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Reaction rate and affecting factors: NEET MCQs with answers

90 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. For the reaction: 5Br⁻(aq) + BrO₃⁻(aq) + 6H⁺(aq) → 3Br₂(aq) + 3H₂O(l), the rate of disappearance of Br⁻ is related to the rate of appearance of Br₂ by the expression:

  1. -Δ[Br⁻]/Δt = (1/5) Δ[Br₂]/Δt
  2. -Δ[Br⁻]/Δt = (3/5) Δ[Br₂]/Δt
  3. -Δ[Br⁻]/Δt = (5/3) Δ[Br₂]/Δt
  4. -Δ[Br⁻]/Δt = 5 Δ[Br₂]/Δt
Show answer and explanation

Answer: (C) -Δ[Br⁻]/Δt = (5/3) Δ[Br₂]/Δt

Rate of reaction = -(1/5) Δ[Br⁻]/Δt = (1/3) Δ[Br₂]/Δt. Rearranging gives -Δ[Br⁻]/Δt = (5/3) Δ[Br₂]/Δt.

medium

2. Given the following two statements: Statement I: A catalyst increases the rate of reaction by decreasing the enthalpy of reaction (ΔH). Statement II: A catalyst provides an alternate reaction pathway with a lower activation energy. Choose the correct option:

  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
Show answer and explanation

Answer: (C) Statement I is incorrect but Statement II is correct

Statement I is incorrect because a catalyst does not alter the thermodynamic parameters like enthalpy of reaction (ΔH) or Gibbs free energy (ΔG). Statement II is correct because a catalyst works by providing an alternate mechanism with lower activation energy.

hard

3. For the liquid phase reaction A → 2B, the concentration of A at t = 0 is 1.00 mol L⁻¹. After 15 minutes, the total concentration of all solutes ([A] + [B]) in the solution is 1.40 mol L⁻¹. What is the average rate of disappearance of A during this period?

  1. 0.0267 mol L⁻¹ min⁻¹
  2. 0.0133 mol L⁻¹ min⁻¹
  3. 0.0400 mol L⁻¹ min⁻¹
  4. 0.0533 mol L⁻¹ min⁻¹
Show answer and explanation

Answer: (A) 0.0267 mol L⁻¹ min⁻¹

Let x mol L⁻¹ of A react in 15 minutes. Then [A] = 1.00 - x and [B] = 2x. Total concentration = (1.00 - x) + 2x = 1.00 + x = 1.40 mol L⁻¹, so x = 0.40 mol L⁻¹. The average rate of disappearance of A is x / Δt = 0.40 / 15 = 0.0267 mol L⁻¹ min⁻¹.

medium

4. Given below are two statements: Statement I: Homogeneous catalysts are generally more selective than heterogeneous catalysts because every catalyst molecule is accessible to reactant molecules. Statement II: Heterogeneous catalysis rates often depend significantly on the surface area of the solid catalyst rather than its mass alone. In light of the above statements, choose the correct 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
Show answer and explanation

Answer: (A) Both Statement I and Statement II are correct

Both statements are correct. In homogeneous catalysis, the catalyst is uniformly dispersed in the same phase, giving high accessibility and specificity. In heterogeneous catalysis, reaction takes place on active surface sites, so increasing surface area (by powdering) enhances the reaction rate.

easy

5. In a hypochlorite oxidation reaction: 2HClO(aq) + 2H⁺(aq) + 2e⁻ → Cl₂(g) + 2H₂O(l), if the rate of formation of Cl₂(g) is measured to be 0.030 mol L⁻¹ min⁻¹, what is the rate of disappearance of HClO(aq)?

  1. 0.015 mol L⁻¹ min⁻¹
  2. 0.060 mol L⁻¹ min⁻¹
  3. 0.030 mol L⁻¹ min⁻¹
  4. 0.120 mol L⁻¹ min⁻¹
Show answer and explanation

Answer: (B) 0.060 mol L⁻¹ min⁻¹

From stoichiometry, Rate = -(1/2) d[HClO]/dt = +d[Cl₂]/dt. Therefore, -d[HClO]/dt = 2 × d[Cl₂]/dt = 2 × 0.030 = 0.060 mol L⁻¹ min⁻¹.

hard

6. In a vessel containing 1.0 L solution, the reaction 2A(aq) → B(aq) + 2C(aq) occurs. At time t, the rate of formation of C is found to be 0.060 mol L⁻¹ s⁻¹. If 0.50 L of pure solvent is suddenly added to the vessel while keeping the total amount of moles unchanged, what will be the new instantaneous rate of formation of C, assuming the rate is second-order with respect to A?

