The trap that costs marks on this topic: writing the rate law directly from the balanced equation's stoichiometric coefficients. For the reaction 2A + B → products, students reflexively write rate = k[A]²[B]. That is wrong unless experimental data confirms those exponents. Order and molecularity look similar but answer fundamentally different questions.
Order is the sum of the exponents in the experimentally determined rate law. For rate = k[A]^x[B]^y, the order with respect to A is x, with respect to B is y, and the overall order is x + y. Order can be zero, fractional, or negative. It comes from experiment — never from the equation's coefficients (NCERT Class 12 Chemistry Chapter 3, page 10).
Molecularity is the number of reacting species (atoms, ions, or molecules) that collide simultaneously in an elementary step. It is always a positive integer: unimolecular (1), bimolecular (2), or trimolecular (3). Molecularity applies only to elementary reactions, never to the overall reaction if it proceeds through multiple steps.
The critical distinction: order describes the overall reaction's kinetic behaviour (experimental), while molecularity describes a single mechanistic step (theoretical). For an elementary reaction, the two happen to coincide — the rate law can be written from the stoichiometry of that single step. For a complex (multi-step) reaction, only the rate-determining step's molecularity matters, and the overall order must still come from experiment.
Watch out: NEET distractors exploit two confusions — (1) treating molecularity as applicable to complex reactions, and (2) assuming order must equal stoichiometric coefficients. Both are traps anchored in the same root misconception: conflating what experiments measure with what mechanisms predict.
Select an option to see the explanation. Wrong answers show why your choice was tempting — and name the exact trap it exploits.
The order of a reaction is determined by:
Molecularity of a reaction can be:
For a complex (multi-step) reaction, which statement is correct?
The experimentally determined rate law for a reaction A + 2B → C is: rate = k[A][B]. What is the overall order and can molecularity of the overall reaction be stated?
For the elementary reaction: NO₂ + CO → NO + CO₂, what are the order and molecularity?
A reaction has the rate law: rate = k[A]^(3/2)[B]^(−1). Which statement is correct?
For the reaction 2NO + O₂ → 2NO₂, the experimentally determined rate law is rate = k[NO]²[O₂]. A student claims: "Since the exponents match the stoichiometric coefficients, this proves the reaction is elementary." Is the student correct?
A reaction proceeds through two elementary steps:
Given
- Balanced equation: 2N₂O₅ → 4NO₂ + O₂ - Experimental rate law: rate = k[N₂O₅]
Required
- The student's error - Correct overall order - Whether molecularity applies to the overall reaction
Concept
Order is determined by experiment, not stoichiometry. Molecularity is defined only for elementary steps.
Formula
Rate law (experimental): rate = k[N₂O₅]^x, where x is determined from data.
Substitution
The experiment gives x = 1. The student assumed x = 2 (the stoichiometric coefficient).
Calculation
Overall order = 1 (the exponent in the experimentally determined rate law). The student's answer of order = 2 is wrong because they read the coefficient of N₂O₅ in the balanced equation as the exponent.
Final answer
- **Error:** The student assumed rate-law exponents equal stoichiometric coefficients. This is valid only for elementary reactions. - **Correct overall order:** 1 (first-order). - **Molecularity:** Not defined for this overall reaction. The rate law (first order despite stoichiometric coefficient 2) proves the reaction is not elementary — it proceeds through multiple steps. Molecularity can be assigned to each elementary step in the mechanism but not to the overall reaction.
Common trap
The most common mark-losing error on this topic: reading the rate law from the balanced equation. The stoichiometric coefficient 2 for N₂O₅ tempts students into writing second-order kinetics. Experimental evidence overrides stoichiometry for complex reactions.
Similar NEET-style question
The reaction H₂ + I₂ → 2HI has rate = k[H₂][I₂]. Another reaction, H₂ + Br₂ → 2HBr, has rate = k[H₂][Br₂]^(1/2). For each, state the overall order and whether the result is consistent with an elementary mechanism. (Answer: H₂ + I₂: order = 2, consistent with bimolecular elementary; H₂ + Br₂: order = 3/2, fractional order proves non-elementary.) ---
Order = sum of exponents in experimental rate law (can be 0, fractional, etc.). Molecularity = number of reacting species in elementary step (always positive integer ≤3). Rate law from experiment; molecularity from mechanism.
-- NCERT Class 12 Chemistry, Ch. 3, p. 10Temperature 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 |
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 |
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 |
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 |
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.
Half-life question with order specified.
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
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
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
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
Rate law from EXPERIMENT, not stoichiometry. Order = sum of exponents in rate = k[A]^x[B]^y. Molecularity follows mechanism.
10 questions from NEET 2021, 2022, 2023, 2024, 2025. Answers verified against NTA official keys.
Activation energy of any chemical reaction can be calculated if one knows the value of
Which reaction is NOT a redox reaction?
Which one is an example of heterogenous catalysis?
Recurring question shapes from past papers. Each pattern shows why wrong options look tempting.
wrong sign of 1 over t
Mixes T1 and T2 in subtraction
uses zero order formula
Plugs into [A]_0/(2k) wrong-order formula
Test yourself on this topic with real past-paper questions:
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