Chemistry · Chemical Kinetics · NEET
| How it is found | Order: experiment (rate law) | Molecularity: theory (balanced single step) |
| Allowed values | Order: 0, fraction, negative, or whole | Molecularity: whole numbers only (1, 2, 3) |
| Applies to | Order: whole reaction (any) | Molecularity: single elementary step only |
| Depends on | Order: measured rate / slow step | Molecularity: number of colliding molecules |
Order is the sum of the powers of concentration in the experimentally measured rate law. Molecularity is the number of reactant molecules that collide together in one elementary (single-step) reaction. Order comes from experiment; molecularity comes from the balanced equation of that single step. This is why the same reaction can have an order that does not match its molecularity.
Order can be zero (rate does not depend on that reactant), a fraction like 0.5 or 1.5, or even negative. Molecularity can NEVER be zero, fractional, or negative. Molecularity is always a whole number: 1, 2, or 3. The reason is that you cannot have half a molecule or zero molecules colliding in a real single step. If you see a fractional value like 1.5, it MUST be an order, not molecularity.
Molecularity is a count of actual particles taking part in one collision, so it is a natural number (1, 2, 3). Order is not a count; it is just a math number that describes how rate changes with concentration. For a multi-step (complex) reaction, the order is decided by the slow step and the equilibria before it, so the powers can turn out fractional or zero. That is why order is not restricted to whole numbers.
They are the same ONLY for a single-step (elementary) reaction. For example, in an elementary reaction A + B -> product, order = 2 and molecularity = 2. But for a complex (multi-step) reaction they are usually different, because order depends on the slow rate-determining step, not on the overall balanced equation. So do not assume they are equal unless the reaction is elementary.
A complex reaction happens in many steps, so there is no single collision to count. Molecularity has meaning only for each individual elementary step, not for the overall reaction. Order, however, can always be found for the overall complex reaction because we measure it from experiment. So for NEET: overall order = yes for any reaction; overall molecularity = only for elementary reactions.
The chance of four or more molecules colliding at the exact same time, with the right energy and direction, is almost zero. So real single steps have molecularity 1, 2, or at most 3. Reactions that look like they need many molecules actually happen in several simple steps. This matters for NEET because a step written with molecularity above 3 is a signal that the reaction is not elementary.
Mechanism of a hypothetical reaction X2 + Y2 -> 2XY is given below: (i) X2 <=> X + X (fast) (ii) X + Y2 <=> XY + Y (slow) (iii) X + Y -> XY (fast). The overall order of the reaction will be:
The decomposition of phosphine PH3 on tungsten at low pressure is a first-order reaction. It is because the:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Order is found by experiment, from the measured rate law. Molecularity is not measured; it is taken from the balanced equation of a single elementary step.
No. Molecularity is a count of colliding molecules, so it is always a whole number (1, 2, or 3). A value like 1.5 can only be an order.
They are equal only for an elementary (single-step) reaction. For complex reactions they usually differ because order depends on the slow step.
A zero order reaction has order 0, meaning rate does not depend on reactant concentration. Molecularity can never be zero, so this again shows the two ideas are different.
No. This is a common NEET trap. Order must come from experiment or from the slow rate-determining step, not from the overall balanced equation.
