CBSE Class 12 Chemistry Chapter 3 Chemical Kinetics — 20 MCQs with Answers

The rate of a chemical reaction quantifies how quickly reactants transform into products. For a reaction aA + bB → cC + dD, the average rate is −(1/a)(Δ[A]/Δt) or +(1/c)(Δ[C]/Δt). NCERT emphasizes that rate depends on concentration, temperature, catalyst presence, and surface area (for heterogeneous reactions). Understanding units—mol L⁻¹ s⁻¹ for concentration-time rate and bar s⁻¹ for gaseous reactions—is crucial. The following MCQs test basic definitions, unit conversions, and conceptual understanding of instantaneous versus average rate. These are commonly the first 1–2 MCQs in Section A of the CBSE paper.

Rate law expresses rate as k[A]ˣ[B]ʸ, where x and y are determined experimentally and their sum is the overall order. Order can be zero, fractional, or integer, but molecularity (number of molecules in an elementary step) is always a positive integer—1 (unimolecular), 2 (bimolecular), or rarely 3 (termolecular). NCERT stresses that order ≠ stoichiometric coefficient unless the reaction is elementary. These distinctions are favorite traps in CBSE MCQs. The questions below test your ability to deduce order from experimental data, recognize elementary steps, and avoid common pitfalls. Expect at least one such MCQ in every CBSE Class 12 Chemistry paper, often worth 1 mark in Section A.

Integrated rate equations relate concentration to time. For zero-order: [A]=[A]₀−kt (linear with time); first-order: ln[A]=ln[A]₀−kt (exponential decay), and t₁/₂=0.693/k independent of [A]₀; second-order: 1/[A]=1/[A]₀+kt. NCERT devotes significant space to deriving these and showing graphical tests (plot [A] vs t for zero-order gives a straight line; ln[A] vs t for first-order; 1/[A] vs t for second-order). Half-life formulas are high-yield: zero-order t₁/₂=[A]₀/(2k), first-order t₁/₂=0.693/k, second-order t₁/₂=1/(k[A]₀). Recognizing which plot is linear for a given order is a classic CBSE trap. Practice these four MCQs to cement the distinctions and avoid mixing up formulas under exam pressure.

The Arrhenius equation k=Ae^(−Ea/RT) links the rate constant k to absolute temperature T and activation energy Ea. Taking natural logarithm: ln k = ln A − Ea/(RT), which is linear in 1/T with slope −Ea/R. NCERT explains that A is the pre-exponential (frequency) factor, and Ea is the minimum energy colliding molecules need to react. A 10°C rise typically doubles or triples the reaction rate for many reactions. The equation also appears in the two-temperature form: ln(k₂/k₁)=(Ea/R)×(1/T₁−1/T₂). CBSE loves numerical-based MCQs here: given k at two temperatures, find Ea, or vice versa. Understanding units (Ea in J mol⁻¹, R=8.314 J K⁻¹ mol⁻¹) and the inverse relationship between Ea and rate is critical. These four questions drill the concepts and common calculation traps that appear in Section A or as assertion-reason pairs.

Collision theory posits that molecules must collide with energy ≥ Ea and proper orientation to react. NCERT defines the fraction of molecules with energy ≥ Ea as e^(−Ea/RT), which explains the exponential term in the Arrhenius equation. Not all collisions are effective; steric factors (orientation) reduce the number of successful collisions, captured by the pre-exponential factor A. Higher Ea means fewer molecules cross the energy barrier at a given T, slowing the reaction. Catalysts lower Ea without being consumed, increasing k. These MCQs test conceptual clarity on effective collisions, the role of Ea, and the distinction between thermodynamic stability (ΔG) and kinetic stability (Ea). Expect one question on collision theory or catalysts in most CBSE papers, sometimes as an assertion-reason type, particularly valuable because assertion-reason MCQs carry specific marking in the new CBSE pattern.

