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 70).
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:
Answer: C. Order is determined experimentally by measuring how rate changes with concentration (NCERT Class 12 Chemistry Chapter 3, page 70). It cannot be deduced from the balanced equation.
Why A is wrong: A is wrong because stoichiometric coefficients determine molecularity of an elementary step, not order. Order comes from experiment, and coefficients may not match rate-law exponents for complex reactions (trap: conflating stoichiometry with kinetics).
Why B is wrong: B is wrong because molecularity applies to a single elementary step and describes the number of colliding species, not the overall kinetic behaviour. Order and molecularity coincide only for elementary reactions (trap: treating the two terms as interchangeable).
Why D is wrong: D is wrong because products do not appear in the rate law (which depends on reactant concentrations). The number of products has no bearing on the reaction order.
Molecularity of a reaction can be:
Answer: D. Molecularity counts the number of species colliding in one elementary step. Since you cannot have a fraction of a molecule colliding, it must be a positive integer — typically 1, 2, or 3 (NCERT Class 12 Chemistry Chapter 3, page 70).
Why A is wrong: A is wrong because molecularity cannot be zero (at least one species must participate) and cannot be fractional (you cannot have a fraction of a molecule colliding). Zero and fractional values are possible for order, not molecularity (trap: confusing order properties with molecularity properties).
Why B is wrong: B is wrong because molecularity is always a whole number, never fractional. Fractional values occur in reaction order, not molecularity (trap: mixing up which quantity can be fractional).
Why C is wrong: C is wrong because molecularity is a theoretical count of colliding species in a single step. It is not measured experimentally — that description fits order (trap: swapping the experimental/theoretical distinction between order and molecularity).
For a complex (multi-step) reaction, which statement is correct?
Answer: B. Molecularity applies only to an elementary step — the single collision event. For a multi-step reaction, you can assign molecularity to each elementary step but not to the overall reaction (NCERT Class 12 Chemistry Chapter 3, page 70).
Why A is wrong: A is wrong because molecularity is not defined for the overall complex reaction at all. Summing stoichiometric coefficients of the balanced equation gives neither the molecularity nor the order (trap: extending molecularity to non-elementary reactions).
Why C is wrong: C is wrong because the rate law of a complex reaction must be determined experimentally. Only for an elementary reaction can the rate law be written from stoichiometry (trap: the core misconception this lesson targets — writing rate law from coefficients).
Why D is wrong: D is wrong because order can be zero, fractional, or even negative. It is molecularity that must be a positive integer (trap: swapping the constraints on order and molecularity).
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?
Answer: A. Overall order = 1 + 1 = 2 (sum of exponents in the rate law). Since the reaction has a rate law inconsistent with the stoichiometry (exponents ≠ coefficients), it is not an elementary reaction, and molecularity is not defined for the overall reaction.
Why B is wrong: B is wrong because while the order is correctly 2, molecularity cannot be assigned to the overall reaction. The mismatched rate-law exponents prove this is not elementary (trap: assigning molecularity from stoichiometric coefficients to a complex reaction).
Why C is wrong: C is wrong on both counts. Order = 1 + 1 = 2 from the rate law exponents (not 1 + 2 = 3 from stoichiometric coefficients). Molecularity cannot be assigned to an overall complex reaction (trap: reading order from stoichiometry and assigning molecularity to a non-elementary reaction).
Why D is wrong: D is wrong because order = 2 (from the rate law exponents 1 + 1), not 3. Adding stoichiometric coefficients (1 + 2) gives 3, but that is not how order works (trap: confusing stoichiometric sum with order).
For the elementary reaction: NO₂ + CO → NO + CO₂, what are the order and molecularity?
Answer: A. Since this is stated to be an elementary reaction, the rate law can be written from stoichiometry: rate = k[NO₂][CO]. Order = 1 + 1 = 2. Two species collide, so molecularity = 2 (bimolecular). For elementary reactions, order equals molecularity.
Why B is wrong: B is wrong because for an elementary bimolecular reaction, both species appear in the rate law with exponent 1. Order = 1 + 1 = 2, not 1 (trap: counting only one reactant's exponent as the overall order).
Why C is wrong: C is wrong because two molecules collide (NO₂ and CO), making it bimolecular (molecularity = 2), not unimolecular (trap: confusing the coefficient of each species with the total number of colliding species).
Why D is wrong: D is wrong because molecularity is always defined for an elementary reaction — it equals the number of reacting species in that single step. It would be undefined only for the overall complex reaction (trap: misapplying the 'undefined molecularity' rule to elementary reactions).
