Q.(i) Generally photochemical reactions follows zero order Kinetics. How? [2 marks]

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1. CBSE 2026Set 56/2/11 markMCQ
   Q.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 $\mathrm{NO_2 + CO \rightarrow NO + CO_2}$ 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. …

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2. CBSE 2026Set 56/2/11 markMCQ
   Q.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 $2HI \rightarrow H_2 + I_2$ 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:

   $2HI \rightarrow H_2 + I_2$

   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 out

   A 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

   1. Identify the nature of the reaction.

      The reaction $2HI \rightarrow H_2 + I_2$ 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.

   2. 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.

   3. Count the reactant molecules in the collision. …

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3. CBSE 2026Set V11 markMCQ
   Q.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. …

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4. CBSE 2026Set ANNUAL1 markMCQ
   Q.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.

   …

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5. CBSE 2026Set ANNUAL1 mark
   Q.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) …

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6. CBSE 2026Set ANNUAL1 markMCQ
   Q.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 …

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7. CBSE 2026Set ANNUAL1 mark
   Q.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). …

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8. CBSE 2026Set ANNUAL1 markMCQ
   Q.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 $= \tfrac12 + \tfrac13 + \tfrac14 = \tfrac{13}{12}$. 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.

   $\text{Order} = \frac12 + \frac13 + \frac14$

   Take LCM $= 12$: …

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9. CBSE 2026Set ANNUAL1 mark
   Q.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.

   …

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10. CBSE 2026Set ANNUAL1 mark
    Q.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. …

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11. CBSE 2026Set ANNUAL1 mark
    Q.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 $mol\,L^{-1}s^{-1}$ and concentration in $mol\,L^{-1}$, the unit of $k$ is $mol^{1/2}\,L^{-1/2}\,s^{-1}$.

    The general rule for an $n^{th}$-order reaction is that the unit of the rate constant is

    $k \;=\; mol^{(1-n)}\,L^{(n-1)}\,s^{-1}$

    Derivation for $n = \tfrac12$: the rate law is

    $\text{rate} = k\,[A]^{1/2}$

    so …

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12. CBSE 2025Set ANNUAL1 markMCQ
    Q.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.

    $\dfrac{2\,\text{rate}}{\text{rate}} = \dfrac{k(4[A])^n}{k[A]^n} = 4^n$

    …

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