The correct statements regarding order and molecularity of the reaction are:
(A) The order of a reaction can be zero or an integer.
(B) The molecularity of a reaction can be zero or an integer.
(C) The molecularity cannot be zero or fraction.
(D) The order of a reaction is decided by the slowest step of the complex reaction.
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → (A), (C) and (D) only
The order of a reaction is the sum of the powers of the concentration terms in the experimental rate law. It is found by experiment and can be zero, a whole number or a fraction. The molecularity is the number of reacting species that collide at the same time in an elementary step; it is a count of molecules, so it can only be 1, 2 or 3, never zero or a fraction.
(A) The order of a reaction can be zero or an integer. — Correct. Zero order reactions (such as the decomposition of ammonia on hot platinum) and first or second order reactions are common; the order can also be fractional.
(B) The molecularity of a reaction can be zero or an integer. — Incorrect. A reaction step needs at least one molecule, so molecularity cannot be zero.
(C) The molecularity cannot be zero or fraction. — Correct. Molecules are counted in whole numbers, and at least one must take part.
(D) The order of a reaction is decided by the slowest step of the complex reaction. — Correct. A complex reaction goes through several steps, and the rate is controlled by the slowest one, the rate determining step; its rate law gives the order. For example, \( \ce{2H2O2 -> 2H2O + O2} \) catalysed by iodide is first order in \( \ce{H2O2} \) because the slow step is \( \ce{H2O2 + I^- -> H2O + IO^-} \).
Shortcut: (B) is false, since molecularity cannot be zero, and only Option (4) leaves out (B).
NCERT (2026-27 edition):
Molecularity of a Reaction – …
Molecularity of a Reaction – “… (iii) For complex reaction, order is given by the slowest step and molecularity of the slowest step is same as the order of the overall reaction.”