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Elementary/complex reactions, order and molecularity: NEET MCQs with answers

89 questions on this topic of Chemical Kinetics (NEET 2027 syllabus). Here are 12; tap one to see its answer and explanation.

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easy

1. Which of the following statements is correct regarding the order and molecularity of a chemical reaction?

  1. Order is an experimentally determined quantity, whereas molecularity is a theoretical concept.
  2. Molecularity can be zero for photochemical reactions.
  3. Order is always an integer whereas molecularity can be fractional.
  4. Molecularity can be determined for complex reactions directly from balanced stoichiometric equations.
Show answer and explanation

Answer: (A) Order is an experimentally determined quantity, whereas molecularity is a theoretical concept.

Order of a reaction is determined strictly by experiment and can be zero, fractional, or an integer. Molecularity is a theoretical concept defined only for elementary steps and cannot be zero or non-integer.

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2. The conversion of ozone into oxygen is catalyzed by chlorine radicals via the following two-step mechanism: Step 1: Cl(g) + O₃(g) → ClO(g) + O₂(g) (Slow) Step 2: ClO(g) + O(g) → Cl(g) + O₂(g) (Fast) What is the molecularity of the rate-determining step and the intermediate species in the reaction?

  1. Molecularity = 2, Intermediate = ClO(g)
  2. Molecularity = 1, Intermediate = Cl(g)
  3. Molecularity = 2, Intermediate = Cl(g)
  4. Molecularity = 3, Intermediate = O(g)
Show answer and explanation

Answer: (A) Molecularity = 2, Intermediate = ClO(g)

Step 1 is the slowest step, hence it is the rate-determining step. It involves two reacting species [Cl(g) and O₃(g)], so its molecularity is 2. ClO(g) is formed in Step 1 and consumed in Step 2, functioning as a reaction intermediate (while Cl is a catalyst).

hard

3. A hypothetical complex reaction proceeds via the following mechanism: Step 1: M₂ ⇌ 2M (Fast equilibrium, equilibrium constant K_eq) Step 2: M + N₂ → MN + N (Slow, rate constant k₂) Step 3: M + N → MN (Fast) What is the overall order of the reaction with respect to M₂ and N₂?

  1. Order with respect to M₂ is 1/2; with respect to N₂ is 1; overall order is 3/2
  2. Order with respect to M₂ is 1; with respect to N₂ is 1; overall order is 2
  3. Order with respect to M₂ is 2; with respect to N₂ is 1; overall order is 3
  4. Order with respect to M₂ is 1/2; with respect to N₂ is 1/2; overall order is 1
Show answer and explanation

Answer: (A) Order with respect to M₂ is 1/2; with respect to N₂ is 1; overall order is 3/2

The rate-determining step is Step 2: Rate = k₂[M][N₂]. From the pre-equilibrium Step 1: K_eq = [M]² / [M₂], so [M] = (K_eq)¹ᐟ² [M₂]¹ᐟ². Substituting [M] into the rate law gives: Rate = k₂(K_eq)¹ᐟ² [M₂]¹ᐟ² [N₂]. Thus, the reaction is of order 1/2 with respect to M₂ and order 1 with respect to N₂, giving an overall order of 3/2.

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4. The gas-phase reaction 2NO(g) + Cl₂(g) → 2NOCl(g) is proposed to occur by the following mechanism: Step 1: NO(g) + Cl₂(g) ⇌ NOCl₂(g) (Fast equilibrium, K = k₁/k₋₁) Step 2: NOCl₂(g) + NO(g) → 2NOCl(g) (Slow, rate constant k₂) Which of the following statements is INCORRECT?

  1. The overall rate law is Rate = k[NO]²[Cl₂], where k = k₂ · K.
  2. NOCl₂ acts as a reaction intermediate.
  3. The overall reaction is trimolecular.
  4. Step 2 is a bimolecular elementary reaction.
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Answer: (C) The overall reaction is trimolecular.

Molecularity is strictly defined only for individual elementary steps and has no physical meaning for an overall complex reaction. Step 2 is indeed bimolecular, NOCl₂ is an unstable intermediate, and the derived rate law gives order 2 with respect to NO and 1 with respect to Cl₂ (overall third-order reaction, but not trimolecular).

easy

5. Which of the following statements about the molecularity of an elementary chemical step is INCORRECT?

  1. It changes when the reaction conditions such as temperature, pressure, or concentration are altered.
  2. It is always a positive integer and can never be zero or fractional.
  3. Reactions with molecularity greater than three are extremely rare due to low collision probability.
  4. It represents the number of reacting species colliding simultaneously to bring about a chemical reaction.
Show answer and explanation

Answer: (A) It changes when the reaction conditions such as temperature, pressure, or concentration are altered.

Molecularity is an intrinsic theoretical property of an elementary step defined by the number of reacting particles participating in the collision. It is completely independent of external experimental variables such as temperature, pressure, or reactant concentrations.

hard

6. Consider the following multi-step mechanism: Step 1: O₃ ⇌ O₂ + O (Fast equilibrium, K = k₁/k₋₁) Step 2: O + O₃ → 2O₂ (Slow, rate constant k₂) If the concentration of O₂ in the reaction mixture is increased by a factor of 2 while keeping [O₃] constant, how does the reaction rate change?

