Definition
Rate of reaction
Rate = -Δ[reactant]/Δt = +Δ[product]/Δt. Average rate over interval; instantaneous = -d[R]/dt at instant. Units: mol·L⁻¹·s⁻¹.
-- NCERT Class 12 Chemistry, Ch. 3, p. 64Rate of Reaction
8 MCQs9-step worked example
Source: NCERT Chemical KineticsOfficial key: NTA-verifiedLast updated: 24 Sep 2026
Rate of Reaction, explained for NEET
A common confusion that costs marks: treating the rate of a reaction as a single fixed number. The rate of a chemical reaction changes with time as reactants are consumed — what NEET tests is whether you understand how rate is defined and what the sign conventions mean.
What is rate of reaction? The rate of a chemical reaction is the change in concentration of a reactant or product per unit time (NCERT Class 12 Chemistry Chapter 3, page 63). For a general reaction aA + bB → cC + dD, the rate is expressed as:
Rate = −(1/a)(Δ[A]/Δt) = −(1/b)(Δ[B]/Δt) = +(1/c)(Δ[C]/Δt) = +(1/d)(Δ[D]/Δt)
The negative sign for reactants accounts for decreasing concentration; the positive sign for products accounts for increasing concentration. The stoichiometric coefficients (1/a, 1/b, etc.) normalise the rate so it is the same regardless of which species you measure.
Average vs. instantaneous rate. Average rate is Δ[concentration]/Δt over a finite interval. Instantaneous rate is the tangent slope (d[concentration]/dt) at a specific moment. NEET questions may give a concentration-time table and ask for the average rate over an interval, or they may ask you to distinguish the two.
The trap that bleeds marks: writing the rate expression without the stoichiometric coefficient. If 2NO₂ → 2NO + O₂, the rate of disappearance of NO₂ is −Δ[NO₂]/Δt, but the rate of reaction is −(1/2)(Δ[NO₂]/Δt). Forgetting the 1/2 gives an answer that is off by a factor of 2 — a distractor NTA loves to plant.
Watch-out: Units of rate are always mol L⁻¹ s⁻¹ (or mol L⁻¹ time⁻¹). If a question gives pressure changes for gaseous reactions, the unit shifts to atm s⁻¹, but the structure of the expression stays the same.
Can you answer these Rate of Reaction MCQs?
Select an option to see the explanation. Wrong answers show why your choice was tempting — and name the exact trap it exploits.
MCQ 1Easy RecallPractice
The rate of a chemical reaction is defined as:
Show answer and why every option is right or wrong
Answer: D. The rate of reaction is defined as the change in concentration of a reactant or product per unit time (NCERT Class 12 Chemistry Chapter 3, page 63). It is a concentration-based quantity, not mass or volume.
Why A is wrong: A is wrong because rate of reaction is defined in terms of concentration (mol/L), not mass. Mass change is not the standard kinetics definition.
Why B is wrong: B is wrong because total moles divided by total time gives an overall average that ignores stoichiometry and concentration — not the standard rate definition.
Why C is wrong: C is wrong because volume of the reaction vessel is not part of the rate definition. Rate tracks concentration change, not container dimensions.
MCQ 2Easy RecallPractice
For the reaction N₂ + 3H₂ → 2NH₃, the rate of reaction in terms of disappearance of H₂ is:
Show answer and why every option is right or wrong
Answer: B. The rate of reaction = −(1/stoichiometric coefficient) × (Δ[reactant]/Δt). For H₂ with coefficient 3: rate = −(1/3)(Δ[H₂]/Δt). The negative sign ensures a positive rate value since [H₂] decreases (NCERT Class 12 Chemistry Chapter 3, page 63).
Why A is wrong: A is wrong because it omits the stoichiometric coefficient 1/3. Without dividing by the coefficient, this gives the rate of disappearance of H₂, not the rate of reaction.
Why C is wrong: C is wrong because for a reactant, the sign must be negative (concentration is decreasing). A positive sign would give a negative rate value, which is physically meaningless.
Why D is wrong: D is wrong because the coefficient should divide, not multiply. Multiplying by 3 would overstate the rate by a factor of 9 compared to the correct expression.
MCQ 3Easy RecallPractice
The SI unit of rate of a chemical reaction is:
Show answer and why every option is right or wrong
Answer: B. Rate = Δ[concentration]/Δt. Concentration is in mol L⁻¹ and time in seconds, so rate has units mol L⁻¹ s⁻¹ (NCERT Class 12 Chemistry Chapter 3, page 63).
Why A is wrong: A is wrong because mol L⁻¹ is the unit of concentration, not rate. Rate requires division by a time unit.
Why C is wrong: C is wrong because mol⁻¹ L s⁻¹ has the inverse relationship for moles — rate is concentration per time, so moles appear in the numerator, not denominator.
Why D is wrong: D is wrong because s⁻¹ is the unit of the rate constant for a first-order reaction, not the unit of rate itself.
