Read the passage given below and answer the questions based on it.
The rate of a reaction is concerned with decrease in concentration of reactants or increase in concentration of products per unit time. It can be expressed as instantaneous rate at a particular instant of time and average rate over a large interval of time. A number of factors such as temperature, concentration of reactants and catalyst affect the rate of a reaction. Mathematical representation of rate of a reaction is given by rate law. It has to be determined experimentally and cannot be predicted. Order of a reaction with respect to a reactant is the power of its concentration which appears in the rate law equation. Molecularity is defined only for an elementary reaction. Molecularity and order of an elementary reaction are the same.
Which of the following statements is incorrect?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Rate law can be written with the help of balanced chemical equation
The rate law of a reaction must be determined experimentally. The powers of the concentrations in it need not be the stoichiometric coefficients of the balanced equation, because most reactions take place in several steps. For example, for 2NO + \( \ce{O2} \) → \( \ce{2NO2} \) the rate is k[NO]2[O2], but for \( \ce{CHCl3} \) + \( \ce{Cl2} \) → \( \ce{CCl4} \) + HCl it is k[CHCl3][Cl2]1/2.
Option (1) Order and molecularity for an elementary reaction are always same — A correct statement. An elementary step’s rate law follows from its own molecularity.
Option (2) Rate of reaction can be expressed as increase in concentration of products formed in unit time — A correct statement. Rate can be expressed as the decrease in concentration of a reactant or the increase in concentration of a product per unit time.
Option (3) Rate law can be written with the help of balanced chemical equation — The incorrect statement. The rate law must be determined experimentally; its exponents need not match the coefficients.
Option (4) Rate of reaction depends upon temperature of the reaction — A correct statement. For many reactions the rate constant nearly doubles for a 10° rise in temperature.
NCERT (2026-27 edition):
Rate Expression and Rate Constant – “… Some other examples are given below:
\( \ce{CHCl3 + Cl2 -> CCl4 + HCl} \qquad \text{Rate} = k[\ce{CHCl3}][\ce{Cl2}]^{1/2} \)
\( \ce{CH3COOC2H5 + H2O -> CH3COOH + C2H5OH} \qquad \text{Rate} = k[\ce{CH3COOC2H5}]^1[\ce{H2O}]^0 \)
In these reactions, the exponents of the concentration terms are not the same as their stoichiometric coefficients. Thus, we can say that: Rate law for any reaction cannot be predicted by merely looking at the balanced chemical equation, i.e., theoretically but must be determined experimentally.”