Question
Explain the difference between order of reaction and molecularity. A reaction has the rate law: Rate = k[A][B]². What is the order of reaction? Can the order of a reaction be a fraction?
(CBSE 2024 Board Exam, 3 marks)
Solution — Step by Step
Molecularity is the number of reacting species (atoms, molecules, or ions) that collide simultaneously in an elementary step — it’s a theoretical concept, determined from the mechanism. Order of reaction is the sum of powers of concentration terms in the experimentally determined rate law.
The rate law is Rate = k[A]¹[B]². The order with respect to A is 1, and with respect to B is 2.
Overall order = 1 + 2 = 3 (third-order reaction).
Yes — order is experimentally determined, so it can be zero, fractional, or even negative. A classic example is the thermal decomposition of acetaldehyde: CH₃CHO → CH₄ + CO, which follows a 1.5 (3/2) order rate law.
Molecularity, however, is always a positive integer (1, 2, or rarely 3) — you cannot have 1.5 molecules colliding.
| Property | Order | Molecularity |
|---|---|---|
| Nature | Experimental | Theoretical |
| Applies to | Overall reaction | Elementary step only |
| Can be fraction/zero? | Yes | No |
| Determined by | Rate law (experiment) | Reaction mechanism |
| Range | 0, fraction, integer | 1, 2, or 3 only |
Why This Works
The rate law Rate = k[A][B]² tells us how fast the reaction actually goes as we change concentrations. The exponents are found by doing experiments — changing [A] while keeping [B] fixed, and vice versa — not by looking at the balanced equation.
Molecularity only makes sense for elementary reactions (single-step, single-collision events). For a complex reaction with multiple steps, we talk about the molecularity of each step separately, never for the overall reaction. The rate law of the overall reaction is controlled by the slowest step (the rate-determining step).
This is why you’ll sometimes see a reaction like 2NO₂ + F₂ → 2NO₂F with an experimentally observed rate law of Rate = k[NO₂][F₂] — order 2, not 3 — because the mechanism has a slow first step that involves only one NO₂ and one F₂.
Alternative Method — Identifying Order from Units of k
If you’re given the rate constant k and asked to find the order, use the units of k.
For an nth order reaction: units of k = mol^(1-n) · L^(n-1) · s⁻¹
| Order | Units of k |
|---|---|
| Zero | mol L⁻¹ s⁻¹ |
| First | s⁻¹ |
| Second | L mol⁻¹ s⁻¹ |
| Third | L² mol⁻² s⁻¹ |
This unit trick is a lifesaver in MCQs. If k = 2.5 × 10⁻³ L mol⁻¹ s⁻¹, that’s second order — no rate law needed. This shortcut has appeared directly in JEE Main multiple times.
Common Mistake
Reading order from the balanced equation. A huge number of students look at 2HI → H₂ + I₂ and write “order = 2” because the coefficient of HI is 2. Wrong. The experimentally determined order for this reaction is actually 2, but that’s a coincidence — it is NOT because of the stoichiometric coefficient. For H₂ + Br₂ → 2HBr, the rate law is Rate = k[H₂][Br₂]^(1/2), giving order = 1.5, nowhere near what the equation suggests.
Order must come from experiment or from a given rate law expression — never from the balanced equation.
- Overall order =
- , are determined experimentally
- Molecularity applies only to elementary steps
- Order: any real number ≥ 0 | Molecularity: positive integer only