Q.ii. Distinguish between order and molecularity of a reaction.

Step 1. ORDER: found by experiment, equal to the sum of the powers of the concentration terms in the (measured) rate law; can be zero, a fraction (e.g. 3/2), or an integer.

Step 2. MOLECULARITY: a purely theoretical quantity, defined as the number of reactant molecules that come together and collide in a single elementary reaction step; it is always a whole number (1, 2, rarely 3).

Step 3. For a genuinely elementary (single-step) reaction the two happen to be numerically equal, but for a complex, multi-step reaction the overall order need not match any individual step's molecularity at all -- molecularity is not even defined for the overall complex reaction, only for each of its elementary steps.

Systematic order-vs-molecularity contrast (experimental vs theoretical, fraction/zero allowed vs integer-only, single-step-only vs any reaction).

• Treating order and molecularity as always interchangeable.
• Trying to assign a molecularity to a complex, multi-step overall reaction -- molecularity is meaningful only for one elementary step at a time.