For a chemical reaction
(A) The order of a reaction is the sum of powers of the concentration of the reactants.
(B) The order of a reaction may be zero, an integer or a fractional value.
(C) Molecularity of an elementary reaction is equal to the number of reacting species.
(D) The molecularity of a reaction may be zero or a fractional value.
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → (A), (B) and (C) only
The order of a reaction is found by experiment from the rate law, while the molecularity is the number of reacting species (atoms, ions or molecules) that collide simultaneously in an elementary reaction. Order can take any value; molecularity is a count of particles.
(A) The order of a reaction is the sum of powers of the concentration of the reactants. — Correct. For Rate \( = k[\mathrm{A}]^{x}[\mathrm{B}]^{y} \), the order is \( x + y \), the sum of the powers of the concentration terms in the rate law.
(B) The order of a reaction may be zero, an integer or a fractional value. — Correct. Since it is found by experiment, the order can be 0, 1, 2, 3 or a fraction, e.g. \( \frac{3}{2} \) for \( \ce{CHCl3 + Cl2 -> CCl4 + HCl} \), Rate \( = k[\ce{CHCl3}][\ce{Cl2}]^{1/2} \).
(C) Molecularity of an elementary reaction is equal to the number of reacting species. — Correct. For example, \( \ce{2HI -> H2 + I2} \) is bimolecular, since two HI molecules collide.
(D) The molecularity of a reaction may be zero or a fractional value. — Incorrect. Molecularity counts colliding particles, so it is always a whole number (1, 2 or, rarely, 3); it cannot be zero or a fraction.
NCERT (2026-27 edition):
Molecularity of a Reaction – “Another property of a reaction called molecularity helps in understanding its mechanism. The number of reacting species (atoms, ions or molecules) taking part in an elementary reaction, which must collide simultaneously in order to bring about a chemical reaction is called molecularity of a reaction.”
Molecularity of a Reaction – “… (i) Order of a reaction is an experimental quantity. It can be zero and even a fraction but molecularity cannot be zero or a non integer.”