CBSE papers increasingly include higher-order thinking (HOTS) and assertion-reason (A-R) MCQs in Section A. HOTS questions require application of two or more concepts—for example, combining integrated rate laws with Arrhenius equation or deducing mechanism from experimental data. Assertion-reason MCQs present two statements: you must decide if both are true, if one is true, and if the reason correctly explains the assertion. NCERT examples show that many students lose marks here by not reading carefully. The two MCQs below are modeled on the 2024–25 CBSE pattern. Practice dissecting each statement independently, then checking logical linkage. These questions often distinguish top scorers from the rest because they test depth, not rote memory. Spending an extra 30 seconds on A-R questions can save a mark, which can matter when cutoffs are 90+.

Section A of the 2025 CBSE Class 12 Chemistry paper carries 20 MCQs (16×1 mark + 4×1 mark assertion-reason), total 20 marks. Timing is tight: aim for 20–25 minutes for the entire section, leaving buffer for numericals and long answers. First, scan all 20 questions and mark the 5–7 you can answer in under 15 seconds (definitions, direct formula recall). Answer those immediately to bank easy marks and build confidence. Next, tackle numerical MCQs (rate law, Arrhenius, half-life calculations) because they have one correct answer—no ambiguity. Show quick working in the margin if allowed; even if you can't finish, you might spot the right option by eliminating impossible values. For assertion-reason, read the assertion first, decide if it's true or false, then read the reason independently. Only then check if R explains A. Beware of statements that are individually true but logically unconnected—CBSE designs these to catch hasty readers. If stuck between two options, use elimination: cross out clearly wrong choices, then guess intelligently from the rest. Never leave an MCQ blank; there is no negative marking in CBSE board exams. Finally, if time remains, revisit any MCQ you flagged. A second read often reveals a missed keyword like 'NOT' or 'EXCEPT'. Practice 20 MCQs under 20-minute time limits weekly in the two months before boards. Use NCERT Exemplar and previous years' CBSE question papers as your primary sources—third-party guides often include non-syllabus or poorly worded questions. Remember, Chemical Kinetics MCQs are usually straightforward if you have clarity on order versus molecularity, half-life formulas, and the Arrhenius equation. Drill these core concepts until they are reflexive, and you will comfortably score full marks in this section on exam day.

Year after year, CBSE examiners report recurring errors in Chemical Kinetics MCQs. First, students confuse order with molecularity. Remember: order is experimental (can be 0, fractional, or integer), while molecularity is theoretical (1, 2, or 3 for elementary steps). Second, half-life formula mix-ups cost marks. Write down the three key formulas on your rough sheet at the start: zero-order [A]₀/(2k), first-order 0.693/k, second-order 1/(k[A]₀). Third, in Arrhenius calculations, unit errors are rampant—Ea must be in J mol⁻¹ if R=8.314 J K⁻¹ mol⁻¹, not kJ. Fourth, many students ignore stoichiometry when relating rates of different species; always divide the rate by the stoichiometric coefficient. Fifth, assertion-reason MCQs trip students who read only the assertion and assume the reason is correct. Treat A and R as independent true/false statements first. Sixth, time pressure leads to misreading 'increases' as 'decreases' or missing a negative sign in −Ea/RT. Underlining key terms as you read can prevent this. Seventh, students apply integrated rate laws without checking which order the reaction is—always identify order from given data before picking a formula. Lastly, neglecting NCERT in-text questions and exercise numericals is a mistake. At least 3–4 MCQs in every CBSE paper are direct adaptations of NCERT exercise problems. Work through all 35 NCERT exercise questions for Chapter 3, especially the ones involving graphical interpretation and two-temperature Arrhenius problems. Avoiding these pitfalls can easily lift your MCQ score from 14–15/20 to 18–19/20. Use CBSETUTOR.ai 24×7 to upload a photo of any MCQ you got wrong and get an instant step-by-step video-style explanation—this rapid feedback loop fixes misconceptions before they cost you marks in the actual exam. Available at a flat ₹999/month for all subjects and classes (6–12), with a 3-day free trial so you can test the AI tutor risk-free.