+]0[NO2−] (D) Rate = k[NH4+][NO2−]1/2 (E) Rate = k[NH4+][NO2−]2
›Reveal solutionSolution
Comparing experiments, halving [NO2−] cuts the rate to one-quarter (second order) and multiplying [NH4+] by 1.5 multiplies the rate by 1.5 (first order). Rate =k[NH4+][NO2−]2.
Reasoning
Assume Rate =k[NH4+]x[NO2−]y.
Order in NH4+ (compare I and II, [NO2−] fixed at 0.020 M):
rateIrateII=0.0200.030=1.5=(0.0100.015)x=(1.5)x⇒x=1
Order in NO2− (compare I and III, [NH4+] fixed at 0.010 M): …
- KEAM 2025Set pha-2025-0424F4 marksMCQQ.What is the unit of rate constant for a second order reaction? (A) mol2L−1s−1 (B) mol−1L−1s−1 (C) mol−1Ls−1 (D) molL2s−1 (E) molL−1s−1
›Reveal solutionSolution
For a second-order reaction k has units of mol−1Ls−1 (i.e. L mol−1s−1).
The general unit of a rate constant for an n-th order reaction is
kunit=(mol L−1)1−ns−1.
For a second-order reaction, rate =k[A]2, so …
- KEAM 2025Set pha-2025-0424A4 marksMCQQ.An example of pseudo first order reaction is (A) Thermal decomposition of N2O5 (B) Inversion of cane sugar (C) Decomposition of gaseous NH3 on hot Pt surface (D) Radioactive decay of 88226R (E) Hydrogenation of ethene
›Reveal solutionSolution
The inversion of cane sugar is a pseudo first order reaction.
A pseudo first order reaction is genuinely higher order but behaves as first order because one reactant is present in large excess (its concentration stays effectively constant).
In the acid-catalysed hydrolysis (inversion) of cane sugar:
C12H22O11+H2OH+C6H12O6+C6H12O6 …
- KEAM 2025Set pha-2025-0429F4 marksMCQQ.The following data were obtained for the reaction , 2NO(g)+O2(g)→2N2O(g) at different concentrations,The rate law of this reaction is (A) Rate = k[NO][O2] (B) Rate = k[NO][O2]2 (C) Rate = k[NO]2[O2]2 (D) Rate = k[NO]2[O2] (E) Rate = k[NO]2[O2]3
Experiment [NO]/ mol L⁻¹ min⁻¹ [O₂]/ mol L⁻¹min⁻¹ Initial rate of formation of [NO₂]/ mol L⁻¹ min⁻¹ 1 0.30 0.30 0.096 2 0.60 0.30 0.384 3 0.30 0.60 0.192 4 0.60 0.60 0.768 ›Reveal solutionSolution
Comparing experiments: [NO]×2 → rate ×4 (second order in NO); [O₂]×2 → rate ×2 (first order in O₂). So Rate =k[NO]2[O2].
Reasoning
Order in NO — compare Exp 1 and 2 (O₂ constant at 0.30): [NO] goes 0.30 → 0.60 (×2), rate goes 0.096 → 0.384 (×4). Since 2x=4, x=2.
Order in O₂ — compare Exp 1 and 3 (NO constant at 0.30): [O₂] goes 0.30 → 0.60 (×2), rate goes 0.096 → 0.192 (×2). Since 2y=2, y=1. …
- KEAM 2024Set eng-2024-06064 marksMCQQ.For the reaction 2A+B→2C+D, the following kinetic data were obtained for three different experiments performed at the same temperature.The total order and order in [B] for the reaction are respectively (A) 2,1 (B) 1,1 (C) 1,2 (D) 2,2 (E) 2,0
Experiment [A]0 /M [B]0 /M Initial rate/ M s−1 I 0.10 0.10 0.10 II 0.20 0.10 0.40 II 0.20 0.20 0.40 ›Reveal solutionSolution
Order in A =2, order in B =0, total order =2.
Compare experiments I and II: [A] doubles (0.10 to 0.20) with [B] fixed, and rate rises fourfold (0.10 to 0.40). Since 2x=4, x=2 (second order in A). …
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