Rate of Chemical Reaction
Chemical reactions are the simplest way to convert one substance into the other substance. The feasibility of a chemical reaction is decided by the concepts of chemical thermodynamics under a defined set of conditions. The extent of a chemical reaction is determined by the principles of chemical equilibrium. But the information that remains undisclosed is the speed or rate of a chemical reaction, path or mechanism of a chemical reaction, time taken for a reaction to attain a state of equilibrium, factors controlling the rate of reaction etc. This information pertaining to the study of a chemical reaction is very pivotal, and it helps in understanding the nature (slow or fast nature) of a chemical reaction. The kinetics of a chemical reaction can be understood with the help of average rate of reaction or a differential rate law or an integrated rate law. There are various factors on which the rate of a chemical reaction depends viz. nature of reactants, temperature, catalyst, pressure and concentration of reactants and products (rate law expression).
Identify the incorrect statement from the following.
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → Order of a reaction cannot be zero or a non integer.
The order of a reaction is the sum of the powers of the concentration terms in the rate law, and that rate law is found by experiment. It is not read from the balanced equation. Molecularity is different: it is the number of species that come together in one elementary step, so it must be a whole number (1, 2 or 3).
Order comes from experiment, not from the balanced equation, so it does not have to be a whole number. It may be:
zero, when the rate does not depend on concentration at all. Examples are the decomposition of ammonia on a hot platinum surface and photochemical reactions.
fractional, as in the thermal decomposition of acetaldehyde, \( \ce{CH3CHO} \) → \( \ce{CH4} \) + CO:
\( \displaystyle \text{Rate}=k[\mathrm{CH_3CHO}]^{3/2}\quad(\text{order }1.5) \)
negative for one species in some complex reactions.
Option (1) Order is applicable to elementary as well as complex reactions. — A correct statement. Order is defined from the rate law, and both elementary and complex reactions have one. Molecularity is the one that applies only to an elementary step.
Option (2) Order of a reaction is an experimental quantity. — A correct statement. It is found by measuring how the rate changes when each concentration is changed, not from the balanced equation.
Option (3) “The overall rate of the reaction is controlled by the slowest step in a reaction.” — A correct statement. In a reaction with several steps, the slowest step is the rate determining step, and the rate law of the whole reaction comes from it.
Option (4) Order of a reaction cannot be zero or a non integer. — An incorrect statement. Order can be zero, fractional or even negative, as shown above.
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.”