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

The concentration of reactants

The correct answer is Option (4) → The concentration of reactants

The rate law is a mathematical expression that shows how the rate of a chemical reaction depends on the concentrations of the reactants (and sometimes products or catalysts).

General form: Rate = k [A]ᵐ [B]ⁿ …

It is determined experimentally and directly relates the reaction rate to the concentration of reactants raised to some power (order of reaction).

Why the other options are incorrect:

• Temperature → Affects the value of rate constant (k) via Arrhenius equation, but rate law itself does not include temperature.
• Activation energy → Influences the magnitude of k, but is not part of the rate law expression.
• Reaction mechanism → The rate law reflects the mechanism (especially the slowest step), but the rate law itself does not relate the rate to the mechanism directly — it relates rate to concentrations.

Hence, the most accurate and direct answer from the given options is: The concentration of reactants