  1. 0.027 mol L⁻¹ s⁻¹
  2. 0.040 mol L⁻¹ s⁻¹
  3. 0.030 mol L⁻¹ s⁻¹
  4. 0.015 mol L⁻¹ s⁻¹
Show answer and explanation

Answer: (A) 0.027 mol L⁻¹ s⁻¹

The new volume becomes 1.5 L. The concentration of A becomes [A]₁ × (1.0 / 1.5) = (2/3)[A]₁. Since the reaction rate is proportional to [A]², the new rate is (2/3)² = 4/9 of the original rate. Therefore, new rate of formation of C = (4/9) × 0.060 = 0.0267 ≈ 0.027 mol L⁻¹ s⁻¹.

medium

7. For a reaction between gases taking place in a closed vessel of fixed volume V at constant temperature, if the volume of the vessel is reduced to V/3, the rate of a bimolecular elementary reaction A(g) + B(g) → Products will increase by a factor of:

  1. 6
  2. 3
  3. 9
  4. 27
Show answer and explanation

Answer: (C) 9

When volume is reduced to V/3, the concentration of each gaseous reactant (n/V) increases by a factor of 3. For an elementary bimolecular reaction, Rate = k[A][B]. The new rate is k(3[A])(3[B]) = 9k[A][B], which is 9 times the initial rate.

easy

8. For the gas-phase reaction 4NH₃(g) + 5O₂(g) → 4NO(g) + 6H₂O(g), if the rate of formation of NO is 3.6 × 10⁻³ mol L⁻¹ s⁻¹, what is the rate of disappearance of O₂?

  1. 2.88 × 10⁻³ mol L⁻¹ s⁻¹
  2. 4.5 × 10⁻³ mol L⁻¹ s⁻¹
  3. 3.6 × 10⁻³ mol L⁻¹ s⁻¹
  4. 1.8 × 10⁻³ mol L⁻¹ s⁻¹
Show answer and explanation

Answer: (B) 4.5 × 10⁻³ mol L⁻¹ s⁻¹

Rate of reaction = -(1/5) d[O₂]/dt = +(1/4) d[NO]/dt. Therefore, the rate of disappearance of O₂, -d[O₂]/dt = (5/4) × (d[NO]/dt) = (5/4) × (3.6 × 10⁻³ mol L⁻¹ s⁻¹) = 4.5 × 10⁻³ mol L⁻¹ s⁻¹.

easy

9. Which of the following statements correctly explains why ionic reactions occurring in aqueous solutions (such as Ag⁺ + Cl⁻ → AgCl) proceed almost instantaneously at room temperature?

  1. The reaction involves only opposite charges and requires no chemical bond cleavage, hence activation energy is nearly zero.
  2. The electrostatic repulsion between ions lowers the overall potential energy barrier.
  3. Water molecules serve as a chemical catalyst by donating electrons to accelerate bond reorganization.
  4. The collision frequency between ions is several orders of magnitude higher than between neutral molecules.
Show answer and explanation

Answer: (A) The reaction involves only opposite charges and requires no chemical bond cleavage, hence activation energy is nearly zero.

Precipitation and neutralization reactions involving simple ions in aqueous solution occur virtually instantaneously because oppositely charged ions attract each other and no covalent bonds need to be broken. Consequently, the activation energy for such processes is practically zero.

medium

10. For the reaction 2SO₂(g) + O₂(g) → 2SO₃(g), the rate of disappearance of SO₂ is 1.6 × 10⁻³ mol L⁻¹ min⁻¹. Which expression correctly correlates the rates of change of concentrations?

  1. -d[SO₂]/dt = -2 d[O₂]/dt = d[SO₃]/dt
  2. -d[SO₂]/dt = -(1/2) d[O₂]/dt = d[SO₃]/dt
  3. -2 d[SO₂]/dt = -d[O₂]/dt = 2 d[SO₃]/dt
  4. -d[SO₂]/dt = -d[O₂]/dt = d[SO₃]/dt
Show answer and explanation

Answer: (A) -d[SO₂]/dt = -2 d[O₂]/dt = d[SO₃]/dt

According to stoichiometry, Rate = -(1/2) d[SO₂]/dt = -d[O₂]/dt = +(1/2) d[SO₃]/dt. Multiplying the entire relation by 2 gives -d[SO₂]/dt = -2 d[O₂]/dt = d[SO₃]/dt.

medium

11. A solid piece of marble (CaCO₃) weighing 10.0 g reacts completely with excess 1.0 M HCl in 120 seconds. If the same mass of marble is crushed into fine powder and added to the same acid solution at identical temperature, which of the following changes will occur?

  1. The total volume of CO₂ evolved increases while the reaction time remains unchanged.
  2. The reaction time decreases and the initial rate of CO₂ evolution increases.
  3. Both the initial rate of reaction and the equilibrium constant of the reaction increase.
  4. The activation energy of the reaction decreases significantly.
Show answer and explanation

Answer: (B) The reaction time decreases and the initial rate of CO₂ evolution increases.

Crushing the solid into a fine powder significantly increases the exposed surface area of the reactant. A higher surface area increases the number of collisions per unit time with HCl molecules, thus increasing the initial reaction rate and decreasing the time taken for completion, without affecting the total yield or activation energy.

easy

12. Given below are two statements: Statement I: Finely divided iron reacts faster with dilute hydrochloric acid than an iron nail of the same mass. Statement II: Increasing the surface area of a solid reactant increases the number of collisions occurring per unit time at the reacting interface. In the light of the above statements, choose the most appropriate answer:

  1. Both Statement I and Statement II are correct and Statement II is the correct explanation of Statement I
  2. Both Statement I and Statement II are correct but Statement II is NOT the correct explanation of Statement I
  3. Statement I is correct but Statement II is incorrect
  4. Statement I is incorrect but Statement II is correct
Show answer and explanation

Answer: (A) Both Statement I and Statement II are correct and Statement II is the correct explanation of Statement I

Finely divided iron has a vastly greater surface area than a solid nail of identical mass. Greater surface area exposes more active sites, increasing collision frequency per unit time and thus increasing the reaction rate.

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