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NEET Chemical Kinetics MCQs with answers

514 questions on Chemical Kinetics for NEET 2027, 1 of them from previous NEET papers. Below are 15 to try; tap a question to see its answer and explanation.

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15 sample questions

mediumNEET 2023

1. Half-life of first order reaction is independent of:

  1. Rate constant
  2. Temperature
  3. Initial concentration
  4. Activation energy
Show answer and explanation

Answer: (C) Initial concentration

t½ = 0.693/k. Depends on k but NOT on initial concentration.

Quick tip: First order: t½ = 0.693/k (concentration independent). Zero order: t½ depends on [A₀].

NCERT Class 12, Chapter 4, page 105

easy

2. Consider the gaseous bimolecular reaction: CH₃Br + OH⁻ → CH₃OH + Br⁻. According to collision theory, an improper collision orientation occurs when:

  1. The OH⁻ ion approaches and collides directly with the bromine atom of CH₃Br
  2. The OH⁻ ion attacks the carbon atom directly from the side opposite to the Br atom
  3. The kinetic energy of the incoming OH⁻ ion exceeds the activation energy
  4. The C-Br bond is elongated during the approach of OH⁻
Show answer and explanation

Answer: (A) The OH⁻ ion approaches and collides directly with the bromine atom of CH₃Br

For substitution to occur, the nucleophile OH⁻ must approach the carbon atom from the side opposite to the leaving group (Br⁻). If OH⁻ collides directly with the electron-rich bromine atom, electrostatic repulsion prevents bond formation, representing an ineffective collision due to improper orientation.

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3. Reactions with molecularity greater than three are very rare because:

  1. They are too exothermic
  2. They have zero activation energy
  3. They are always reversible
  4. The chance of more than three molecules colliding simultaneously is very small
Show answer and explanation

Answer: (D) The chance of more than three molecules colliding simultaneously is very small

Simultaneous collision of more than three molecules is highly improbable, so elementary reactions have molecularity of at most three.

NCERT Class 12

hard

4. A composite reaction mechanism has an effective rate constant k given by k = (k₁ · k₂ / k₃)^(1/2). If the activation energies for the elementary steps are Ea₁ = 60 kJ mol⁻¹, Ea₂ = 40 kJ mol⁻¹, and Ea₃ = 20 kJ mol⁻¹, what is the overall activation energy of the reaction?

  1. 30 kJ mol⁻¹
  2. 80 kJ mol⁻¹
  3. 50 kJ mol⁻¹
  4. 40 kJ mol⁻¹
Show answer and explanation

Answer: (D) 40 kJ mol⁻¹

Taking natural logs, ln k = 1/2 (ln k₁ + ln k₂ - ln k₃). Differentiating with respect to temperature gives the overall activation energy: Ea = 1/2 (Ea₁ + Ea₂ - Ea₃). Substituting the given values: Ea = 1/2 (60 + 40 - 20) = 1/2 (80) = 40 kJ mol⁻¹.

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5. The rate constants for the decomposition of a compound are 1.6 × 10⁻³ s⁻¹ at 400 K and 6.4 × 10⁻² s⁻¹ at 500 K. What is the activation energy of this reaction? (Take: R = 8.314 J K⁻¹ mol⁻¹, ln 40 = 3.69)

  1. 30.7 kJ mol⁻¹
  2. 61.4 kJ mol⁻¹
  3. 92.1 kJ mol⁻¹
  4. 122.8 kJ mol⁻¹
Show answer and explanation

Answer: (B) 61.4 kJ mol⁻¹

ln(k₂/k₁) = ln(6.4 × 10⁻² / 1.6 × 10⁻³) = ln(40) = 3.69. Using ln(k₂/k₁) = (Ea / R) × [(T₂ - T₁) / (T₁ × T₂)], we have 3.69 = (Ea / 8.314) × [100 / (400 × 500)]. Thus, Ea = 3.69 × 8.314 × 2000 = 61357 J mol⁻¹ ≈ 61.4 kJ mol⁻¹.

easy

6. For the reaction: CH₃Br(aq) + OH⁻(aq) → CH₃OH(aq) + Br⁻(aq), the instantaneous rate at time t is found to be 1.5 × 10⁻⁴ mol L⁻¹ s⁻¹. Which of the following expressions represents the rate of change of bromide ion concentration at that moment?

