CHEMICAL KINETICS (Class XII CBSE)

Kinetics deals with the rate of chemical reactions, factors affecting it, and the reaction mechanism.

1. Rate of Reaction

Change in concentration per unit time.

Rate = -(1/a)d[A]/dt = (1/c)d[C]/dt

Units: mol L⁻¹ s⁻¹

Factors affecting:

• Nature & Conc. of reactants

• Temperature & Catalyst

• Surface Area

2. Order vs Molecularity

Rate Law: Rate = k[A]ˣ[B]ʸ

Order = x + y

Zero Order Reaction

• Integrated Eq: [A] = [A]₀ - kt

• Half-life (t₁/₂): [A]₀ / 2k

• Units of k: mol L⁻¹ s⁻¹

First Order Reaction

• Integrated Eq: k = (2.303/t) log([A]₀/[A])

• Half-life (t₁/₂): 0.693 / k (Constant!)

• Units of k: s⁻¹

3. Temperature Dependence

Arrhenius Equation: k = Ae^(-Ea/RT)

• Ea (Activation Energy): Minimum energy to initiate reaction.

• Catalyst: Lowers Ea by providing an alternative pathway.

• Collision Theory: Effective collisions + Proper orientation + Threshold energy.

★ Key Examination Points:

1. Half-life of 1st order is independent of initial concentration.

2. Pseudo 1st order occurs when one reactant is in large excess.

3. Units of 'k' are the best way to identify reaction order.