Class 12 Chemistry Chemical Kinetics Notes

3.1 Rate of a Chemical Reaction

Chemical kinetics deals with the speed (rate) of chemical reactions and the factors affecting it.

Rate of Reaction

Rate is the change in concentration of reactant or product per unit time. $\text{Rate} = \frac{\Delta[\text{Concentration}]}{\Delta t}$ Rate=ΔtΔ[Concentration]

Units of Rate

mol L⁻¹ s⁻¹

Average Rate

Calculated over a time interval.

Instantaneous Rate

Rate at a particular moment.

3.2 Factors Influencing Rate of a Reaction

Nature of reactants
Ionic reactions are faster than covalent reactions.
Concentration of reactants
Higher concentration → higher rate.
Temperature
Increase in temperature increases rate.
Catalyst
Increases rate without being consumed.
Surface area
Finely divided solids react faster.
Pressure (for gases)
Higher pressure increases rate.

3.3 Integrated Rate Equations

These equations relate concentration and time.

Zero Order Reaction

$[\text{R}] = [\text{R}_0] – kt$ [R]=[R0]−kt

Rate independent of concentration
Unit of k: mol L⁻¹ s⁻¹

First Order Reaction

$\ln \frac{[\text{R}_0]}{[\text{R}]} = kt$ ln[R][R0]=kt

Rate ∝ concentration
Unit of k: s⁻¹
Half-life:

$t_{1/2} = \frac{0.693}{k}$ t1/2=k0.693

Second Order Reaction

$\frac{1}{[\text{R}]} = kt + \frac{1}{[\text{R}_0]}$ [R]1=kt+[R0]1

Unit of k: L mol⁻¹ s⁻¹

3.4 Temperature Dependence of Rate of Reaction

Arrhenius Equation

$k = A e^{-E_a/RT}$ k=Ae−Ea/RT

Where:

$k$ k = rate constant
$A$ A = frequency factor
$E_a$ Ea = activation energy
$R$ R = gas constant
$T$ T = temperature

Effect of Temperature

Rate approximately doubles for every 10°C rise.

3.5 Collision Theory of Chemical Reactions

Basic Idea

Reaction occurs when reactant molecules:

Collide with each other
Have sufficient energy (≥ activation energy)
Have proper orientation

Effective Collisions

Only collisions that lead to product formation.

Role of Catalyst

Lowers activation energy
Increases number of effective collisions