Order and Molecularity: The Two Faces of a Reaction
Imagine you are watching a chemical reaction happen. Two molecules of A collide, rearrange, and become B. That collision — the actual event where bonds break and form — is an elementary step. The number of molecules that must come together in that single step is its molecularity.
Now imagine a different reaction. You mix A and B, but the product appears at a rate that depends on the square of A and not at all on B. That dependence — how the speed of the overall reaction changes when you change concentrations — is its order.
These two ideas answer different questions. Molecularity asks: How many particles actually meet in one step? Order asks: What is the mathematical relationship between concentration and rate for the overall reaction?
Molecularity — The Microscope View
Molecularity is a property of an elementary reaction only. It is the number of reactant particles (atoms, ions, molecules) that collide in that step.
- Unimolecular (1): A single molecule decomposes or rearranges. Example: N2O5→NO2+NO3
- Bimolecular (2): Two particles collide. Example: NO2+CO→NO+CO2
- Termolecular (3): Three particles collide simultaneously — very rare because three-way collisions are improbable. Example: 2NO+O2→2NO2
Molecularity is never zero, never fractional, and never greater than 3. You cannot have a molecularity of 1.5 or 4 — collisions of four particles at once are astronomically unlikely.
Order — The Macroscope View
Order is an experimental quantity. You run the reaction, measure the initial rate at different concentrations, and deduce the rate law:
Rate=k[A]m[B]n
The order with respect to A is m, the order with respect to B is n, and the overall order is m+n.
Order can be:
- Integer (0, 1, 2, 3)
- Fractional (e.g., 0.5 for a reaction like H2+Br2→2HBr)
- Negative (if increasing a reactant slows the reaction)
- Zero (if concentration does not affect rate)
Order is determined by experiment, not by the balanced chemical equation. A reaction written as 2A+B→C does not automatically have order 3. The actual rate law could be Rate=k[A]2 (order 2) or Rate=k[A] (order 1) — only the lab can tell.
The Critical Difference
| Property | Molecularity | Order |
|---|
| Applies to | Elementary steps only | Overall reaction (or any step) |
| Determined by | Stoichiometry of the step | Experiment |
| Can be fractional? | No | Yes |
| Can be zero? | No | Yes |
| Can be negative? | No | Yes |
| Maximum value | 3 (rarely) | Any number |
Why the Confusion?
Many students think: "If the balanced equation says 2A + B, then molecularity is 3 and order is 3." This is wrong for two reasons.
First, most reactions are multi-step. The balanced equation shows only the net change, not the actual collision events. A reaction like 2NO+O2→2NO2 appears termolecular, but it actually happens in two bimolecular steps:
NO+NO→N2O2(fast)
N2O2+O2→2NO2(slow)
The molecularity of each step is 2 (bimolecular). The overall order, determined by the slow step, is 2+1=3 — but that is a coincidence, not a rule. …