Read the passage carefully and answer the questions.
The chemical reactions proceed at different rates which depend on various factors like concentration, temperature, etc. Rate law is an expression which relates the rate of reaction with concentration of various reacting species in the reaction. For a first order reaction, the concentration of the reactant after time \( t \) is related with its initial concentration by the relation, \( C = C_0e^{-kt} \). The rate constants for different orders have different expressions. For a first order reaction, \( k =\frac{2.303}{t}\log\frac{a}{a-x} \), where \( a \) is the initial concentration and \( x \) is the extent of reaction.
The Rate law relates the rate of a chemical reaction to
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → The concentration of reactants
A rate law (rate expression) gives the rate of a reaction in terms of the molar concentrations of the reactants, each raised to some power. For a general reaction aA + bB → products:
\( \displaystyle \text{Rate} = k\,[\mathrm{A}]^{x}[\mathrm{B}]^{y} \)
The rate law is found by experiment. For example, for \( \ce{2NO(g) + O2(g) -> 2NO2(g)} \) experiment gives:
\( \displaystyle \text{Rate} = k\,[\ce{NO}]^{2}[\ce{O2}] \)
So the rate law relates the rate to the concentration of reactants.
Why the other options are incorrect
Option (1) The temperature — Incorrect. Temperature changes the rate through the rate constant; this is described by the Arrhenius equation, \( k = A\,e^{-E_a/RT} \), not by the rate law.
Option (2) The activation energy — Incorrect. The activation energy also appears in the Arrhenius equation, which relates k to temperature; the rate law contains only concentrations and k.
Option (3) The reaction mechanism — Incorrect. A rate law can give clues about the mechanism (the slowest step decides it), but it does not express the rate in terms of the mechanism; it expresses it in terms of concentrations.
NCERT (2026-27 edition):
Rate Expression and Rate Constant – “… This form of equation (3.4 b) is known as differential rate equation, where k is a proportionality constant called rate constant. The equation like (3.4), which relates the rate of a reaction to concentration of reactants is called rate law or rate expression. Thus, rate law is the expression in which reaction rate is given in terms of molar concentration of reactants with each term raised to some power, which may or may not be same as the stoichiometric coefficient of the reacting species in a balanced chemical equation.”