MCQ 4Direct ApplicationPractice
For 2SO₂(g) + O₂(g) → 2SO₃(g), if the rate of formation of SO₃ is 4.0 × 10⁻² mol L⁻¹ s⁻¹, the rate of disappearance of O₂ is:
Show answer and why every option is right or wrong
Answer: D. Rate of reaction = +(1/2)(Δ[SO₃]/Δt) = −Δ[O₂]/Δt. So rate of reaction = (1/2)(4.0 × 10⁻²) = 2.0 × 10⁻² mol L⁻¹ s⁻¹. Since O₂ has coefficient 1, rate of disappearance of O₂ = rate of reaction = 2.0 × 10⁻² mol L⁻¹ s⁻¹.
Why A is wrong: A is wrong because it equates the rate of disappearance of O₂ directly to the rate of formation of SO₃ without adjusting for the stoichiometric coefficient of SO₃ (which is 2). This is the stoichiometric-coefficient-omission trap.
Why B is wrong: B is wrong because dividing by 4 instead of 2 over-corrects. The relevant coefficient for SO₃ is 2, not 4.
Why C is wrong: C is wrong because it doubles the rate of formation of SO₃ instead of halving it. This reverses the stoichiometric adjustment (multiplying by 2 instead of dividing by 2).
MCQ 5Direct ApplicationPractice
For the reaction A → 2B, if [A] decreases from 0.50 mol L⁻¹ to 0.40 mol L⁻¹ in 10 s, the average rate of formation of B is:
Show answer and why every option is right or wrong
Answer: A. Rate of disappearance of A = (0.50 − 0.40)/10 = 0.010 mol L⁻¹ s⁻¹. Rate of reaction = 0.010 mol L⁻¹ s⁻¹ (coefficient of A is 1). Rate of formation of B = 2 × rate of reaction = 2 × 0.010 = 0.020 mol L⁻¹ s⁻¹ (since coefficient of B is 2).
Why B is wrong: B is wrong because 0.010 mol L⁻¹ s⁻¹ is the rate of disappearance of A (or the rate of reaction), not the rate of formation of B. B forms at twice this rate because its stoichiometric coefficient is 2.
Why C is wrong: C is wrong because it divides the rate of disappearance of A by 2 instead of multiplying. Since B has coefficient 2, B forms twice as fast as A disappears — not half.
Why D is wrong: D is wrong because it uses the total concentration change (0.10) divided by 2 instead of dividing by the time interval (10 s) first and then multiplying by the coefficient.
MCQ 6Direct ApplicationPractice
The concentration of a reactant drops from 0.80 mol L⁻¹ to 0.60 mol L⁻¹ in the first 20 s, and from 0.60 mol L⁻¹ to 0.45 mol L⁻¹ in the next 20 s. Which statement is correct?
Show answer and why every option is right or wrong
Answer: C. First interval: average rate = (0.80 − 0.60)/20 = 0.010 mol L⁻¹ s⁻¹. Second interval: average rate = (0.60 − 0.45)/20 = 0.0075 mol L⁻¹ s⁻¹. The first interval has a higher average rate, which is expected since rate generally decreases as reactant concentration drops.
Why A is wrong: A is wrong because the concentration change is different in each interval (0.20 vs 0.15 mol L⁻¹ over the same time span), so the average rates differ.
Why B is wrong: B is wrong because the rate decreases as reactant is consumed. The second interval shows a smaller concentration drop (0.15 mol L⁻¹) than the first (0.20 mol L⁻¹) in the same time, meaning a lower rate.
Why D is wrong: D is wrong because average rate = Δ[concentration]/Δt, and both intervals provide the concentration values and time span needed for this calculation.
MCQ 7Concept TrapPractice
For a gaseous reaction A(g) → B(g) + C(g), the rate of reaction can be expressed in terms of pressure change. If the total pressure increases at a rate of dp/dt, what is the rate of disappearance of A in terms of dp/dt?
Show answer and why every option is right or wrong
Answer: C. A → B + C. Let initial pressure of A = P₀. At time t, if x mol of A decomposes: P_A = P₀ − x, P_B = x, P_C = x. Total pressure = P₀ + x. So dP_total/dt = dx/dt. Rate of disappearance of A = dx/dt = dp/dt. The rate of disappearance of A equals the rate of total pressure increase because stoichiometric coefficients of products sum to 2 while reactant is 1, giving a net gain of 1 mole of gas per mole reacted.
Why A is wrong: A is wrong because dividing by 2 confuses the number of product species (2) with the net mole change. The net increase in total moles per mole of A reacted is 1 (from 1 mole to 2 moles), not 2.
Why B is wrong: B is wrong because multiplying by 2 would apply if 2 moles of A decomposed per mole of pressure change, but here 1 mole of A produces exactly 1 extra mole of gas (net), so dp/dt already equals the rate of disappearance of A.