  1. It increases by a factor of 2
  2. It decreases to one-fourth of its initial value
  3. It decreases to half of its initial value
  4. It remains unchanged
Show answer and explanation

Answer: (C) It decreases to half of its initial value

From Step 2: Rate = k₂[O][O₃]. From Step 1 equilibrium: K = ([O₂][O]) / [O₃], hence [O] = K [O₃] / [O₂]. Substituting into the rate law gives Rate = k₂ K [O₃]² / [O₂]. Because rate is inversely proportional to [O₂], doubling [O₂] reduces the reaction rate to half of its original value.

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7. The reaction 2NO₂(g) + F₂(g) → 2NO₂F(g) follows the experimental rate law Rate = k[NO₂][F₂]. Which of the following mechanisms is consistent with this rate law?

  1. NO₂ + F₂ → NO₂F + F (Slow); NO₂ + F → NO₂F (Fast)
  2. NO₂ + F₂ ⇌ NO₂F₂ (Fast equilibrium); NO₂F₂ + NO₂ → 2NO₂F (Slow)
  3. 2NO₂ ⇌ N₂O₄ (Fast equilibrium); N₂O₄ + F₂ → 2NO₂F (Slow)
  4. 2NO₂ + F₂ → 2NO₂F (Single elementary step)
Show answer and explanation

Answer: (A) NO₂ + F₂ → NO₂F + F (Slow); NO₂ + F → NO₂F (Fast)

For Mechanism A, the first step is the slow step: Rate = k₁[NO₂][F₂]. Adding the two steps yields 2NO₂ + F₂ → 2NO₂F, which perfectly matches both the overall stoichiometry and the observed first-order dependence in each reactant.

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8. The thermal decomposition of dinitrogen pentoxide proceeds via the following mechanism: Step 1: N₂O₅ ⇌ NO₂ + NO₃ (Fast equilibrium, K) Step 2: NO₂ + NO₃ → NO₂ + O₂ + NO (Slow, k₂) Step 3: NO + NO₃ → 2NO₂ (Fast, k₃) What is the overall order of the reaction with respect to N₂O₅?

  1. 0
  2. 2
  3. 1
  4. 3
Show answer and explanation

Answer: (C) 1

The rate-determining step is Step 2: Rate = k₂[NO₂][NO₃]. From the fast equilibrium in Step 1, K = [NO₂][NO₃] / [N₂O₅], which yields [NO₂][NO₃] = K[N₂O₅]. Substituting this into the rate equation gives Rate = k₂K[N₂O₅] = k_eff[N₂O₅]. Hence, the reaction is first-order with respect to N₂O₅.

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9. Given below are two statements: Statement I: A complex reaction occurring in multiple steps can have an overall molecularity equal to the sum of the molecularities of all steps. Statement II: The order of an elementary step is always equal to its molecularity. In the light of the above statements, choose the most appropriate answer:

  1. Both Statement I and Statement II are correct
  2. Statement I is correct but Statement II is incorrect
  3. Statement I is incorrect but Statement II is correct
  4. Both Statement I and Statement II are incorrect
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Answer: (C) Statement I is incorrect but Statement II is correct

Molecularity has no meaning for an overall complex reaction; it is only defined for individual elementary steps, so Statement I is incorrect. For an elementary step, the stoichiometric coefficients represent the number of colliding species, so its order is strictly equal to its molecularity, making Statement II correct.

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10. Acid hydrolysis of ethyl acetate in excess water behaves as a first-order reaction because:

  1. The reaction is unimolecular
  2. Ester concentration is constant
  3. Water is in large excess, so its concentration remains practically constant
  4. Water does not take part
Show answer and explanation

Answer: (C) Water is in large excess, so its concentration remains practically constant

Rate = k[ester][H₂O], but with water in large excess [H₂O] is nearly constant, so rate = k'[ester]. Such reactions are pseudo first-order.

NCERT Class 12

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11. A hypothetical gaseous reaction follows the stoichiometry 2A + B → 2C. When the partial pressure of A is doubled keeping the partial pressure of B constant, the rate increases 4-fold. When the partial pressures of both A and B are doubled, the rate increases 8-fold. What can be concluded about this reaction?

  1. The reaction must be elementary.
  2. The reaction must be a complex reaction because its overall order is 3.
  3. The reaction cannot proceed via collisions.
  4. The reaction may be elementary because its rate law matches the molecularity expected from stoichiometry.
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Answer: (D) The reaction may be elementary because its rate law matches the molecularity expected from stoichiometry.

Rate ∝ [A]² because doubling [A] increases rate 4-fold (2² = 4). When both are doubled, rate increases 8-fold: 2² × 2ⁿ = 8, so 4 × 2ⁿ = 8, giving n = 1. The rate law is Rate = k[A]²[B]. Since the rate law coefficients match the stoichiometric coefficients (2 for A, 1 for B), this reaction is consistent with a trimolecular elementary step (or it may be elementary).

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12. In the formation of HBr from H₂ and Br₂, the reaction mechanism involves free radicals. Under specific conditions with large excess of H₂, the reaction rate is given by Rate = k [H₂][Br₂]^(1/2). What are the overall order and the molecularity of this reaction?

  1. Order = 1.5, Molecularity = 2
  2. Order = 1.5, Molecularity is undefined
  3. Order = 2, Molecularity = 2
  4. Order = 0.5, Molecularity is undefined
Show answer and explanation

Answer: (B) Order = 1.5, Molecularity is undefined

The overall order is the sum of exponents: 1 + 0.5 = 1.5. Because this reaction proceeds via a multi-step chain mechanism, molecularity cannot be assigned to the overall process and is undefined.

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