NCERT MCQ Solutions for Class 12 Chemistry Chapter 3 Chemical Kinetics

Chemical kinetics as the branch of chemistry dealing with the study of reaction rates and their mechanisms. Thermodynamics deals with feasibility and equilibrium deals with extent.

The rate of reaction as the change in concentration of a reactant or product in unit time.

Since the concentration of reactant R decreases with time, Δ[R] ([R]₂ – [R]₁) is negative.
A negative sign is added to make the rate positive: Rate = -Δ[R] / Δt.

Rate is change in concentration divided by change in time, so the units are concentration/time, typically mol L⁻¹ s⁻¹.

For expressing the rate where stoichiometric coefficients are not one, the rate of change of concentration is divided by the stoichiometric coefficient. Rate = -(1/2) × Δ[HI]/Δt = Δ[H₂]/Δt = Δ[I₂]/Δt.

Rate law (or rate equation/expression) as the representation of reaction rate in terms of concentration of the reactants.

The overall order of a reaction as the sum of the powers (exponents) of the concentration terms in the rate law expression (x + y).

For a zero-order reaction, Rate = k[Reactant]⁰ = k. The rate is constant and independent of the reactant’s concentration.

For a first-order reaction, Rate = k[Concentration]¹.
So, k = Rate / [Concentration] = (mol L⁻¹ s⁻¹) / (mol L⁻¹) = s⁻¹.

Molecularity as the number of reacting species (atoms, ions or molecules) taking part in an elementary reaction, which must collide simultaneously.

Molecularity is applicable only for elementary reactions, cannot be zero or non-integer, while order is experimental and can be zero or fractional.

The overall rate of a complex reaction is controlled by the slowest step, called the rate-determining step.

Integrating the differential rate law -d[R]/dt = k gives [R] = -kt + I. Using the initial condition [R] = [R]₀ at t = 0, we get the integrated form [R] = -kt + [R]₀.

The half-life for a first-order reaction as t₁/₂ = (2.303 × log2) / k = 0.693 / k. It is independent of the initial concentration.

Pseudo first-order reactions (like hydrolysis of ethyl acetate in excess water) as higher order reactions that follow first-order kinetics because the concentration of one reactant is in large excess and remains effectively constant.

The Arrhenius equation, k = A × exp(-Eₐ / RT), explicitly relates the rate constant (k) to temperature (T) and activation energy (Eₐ).

Eₐ in the Arrhenius equation is the activation energy, the minimum energy required for reactants to form the activated complex and proceed to products.

Collision theory states that effective collisions, which lead to product formation, require molecules to collide with sufficient energy (threshold energy) and with the proper orientation.

A catalyst provides an alternate reaction pathway with a lower activation energy, thereby increasing the reaction rate.

the factor P, called the probability or steric factor, to account for the requirement that colliding molecules must have the proper orientation for a collision to be effective.