Free NEET Chemistry practice
Chemical Kinetics: MCQs with Solutions
A rate law is determined experimentally; it cannot generally be read from an overall balanced equation. These five original questions test the assumptions behind rate calculations and the difference between changing reaction speed and changing equilibrium.
5 original practice questions — not previous-year paper reproductions. No account required.
By NEET Ninja · Updated
Before you answer
- For rate = k[A]ᵐ[B]ⁿ, overall order is m + n. Rate-constant units depend on this order and the units used for concentration and time.
- A first-order reaction has t½ = ln 2/k. Successive equal half-life intervals leave half of the amount present at the start of each interval.
- A catalyst provides a different reaction pathway and accelerates approach to equilibrium. It does not change the equilibrium constant at a fixed temperature.
Questions and worked answers
Choose an option before reading the answer. For a wrong answer, identify the step or concept that changed the result.
QUESTION 1
A reaction follows rate = k[A]²[B]. What is its overall order?
- 1
- 2
- 3
- 4
Answer: C — 3
Overall order is the sum of the powers of concentration in the experimentally determined rate law. Here the order is 2 + 1 = 3. The exponent on A alone gives only the order with respect to A, not the order of the entire reaction.
QUESTION 2
For a first-order reaction with time measured in seconds, what are the units of the rate constant?
- mol L⁻¹ s⁻¹
- s⁻¹
- L mol⁻¹ s⁻¹
- mol² L⁻² s⁻¹
Answer: B — s⁻¹
Rate has units mol L⁻¹ s⁻¹ and, for a first-order reaction, rate = k[A]. Dividing rate units by concentration units mol L⁻¹ leaves s⁻¹. The units L mol⁻¹ s⁻¹ instead correspond to an overall second-order rate law.
QUESTION 3
A first-order reaction has k = 0.0231 min⁻¹. Using ln 2 = 0.693, what is its half-life?
- 15 min
- 30 min
- 43.3 min
- 60 min
Answer: B — 30 min
For a first-order reaction, t½ = ln 2/k = 0.693/0.0231 = 30 min. The time unit comes from the reciprocal of min⁻¹. This expression is independent of initial concentration and must not be applied automatically to zero- or second-order reactions.
QUESTION 4
What fraction of a reactant remains after three half-lives in a first-order reaction?
- 1/2
- 1/3
- 1/6
- 1/8
Answer: D — 1/8
After each half-life, the amount remaining is multiplied by one half. After three half-lives the fraction is (1/2)³ = 1/8. Thus 7/8 has reacted. Subtracting a fixed amount after each interval would confuse first-order decay with a linear decrease.
QUESTION 5
At a fixed temperature, what does a catalyst change for a reversible reaction?
- The equilibrium constant
- The standard Gibbs energy change
- The time needed to approach equilibrium
- The equilibrium composition by favouring only the forward reaction
Answer: C — The time needed to approach equilibrium
A catalyst accelerates both forward and reverse reactions through an alternative pathway, reducing the time needed to approach equilibrium. It does not alter the standard Gibbs energy difference between reactants and products, so the equilibrium constant and equilibrium composition at that temperature remain unchanged.
Study source and corrections
Use the matching chapter in NCERT Class XII Chemistry, Part I — Chemical Kinetics for the underlying concepts. These questions are written for NEET Ninja, not endorsed or supplied by NCERT or NTA.
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