Question
What is the order of reaction? How is it different from molecularity?
This distinction shows up directly in NCERT Class 12 and has appeared in multiple JEE Main and NEET papers. Getting this conceptually clear — not just memorised — saves you marks.
Solution — Step by Step
Order of reaction is the sum of the powers of the concentration terms in the experimentally determined rate law. If rate , then order .
The key word is experimentally — we don’t derive order from the balanced equation. We measure it from concentration vs. rate data.
Molecularity is the number of reacting species (atoms, molecules, or ions) that actually collide in a single elementary step of the reaction mechanism.
It’s a theoretical concept — you read it off from the mechanism, not from an experiment.
| Property | Order | Molecularity |
|---|---|---|
| Determined by | Experiment (rate law) | Mechanism (theory) |
| Can be fraction? | Yes (e.g., 0.5, 1.5) | No — always integer |
| Can be zero? | Yes | No (minimum is 1) |
| Applies to | Overall reaction | Single elementary step only |
| Can be negative? | Yes (inhibition cases) | No |
Order can be 0, fractional, or even negative. Molecularity is always a positive integer — 1, 2, or rarely 3.
For an elementary reaction (a reaction that occurs in a single step), molecularity and order are equal.
For a complex reaction (multiple steps), the order is determined by the rate-determining step, and it need not equal the stoichiometry of the overall equation.
Consider the reaction:
Experimentally, rate , so order = 2.
If this happens via a single bimolecular collision, molecularity is also 2. But if there’s a complex mechanism, we can’t assume this — we’d need the mechanism to comment on molecularity.
Why This Works
Order comes from the rate law, which we build from kinetic experiments — varying concentrations and measuring how rate changes. The balanced equation tells us what reacts and how much, but not how fast or by what pathway.
Molecularity, on the other hand, describes the molecular-level picture of one step in the mechanism. A termolecular reaction (molecularity = 3) is possible but rare because the probability of three molecules colliding simultaneously is very low.
This is why order can be fractional or zero: these arise from mechanisms with multiple steps, pre-equilibria, or intermediate species. A rate expression like rate simply cannot come from a single collision — it’s telling you the mechanism is complex.
Alternative Method — Recognising from the Rate Law
If you’re given the rate law directly:
Order with respect to = 1, order with respect to = 1, overall order = 2.
This is a second-order reaction. If this is also an elementary step in the mechanism, molecularity = 2 (bimolecular).
For NEET/JEE MCQs: if the question says “elementary reaction”, you can directly say order = molecularity. If it just gives a balanced equation without saying “elementary”, never assume order equals stoichiometric coefficients.
Common Mistake
Reading order directly from the balanced equation.
Students see and write order = 2 because of the coefficient. Wrong. The decomposition of HI is experimentally found to be second order, yes — but that’s a coincidence here, not a rule. For the decomposition of , the equation suggests 2 molecules, but the experimental order is 1. Never copy coefficients as order unless explicitly told the reaction is elementary.
Summary: Order is experimental, applies to the full reaction, and can be any real number. Molecularity is theoretical, applies only to elementary steps, and is always a small positive integer. When both terms apply (elementary reactions only), they are equal.