This distinction is among the most frequently tested concepts in NEET Chemical Kinetics.
Complete Comparison Table
| Feature | Order | Molecularity |
|---|---|---|
| Definition | Sum of exponents in rate law | Number of molecules in elementary step |
| Determined by | Experiment | Theoretical (mechanism) |
| Values allowed | 0, fraction, integer (any) | 1, 2, or 3 (only) |
| Applies to | Overall and elementary reactions | Elementary steps ONLY |
| Can be zero? | Yes (zero-order reactions) | No |
| Can be fractional? | Yes (e.g., 3/2, 1/2) | No |
| Can change? | Yes (with conditions, temperature) | No (fixed for a given elementary step) |
| Example | + → 2HBr: order = 3/2 | → 2NO + : molecularity = 2 |
Mnemonic
OEM: Order Experimental, Molecularity Mechanical
- ORDER = EXPERIMENTAL (can be 0, fractional, changes with conditions)
- MOLECULARITY = MECHANISTIC (always positive integer, fixed for a given elementary step)
When Do They Match?
For elementary reactions only: order = molecularity. For example:
- Unimolecular: A → B; rate = k[A]; order = 1 = molecularity
- Bimolecular: A + B → C; rate = k[A][B]; order = 2 = molecularity
For complex (non-elementary) reactions: order ≠ molecularity (in general).
Rate-Determining Step
In a multi-step mechanism, the slowest step (rate-determining step, RDS) controls the overall rate. The rate law of the overall reaction is determined by the RDS.