Why D is wrong: D is wrong because while the pressure of A decreases, the question asks for the 'rate of disappearance' which is a positive quantity by convention. The magnitude dp/dt is already positive.
MCQ 8CalculationPractice
For the reaction 2N₂O₅(g) → 4NO₂(g) + O₂(g), at a certain moment the rate of disappearance of N₂O₅ is 1.2 × 10⁻² mol L⁻¹ s⁻¹. The rate of formation of NO₂ and the rate of reaction are, respectively:
Show answer and why every option is right or wrong
Answer: A. Rate of reaction = (1/2) × rate of disappearance of N₂O₅ = (1/2)(1.2 × 10⁻²) = 6.0 × 10⁻³ mol L⁻¹ s⁻¹. Rate of formation of NO₂ = 4 × rate of reaction = 4 × 6.0 × 10⁻³ = 2.4 × 10⁻² mol L⁻¹ s⁻¹. This requires two linked steps: finding the rate of reaction first, then scaling to NO₂.
Why B is wrong: B is wrong because it correctly computes the rate of formation of NO₂ but equates the rate of reaction to the rate of disappearance of N₂O₅ without dividing by its coefficient 2. The stoichiometric-coefficient-omission trap costs the rate-of-reaction value.
Why C is wrong: C is wrong because it doubles the NO₂ formation rate incorrectly (multiplying by 4 twice, effectively) while getting the rate of reaction correct. The 4:2 ratio means NO₂ forms at twice the rate of N₂O₅ disappearance, not four times.
Why D is wrong: D is wrong because both values are off — it over-multiplies for NO₂ formation AND omits the 1/2 factor for the rate of reaction. This is a double application of the stoichiometric-coefficient-omission error.
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How do you solve a Rate of Reaction question? A worked example
- 1
Given
• Reaction: 5Br⁻ + BrO₃⁻ + 6H⁺ → 3Br₂ + 3H₂O• Rate of disappearance of Br⁻ = 5.0 × 10⁻³ mol L⁻¹ s⁻¹
- 2
Required
(a) Rate of reaction
(b) Rate of formation of Br₂ - 3
Concept
The rate of reaction is related to the rate of change of any species by dividing by the stoichiometric coefficient (with a negative sign for reactants): Rate = −(1/coefficient)(Δ[species]/Δt) for reactants, and +(1/coefficient)(Δ[species]/Δt) for products.
- 4
Formula
Rate of reaction = (1/5) × rate of disappearance of Br⁻
Rate of formation of Br₂ = 3 × rate of reaction - 5
Substitution
Rate of reaction = (1/5) × 5.0 × 10⁻³
Rate of formation of Br₂ = 3 × rate of reaction - 6
Calculation
Rate of reaction = (1/5)(5.0 × 10⁻³) = 1.0 × 10⁻³ mol L⁻¹ s⁻¹
Rate of formation of Br₂ = 3 × 1.0 × 10⁻³ = 3.0 × 10⁻³ mol L⁻¹ s⁻¹
Note: The stoichiometric coefficients (5, 3) are exact integers and do not affect significant figures. The answer retains 2 significant figures, matching the given data. - 7
Final answer
(a) Rate of reaction = 1.0 × 10⁻³ mol L⁻¹ s⁻¹
(b) Rate of formation of Br₂ = 3.0 × 10⁻³ mol L⁻¹ s⁻¹ - 8
Common trap
A frequent error is skipping the stoichiometric coefficient and writing rate of reaction = 5.0 × 10⁻³ mol L⁻¹ s⁻¹ (equating it directly to the disappearance rate of Br⁻). This gives a value 5 times too large and leads to an incorrect Br₂ formation rate of 1.5 × 10⁻² mol L⁻¹ s⁻¹.
- 9
Similar NEET-style question
For 4NH₃(g) + 5O₂(g) → 4NO(g) + 6H₂O(g), if the rate of formation of NO is 2.0 × 10⁻⁴ mol L⁻¹ s⁻¹, calculate the rate of disappearance of O₂. [Answer: Rate of reaction = (1/4)(2.0 × 10⁻⁴) = 5.0 × 10⁻⁵ mol L⁻¹ s⁻¹. Rate of disappearance of O₂ = 5 × 5.0 × 10⁻⁵ = 2.5 × 10⁻⁴ mol L⁻¹ s⁻¹.]
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What to remember before solving Rate of Reaction questions
More in Chemical Kinetics: 5 exam traps and mistakes · 4 formulas · 2 question patterns from its other lessons.
Rate of Reaction questions from past NEET papers
No question in our NEET 2020–2025 set targets this topic directly.
Sources
NCERT refs: Class 12 Chemistry Chapter 3, p.63
Page numbers are the ones printed in the current NCERT textbook (2023 rationalised edition), unless marked pre-2023. The books are free at ncert.nic.in.
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