Read the passage carefully and answer the questions based on the passage:
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. In the rate equation Rate = \( k [A]^x [B]^y \)
\( x \) and \( y \) indicate how sensitive the rate is to the change in concentration of A and B, respectively. Sum of these exponents, i.e., \( x + y \) gives the overall order of a reaction where \( x \) and \( y \) represent the order with respect to the reactants A and B, respectively. Hence, the sum of powers of the concentration of the reactants in the rate law expression is called the order of that chemical reaction. For a first order reaction, the concentration of the reactant varies as \( [R] = [R]_0e^{-kt} \)
Which of the following statements is not true for order of a reaction?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Order of a reaction cannot be fraction.
The order of a reaction is the sum of the powers of the concentration terms in the experimentally found rate law. For Rate \( = k[\mathrm{A}]^{x}[\mathrm{B}]^{y} \), the order is \( x + y \). The powers are found by experiment, not read off the balanced equation, so they can be zero, a whole number or a fraction. Molecularity, the number of species colliding in an elementary step, is always a whole number.
For example, the decomposition of acetaldehyde follows
\( \displaystyle \ce{CH3CHO -> CH4 + CO} \qquad \text{Rate} = k[\ce{CH3CHO}]^{3/2} \)
so its order is 3/2.
Option (1) A balanced chemical equation does not give the correct order of reaction every time. — True for the order of a reaction. The rate law depends on the mechanism; for \( \ce{2NO + O2 -> 2NO2} \) the coefficients happen to match the order, but for many reactions (such as the one above) they do not.
Option (2) Order of a reaction cannot be fraction. — Not true for the order of a reaction. The order can be a fraction, as 3/2 for acetaldehyde.
Option (3) Order of a reaction can be determined experimentally only. — True for the order of a reaction. The order is an experimental quantity, found by measuring how the rate changes with concentration.
Option (4) Order of a reaction can be zero also. — True for the order of a reaction. The decomposition of ammonia on a hot platinum surface at high pressure has Rate = \( k[\ce{NH3}]^{0} = k \).
NCERT (2026-27 edition):
Molecularity of a Reaction – …