  1. -1.5 × 10⁻⁴ mol L⁻¹ s⁻¹
  2. +1.5 × 10⁻⁴ mol L⁻¹ s⁻¹
  3. +3.0 × 10⁻⁴ mol L⁻¹ s⁻¹
  4. -3.0 × 10⁻⁴ mol L⁻¹ s⁻¹
Show answer and explanation

Answer: (B) +1.5 × 10⁻⁴ mol L⁻¹ s⁻¹

The stoichiometric coefficient of Br⁻ is 1. Hence, Rate of reaction = +d[Br⁻]/dt. Since the reaction rate is 1.5 × 10⁻⁴ mol L⁻¹ s⁻¹, the rate of formation of Br⁻ is +1.5 × 10⁻⁴ mol L⁻¹ s⁻¹.

hard

7. For an elementary gaseous reaction A + B → Products with an activation energy of 11.51 kJ mol⁻¹, what percentage of the total collisions possess sufficient energy to react at 300 K? (Given: R = 8.314 J K⁻¹ mol⁻¹, e^(-4.615) = 0.010)

  1. 1.0%
  2. 0.10%
  3. 0.01%
  4. 10.0%
Show answer and explanation

Answer: (A) 1.0%

The fraction of collisions with energy ≥ Eₐ is f = e^(-Eₐ/RT). Here, Eₐ/(RT) = 11510 / (8.314 × 300) = 11510 / 2494.2 ≈ 4.615. Thus, f = e^(-4.615) = 0.010. Expressed as a percentage, 0.010 × 100 = 1.0%.

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8. A first-order reaction has a half-life of 69.3 min. The time needed for 50% of the reactant to decompose is:

  1. 34.6 min
  2. 104 min
  3. 139 min
  4. 69.3 min
Show answer and explanation

Answer: (D) 69.3 min

50% decomposed means 1/2 remains, which takes 1 half-lives: 1 × 69.3 = 69.3 min.

easy

9. Consider the reaction: 2KMnO₄ + 5H₂C₂O₄ + 3H₂SO₄ → K₂SO₄ + 2MnSO₄ + 10CO₂ + 8H₂O. The reaction is initially slow but speeds up significantly after some time without any external intervention. Which phenomenon explains this behavior?

  1. Heterogeneous catalysis
  2. Enzyme catalysis
  3. Auto-catalysis by Mn²⁺ ions
  4. Thermal dissociation of oxalic acid
Show answer and explanation

Answer: (C) Auto-catalysis by Mn²⁺ ions

In the reaction between acidified KMnO₄ and oxalic acid, Mn²⁺ ions produced during the reaction act as an auto-catalyst, drastically increasing the reaction rate as the reaction progresses.

easy

10. For a first-order process with rate constant k = 0.05 min⁻¹, the half-life is:

  1. 0.0722 min
  2. 6.93 min
  3. 13.86 min
  4. 20 min
Show answer and explanation

Answer: (C) 13.86 min

For a first-order reaction, t½ = 0.693/k = 0.693/0.05 = 13.86 min.

Quick tip: First-order half-life is independent of initial concentration.

hard

11. For a gaseous reaction between two non-spherical polyatomic molecules, the experimentally observed rate is 1000 times slower than that predicted by the simple collision expression rate = Z_AB e^(-E_a/RT). What conclusion can be drawn regarding the reaction?