A reaction has the rate law: rate = k[A]^(3/2)[B]^(−1). Which statement is correct?
Answer: D. Overall order = 3/2 + (−1) = 1/2. Since the order is fractional (and one exponent is negative), the rate law cannot correspond to an elementary step. Molecularity must be a positive integer, so this reaction must proceed through multiple steps (complex reaction).
Why A is wrong: A correctly computes order as 1/2 but wrongly calls the reaction elementary. An elementary reaction cannot have fractional or negative exponents in its rate law — those signal a multi-step mechanism (trap: not recognising that fractional/negative order rules out an elementary reaction).
Why B is wrong: B is wrong for two reasons: the order is 1/2 (not 5/2), and molecularity cannot be fractional — it must be a positive integer. Fractional molecularity is meaningless (trap: equating order with molecularity and computing the sum incorrectly).
Why C is wrong: C is wrong because 3/2 + (−1) = 1/2, not 5/2. The negative exponent must be subtracted, not added as a positive value. Additionally, a reaction with fractional order cannot be elementary (trap: adding magnitudes instead of algebraic values for order).
For a reaction A + 2B → C, the experimentally determined rate law is rate = k[A][B]². A student claims: "Since the exponents match the stoichiometric coefficients, this proves the reaction is elementary." Is the student's reasoning valid?
Answer: B. B is correct. Order is an experimental quantity, and NCERT warns that a balanced equation "never gives us a true picture of how a reaction takes place" (NCERT Class 12 Chemistry, Chapter 3, page 70). A multi-step reaction can still show exponents that happen to equal the coefficients — the slow step may simply involve those species in those numbers — so the match on its own proves nothing. The mechanism has to be established independently, by detecting intermediates or identifying the rate-determining step.
Why A is wrong: A is wrong because matching exponents can occur by coincidence in complex reactions. The rate law alone cannot confirm mechanism — independent evidence is required (trap: the exact misconception this topic targets, where students equate coefficient-matching with proof of elementary nature).
Why C is wrong: C is wrong because it inverts the rule. For a genuinely elementary step the exponents DO equal the coefficients — that is exactly what molecularity means. The flaw in the student's argument is the converse: matching exponents can ALSO arise in a complex reaction, so the match cannot single out an elementary one (trap: over-correcting a misconception into an equally false opposite).
Why D is wrong: D is wrong because trimolecular (termolecular) elementary reactions are extremely rare due to the low probability of three-body simultaneous collisions. Moreover, the coincidence of exponents matching coefficients does not establish elementary nature (trap: assuming any reaction with three reactant molecules in the balanced equation must be trimolecular and elementary).
A reaction proceeds through two elementary steps:
Step 1 (slow): A + B → X (intermediate)
Step 2 (fast): X + A → C
The overall reaction is 2A + B → C. What is the molecularity of the rate-determining step, and what is the overall order of the reaction?
Answer: C. The rate-determining step (Step 1) involves A + B — two species collide, so molecularity = 2 (bimolecular). The rate law is determined by the slow step: rate = k[A][B]. Overall order = 1 + 1 = 2. The stoichiometric coefficients of the overall reaction (2 + 1 = 3) are irrelevant for determining order.
Why A is wrong: A is wrong because while molecularity of Step 1 is correctly 2, the overall order is 2 (from the rate-determining step's rate law: rate = k[A][B]), not 3. Using stoichiometric coefficients of the overall reaction gives 3 — the central trap of this topic (trap: reading order from the overall balanced equation instead of the rate-determining step).
Why B is wrong: B is wrong on both counts. Molecularity of Step 1 is 2 (A + B), not 3 (the student is counting the overall stoichiometry 2A + B). And order is 2, not 3 (trap: conflating overall stoichiometry with both molecularity and order).
Why D is wrong: D is wrong because while molecularity is correctly 2, the order is 2 (rate = k[A][B], so 1 + 1 = 2), not 1. The student may be counting only one reactant (trap: incomplete reading of the rate-determining step's rate law).
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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.)
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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. 70These are the exact patterns that cause wrong answers in NEET. Each trap includes when it triggers and how to avoid it.
Root cause: concept gap
Rate law from EXPERIMENT, not stoichiometry. Order = sum of exponents in rate = k[A]^x[B]^y. Molecularity follows mechanism.
More in Chemical Kinetics: 4 exam traps and mistakes · 4 formulas · 2 question patterns from its other lessons.
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