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Rate law, rate constant and units: NEET MCQs with answers

73 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 a zero-order reaction A → Products, which of the following statements is correct?

  1. The rate constant has the unit s⁻¹.
  2. The numerical value of the rate of reaction is equal to the rate constant at all reactant concentrations.
  3. A plot of rate versus reactant concentration gives a line passing through the origin.
  4. The reaction stops immediately if the concentration of A is doubled.
Show answer and explanation

Answer: (B) The numerical value of the rate of reaction is equal to the rate constant at all reactant concentrations.

For a zero-order reaction, Rate = k[A]⁰ = k. Therefore, the rate of reaction is independent of reactant concentration and always numerically equal to the rate constant.

medium

2. The rate law of a reaction is rate = k[A][B]². If [A] is multiplied by 3 and [B] by 2, the rate becomes how many times the original?

  1. 18
  2. 12
  3. 7
  4. 6
Show answer and explanation

Answer: (B) 12

Rate scales with each concentration raised to its order: new rate/old rate = 3¹ × 2² = 12.

hard

3. For the catalytic decomposition of ozone in the presence of atomic oxygen, the rate law is experimentally found to be Rate = k[O₃][O]⁻¹. What happens to the rate of reaction if the concentration of O₃ is doubled and the concentration of O is halved?

  1. The rate remains unchanged
  2. The rate increases by a factor of 2
  3. The rate decreases by a factor of 4
  4. The rate increases by a factor of 4
Show answer and explanation

Answer: (D) The rate increases by a factor of 4

Given Rate = k[O₃]/[O]. When [O₃]' = 2[O₃] and [O]' = 0.5[O], the new rate becomes Rate' = k(2[O₃])/(0.5[O]) = 4 × (k[O₃]/[O]) = 4 × Rate. Therefore, the rate increases by a factor of 4.

medium

4. The units of the rate constant of a zero-order reaction are:

  1. mol L⁻¹ s⁻¹
  2. s⁻¹
  3. L mol⁻¹ s⁻¹
  4. L² mol⁻² s⁻¹
Show answer and explanation

Answer: (A) mol L⁻¹ s⁻¹

k has units (mol L⁻¹)^(1−n) s⁻¹; for zero order this is mol L⁻¹ s⁻¹.

easy

5. Which of the following correctly describes the relationship between the rate of a reaction and its rate constant k?

  1. The rate of a reaction is numerically equal to k when the concentration of each reactant is 1 mol L⁻¹.
  2. The rate constant k decreases as the concentration of reactants increases at constant temperature.
  3. The units of the rate constant k are always identical to the units of the rate of reaction.
  4. The rate constant k remains constant when the temperature of the reaction mixture changes.
Show answer and explanation

Answer: (A) The rate of a reaction is numerically equal to k when the concentration of each reactant is 1 mol L⁻¹.

By definition, Rate = k[Reactants]ⁿ. When each reactant concentration is unity (1 mol L⁻¹), Rate = k, which is why k is also referred to as the specific reaction rate. k is independent of concentration but depends on temperature.

hard

6. For a chemical reaction: X + 2Y → Z, the following rate data were obtained at 300 K: When [X] = 0.1 M and [Y] = 0.1 M, Rate = 2.0 × 10⁻³ M s⁻¹. When [X] is doubled and [Y] is quadrupled, the rate increases by a factor of 8. When [X] is doubled and [Y] is kept constant, the rate remains unchanged. What is the overall rate law for the reaction?

  1. Rate = k[X][Y]^(1/2)
  2. Rate = k[Y]^(3/2)
  3. Rate = k[Y]²
  4. Rate = k[X]⁰[Y]
Show answer and explanation

Answer: (B) Rate = k[Y]^(3/2)

Let Rate = k[X]^a [Y]^b. When [X] is doubled keeping [Y] constant, the rate is unchanged, which means 2^a = 1, so a = 0. When [X] is doubled and [Y] is quadrupled, the rate increases by 8 times: (4)^b = 8 ⇒ (2²)^b = 2³ ⇒ 2b = 3 ⇒ b = 3/2. Therefore, the rate law is Rate = k[X]⁰[Y]^(3/2) = k[Y]^(3/2).

easy

7. The rate of a certain reaction is given by Rate = k[A]²[B]³. If the concentration of A is halved while the concentration of B is doubled, the reaction rate will:

  1. decrease by a factor of 2
  2. increase by a factor of 4
  3. increase by a factor of 2
  4. remain unchanged
Show answer and explanation

Answer: (C) increase by a factor of 2

The new rate is Rate' = k([A]/2)²(2[B])³ = k([A]²/4)(8[B]³) = 2k[A]²[B]³ = 2 × Rate. Thus, the rate increases by a factor of 2.

easy

8. Given below are two statements: Statement I: A negative order with respect to a reactant or substance indicates that increasing its concentration retards the rate of reaction. Statement II: Rate law is an experimentally determined expression and cannot necessarily be inferred from the stoichiometric equation of a balanced reaction. 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
Show answer and explanation

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

When an order with respect to a substance is negative, an increase in its concentration decreases the reaction rate (inhibition). Furthermore, rate laws are determined strictly by experiment and cannot generally be deduced from stoichiometry alone.

easy

9. For a reaction 2NO(g) + 2H₂(g) → N₂(g) + 2H₂O(g), the rate law is Rate = k[NO]²[H₂]. If the reaction is carried out in a vessel whose volume is suddenly decreased to one-third of its initial volume, how many times will the rate of the reaction increase?

  1. 9 times
  2. 3 times
  3. 81 times
  4. 27 times
Show answer and explanation

Answer: (D) 27 times

When the volume of the reaction vessel is reduced to 1/3 of its initial volume, the concentration of each gaseous reactant becomes 3 times the initial value. Since Rate = k[NO]²[H₂], the new rate becomes k(3[NO])²(3[H₂]) = 27 k[NO]²[H₂], which is 27 times the initial rate.

easy

10. For an elementary reaction 2P + Q → Products, what is the unit of the rate constant if concentration is expressed in mol L⁻¹ and time in seconds?

  1. s⁻¹
  2. L² mol⁻² s⁻¹
  3. L mol⁻¹ s⁻¹
  4. L³ mol⁻³ s⁻¹
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Answer: (B) L² mol⁻² s⁻¹

For an elementary reaction, the order equals the molecularity, so overall order n = 2 + 1 = 3. The general unit of k is (mol L⁻¹)¹⁻ⁿ s⁻¹ = (mol L⁻¹)⁻² s⁻¹ = L² mol⁻² s⁻¹.

medium

11. The rate law of a reaction is rate = k[A][B]. If [A] is multiplied by 3 and [B] by 3, the rate becomes how many times the original?

  1. 10
  2. 6
  3. 11
  4. 9
Show answer and explanation

Answer: (D) 9

Rate scales with each concentration raised to its order: new rate/old rate = 3¹ × 3¹ = 9.

medium

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

  1. 6
  2. 9
  3. 4
  4. 8
Show answer and explanation

Answer: (D) 8

Rate scales with each concentration raised to its order: new rate/old rate = 2¹ × 2² = 8.

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