  1. The orientation requirement is stringent, corresponding to a steric factor P = 10⁻³
  2. The reaction follows termolecular kinetics rather than bimolecular kinetics
  3. The activation energy of the reaction is negative
  4. The reaction cannot occur in the gas phase
Show answer and explanation

Answer: (A) The orientation requirement is stringent, corresponding to a steric factor P = 10⁻³

When experimental rate is smaller than the rate predicted by Z_AB e^(-E_a/RT), the steric factor P accounts for the discrepancy: Rate = P × Z_AB e^(-E_a/RT). Since experimental rate is 10⁻³ times the simple collision theory rate, P = 10⁻³. This indicates that only 1 in 1000 properly energized collisions has the necessary spatial orientation.

hard

12. For two gaseous bimolecular reactions, Reaction 1 and Reaction 2, both have identical collision frequencies Z_AB and identical activation energies Eₐ at 400 K. However, the experimental rate of Reaction 1 is found to be 250 times faster than that of Reaction 2. What is the most plausible explanation according to collision theory?

  1. Molecules of Reaction 2 have a threshold energy 250 times greater than Reaction 1.
  2. Reaction 2 involves monoatomic gas species whereas Reaction 1 involves complex polymers.
  3. Reaction 1 has a steric factor (P₁) that is 250 times larger than that of Reaction 2 (P₂).
  4. The fraction of molecules with energy ≥ Eₐ is 250 times higher in Reaction 1.
Show answer and explanation

Answer: (C) Reaction 1 has a steric factor (P₁) that is 250 times larger than that of Reaction 2 (P₂).

Since both reactions have the same Z_AB and identical Eₐ at the same temperature, the terms Z_AB and e^(-Eₐ/RT) are identical. According to Rate = P · Z_AB · e^(-Eₐ/RT), the ratio of rates equals the ratio of steric factors (Rate₁ / Rate₂ = P₁ / P₂ = 250), indicating that Reaction 1 has far fewer steric restrictions (a 250-fold larger steric factor) than Reaction 2.

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13. The rate law of a reaction is rate = k[A]²[B]². If [A] is multiplied by 2 and [B] by 2, the rate becomes how many times the original?

  1. 8
  2. 16
  3. 17
  4. 4
Show answer and explanation

Answer: (B) 16

Rate scales with each concentration raised to its order: new rate/old rate = 2² × 2² = 16.

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14. For a general reaction R → P, the concentration of reactant R was monitored over time: at t = 0 min, [R] = 0.50 mol L⁻¹; at t = 10 min, [R] = 0.38 mol L⁻¹; at t = 30 min, [R] = 0.20 mol L⁻¹. What is the average rate of the reaction during the time interval between 10 minutes and 30 minutes?

  1. 0.0180 mol L⁻¹ min⁻¹
  2. 0.0150 mol L⁻¹ min⁻¹
  3. 0.0090 mol L⁻¹ min⁻¹
  4. 0.0100 mol L⁻¹ min⁻¹
Show answer and explanation

Answer: (C) 0.0090 mol L⁻¹ min⁻¹

Average rate = -Δ[R]/Δt = -([R]₃₀ - [R]₁₀)/(30 - 10) = -(0.20 - 0.38)/20 = 0.18/20 = 0.0090 mol L⁻¹ min⁻¹.

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15. A reaction A + B → Products follows the rate law: Rate = k[A]^(3/2) [B]^(-1/2). What are the SI units of its rate constant?

  1. mol^(-1/2) L^(1/2) s⁻¹
  2. mol L⁻¹ s⁻¹
  3. mol^(1/2) L^(-1/2) s⁻¹
  4. s⁻¹
Show answer and explanation

Answer: (D) s⁻¹

Overall order n = (3/2) + (-1/2) = 1. The unit of the rate constant for any overall first-order reaction is (concentration)^(1-n) time⁻¹ = (mol L⁻¹)^(1-1) s⁻¹ = s⁻¹.

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