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. 4Rate of Reaction
8 MCQs9-step worked example
Source: NCERT Organic Chemistry — Basic PrinciplesPYQ coverage: NEET 2021, 2022, 2023, 2024, 2025Official key: NTA-verifiedLast reviewed: May 2026
Lesson
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 4). 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.
Practice 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:
MCQ 2Easy RecallPractice
For the reaction N₂ + 3H₂ → 2NH₃, the rate of reaction in terms of disappearance of H₂ is:
MCQ 3Easy RecallPractice
The SI unit of rate of a chemical reaction is:
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:
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:
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?
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?
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:
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.] ---
Before solving, remember these
Formulas
Arrhenius equation
Temperature dependence of rate constant. Higher Ea → more T-sensitive rate.
| Symbol | Quantity | SI Unit |
|---|---|---|
| A | frequency factor | same as k |
| Ea | activation energy | J/mol |
| R | gas constant | J/mol/K |
| T | temp | K |
Valid when
- T in kelvins
- Most reactions in modest T range
Arrhenius for two temperatures
Compare rate constants at two temperatures to find Ea.
| Symbol | Quantity | SI Unit |
|---|---|---|
| k1, k2 | rate constants | same units |
| T1, T2 | temperatures | K |
| Ea | activation energy | J/mol |
Valid when
- A constant across temperature range
- T in kelvins
First-order kinetics
Concentration decays exponentially. Half-life independent of [A]_0.
| Symbol | Quantity | SI Unit |
|---|---|---|
| [A] | conc at time t | mol/L |
| k | rate constant | 1/s |
| t | time | s |
Valid when
- First-order reaction (rate = k[A])
Zero-order kinetics
Concentration decays linearly. Half-life depends on initial concentration.
| Symbol | Quantity | SI Unit |
|---|---|---|
| [A]_0 | initial conc | mol/L |
| k | rate constant | mol/L/s |
| t | time | s |
Valid when
- Zero-order reaction (rate = k, no concentration dependence)
Exam Traps & Common Mistakes
These are the exact patterns that cause wrong answers in NEET. Each trap includes when it triggers and how to avoid it.
Category: Similar Terms
Zero-order t_1/2 depends on [A]_0. First-order t_1/2 INDEPENDENT of [A]_0. Student uses wrong formula.
When it triggers
Half-life question with order specified.
How to avoid
1st order: t_1/2 = 0.693/k (constant). Zero order: t_1/2 = [A]_0/(2k) (varies with initial conc). Second order: t_1/2 = 1/(k[A]_0).
Root cause: sign error
Correction
ln(k2/k1) = (Ea/R) × (1/T1 - 1/T2). At higher T2, k2 > k1, so ln(k2/k1) > 0. Need 1/T1 > 1/T2.
Root cause: formula misuse
Correction
First order: t_1/2 = 0.693/k (constant). Zero order: t_1/2 = [A]_0/(2k) (depends on [A]_0). Second order: t_1/2 = 1/(k[A]_0).
Root cause: formula misuse
Correction
Zero-order: t_1/2 = [A]_0/(2k) (depends on initial conc). First-order: t_1/2 = 0.693/k (independent of [A]_0).
Root cause: concept gap
Correction
Rate law from EXPERIMENT, not stoichiometry. Order = sum of exponents in rate = k[A]^x[B]^y. Molecularity follows mechanism.
Past Year Questions
10 questions from NEET 2021, 2022, 2023, 2024, 2025. Answers verified against NTA official keys.
NEET 2025
1[Co(NH 3 ) 5 Cl]Cl
2[Co(NH 3 ) 3 Cl 3 ]
3[Co(NH ) Cl ]
4[Co(NH ) ]Cl 3 4 2 3 6 3 Answer (2, 3)
NTA Answer: Option multi:2,3(final)
NEET 2025
110 minutes
22 minutes
34 minutes
45 minutes
NTA Answer: Option 1(final)
NEET 2024Revised key
Activation energy of any chemical reaction can be calculated if one knows the value of
1rate constant at standard temperature
2probability of collision
3orientation of reactant molecules during collision
4rate constant at two different temperatures
NTA Answer: Option 4(revised_final)
NEET 2024Revised key
Which reaction is NOT a redox reaction?
1Zn + CuSO → ZnSO + Cu 4 4
22KClO 3 + I 2 → 2KIO 3 + Cl 2
3H + Cl → 2HCl 2 2
4BaCl + Na SO → BaSO + 2NaCl 2 2 4 4
NTA Answer: Option 4(revised_final)
NEET 2024Revised key
138.04 kJ/mol
2380.4 kJ/mol
33.80 kJ/mol
43804 kJ/mol
NTA Answer: Option 1(revised_final)
NEET 2023
Which one is an example of heterogenous catalysis?
1Hydrolysis of sugar catalysed by H+ ions
2Decomposition of ozone in presence of nitrogen monoxide
3Combination between dinitrogen and dihydrogen to form ammonia in the presence of finely divided iron
4Oxidation of sulphur dioxide into sulphur trioxide in the presence of oxides of nitrogen
NTA Answer: Option 3(final)
NEET 2023
1Increase by a factor of six
2Increase by a factor of nine
3Increase by a factor of three
4Decrease by a factor of nine
NTA Answer: Option 2(final)
NEET 2022
1Statement I is incorrect but Statement II is correct.
2Both Statement I and Statement II are correct.
3Both Statement I and Statement II are incorrect.
4Statement I is correct but Statement II is incorrect.
NTA Answer: Option 4(final)
NEET 2021
1Zone refining
2Electrolysis
3Chromatography
4Distillation
NTA Answer: Option 4(final)
NEET 2021
1–83 kJ mol–1
241.5 kJ mol–1
383.0 kJ mol–1
4166 kJ
NTA Answer: Option 2(final)
How NEET usually asks this
Recurring question shapes from past papers. Each pattern shows why wrong options look tempting.
Find Ea from rate-constant ratio at two temperatures using ln(k2/k1) = Ea/R · (1/T1 − 1/T2).
Multi StepMedium
Common distractors
wrong sign of 1 over t
Mixes T1 and T2 in subtraction
First-order: t_1/2 = 0.693/k. Apply or invert to find k or remaining concentration.
Direct ApplicationEasy
Common distractors
uses zero order formula
Plugs into [A]_0/(2k) wrong-order formula
Sources
NCERT refs: Class 12 Chemistry Chapter 3, p.4
Test yourself on this topic with real past-paper questions:
Practice this topic →Free NEET study resources
Get a structured 30-day study plan and a complete formula booklet — delivered to your inbox instantly.