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Which of the following best describes the difference between ORDER and MOLECULARITY of a reaction?
AOrder is theoretical; molecularity is experimental
BOrder and molecularity always have the same value
CMolecularity can be fractional; order must be an integer
DOrder is experimental; molecularity is a property of elementary steps
Answer & Solution
Correct answer: D. Order is experimental; molecularity is a property of elementary steps
1. NCERT §3.2.4 distinguishes the two concepts.
2. ORDER: experimental, deduced from the rate law $\text{rate} = k[A]^a[B]^b$. The total order is $a + b$, which CAN be zero, fractional, or any real number.
3. MOLECULARITY: theoretical, defined ONLY for elementary (single-step) reactions. It is the number of reactant species colliding in that elementary step. Therefore molecularity is ALWAYS A POSITIVE INTEGER (1, 2, occasionally 3).
4. For an elementary reaction, order = molecularity. For a multi-step reaction, the OVERALL order need not equal the sum of molecularities of all steps — usually the slowest step (rate-determining) dictates the order.
5. Option A inverts the two. Option C is wrong (they can differ). Option D inverts the integer rule (it's molecularity that must be integer, not order).
_Source: NCERT Class 12 Chemistry Part 1, Ch 3, §3.2.4 (Molecularity of a Reaction), p. 9._
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