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Class 12 Chemistry Chapter 3: Chemical Kinetics MCQs with answers

10 multiple-choice questions on Chemical Kinetics, each with its answer and the reason, written from the NCERT chapter. For the CBSE board, JEE Main and NEET, which are set from the same NCERT chapter. Open a question's solution to check your working; then take the free test to find your next chapter.

Question 1
In the synthesis N₂(g) + 3H₂(g) → 2NH₃(g), ammonia is formed at 4.0 × 10⁻⁴ mol L⁻¹ s⁻¹. The rate at which H₂ disappears is
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Answer: (a) 6.0 × 10⁻⁴ mol L⁻¹ s⁻¹

Rate = −⅓ Δ[H₂]/Δt = ½ Δ[NH₃]/Δt, so −Δ[H₂]/Δt = (3/2)(4.0 × 10⁻⁴) = 6.0 × 10⁻⁴ mol L⁻¹ s⁻¹.

Question 2
The gas-phase reaction 2NO(g) + 2H₂(g) → N₂(g) + 2H₂O(g) follows Rate = k[NO]²[H₂]. With concentration in mol L⁻¹ and time in seconds, the unit of k is
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Answer: (d) mol⁻² L² s⁻¹

The order is 2 + 1 = 3, so k = Rate/[conc]³ has unit (mol L⁻¹ s⁻¹)/(mol L⁻¹)³ = mol⁻² L² s⁻¹.

Question 3
For CHCl₃ + Cl₂ → CCl₄ + HCl, the experimental rate law is Rate = k[CHCl₃][Cl₂]1/2. The overall order of the reaction is
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Answer: (c) 3/2

Order = sum of the powers in the rate law = 1 + ½ = 3/2, although the coefficients in the equation add to 2.

Question 4
Which statement about order and molecularity is correct?
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Answer: (d) Order can be zero or a fraction, but molecularity cannot.

Order is an experimental quantity and may be zero or fractional; molecularity counts colliding species in an elementary step, so it is a whole number and has no meaning for a complex reaction.

Question 5
In the iodide-catalysed decomposition of H₂O₂, the fast step is the elementary reaction H₂O₂ + IO⁻ → H₂O + I⁻ + O₂. The molecularity of this step is
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Answer: (a) 2

Two species, H₂O₂ and IO⁻, must collide together; the step is bimolecular.

Question 6
A reaction goes by three elementary steps. Run on its own, step 1 is over in about 2 s, step 2 takes about 40 min and step 3 about 5 s. The overall rate of the reaction is decided by
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Answer: (a) step 2, because it is the slowest

The slowest step is the rate determining step; the overall reaction cannot go faster than it, as a relay team is held to its slowest runner.

Question 7
For a zero order reaction R → P, which plot is a straight line of slope −k?
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Answer: (c) [R] against t

The integrated zero order law is [R] = −kt + [R]₀: [R] against t is a straight line of slope −k and intercept [R]₀.

Question 8
A first order reaction has a half-life of 20 min. Starting from 0.80 mol L⁻¹, the time taken for the concentration to fall to 0.10 mol L⁻¹ is
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Answer: (c) 60 min

First order t½ does not depend on concentration: 0.80 → 0.40 → 0.20 → 0.10 is three half-lives, 3 × 20 = 60 min.

Question 9
A zero order reaction has k = 0.005 mol L⁻¹ min⁻¹. If the initial concentration is 0.40 mol L⁻¹, the half-life is
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Answer: (b) 40 min

For zero order t½ = [R]₀/2k = 0.40/(2 × 0.005) = 40 min. (138.6 min is 0.693/k, the first order formula wrongly applied.)

Question 10
After a single dose, the concentration of a drug in blood plasma falls by first order kinetics with k = 2.303 × 10⁻² h⁻¹. The time for it to fall from 40 mg L⁻¹ to 4 mg L⁻¹ is (log 10 = 1)
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Answer: (d) 100 h

t = (2.303/k) log([R]₀/[R]) = (2.303/2.303 × 10⁻²) × log 10 = 100 h.

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