CBSE Class 12 Chemistry Chemical Kinetics Notes Set D

CHEMICAL KINETICS

CONCEPT

Thermodynamics helps us to predict the feasibility of chemical reaction by using ΔG as parameter but it cannot tell everything about the rate of reaction. Rate of chemical reaction is studied in another branch of chemistry called Chemical Kinetics.

Chemical kinetics- The branch of physical chemistry which deals with the study of rate of reaction and their mechanism is called chemical kinetics.

Rate of chemical reaction- The change in concentration of any reactant or product per unit time is called rate of reaction.

TYPES OF RATE OF REACTION-

1. Average rate of reaction- The rate of reaction measured over the long time interval is called average rate of reaction.

Avg rate Δx/Δt = -Δ[R]/Δt = +Δ[p]/Δt

2. Instantaneous rate of reaction- The rate of reaction measured at a particular time is called instantaneous rate of reaction.

Instantaneous rate dx/dt= -d[R]/dt=+d[P]/dt

FACTORS AFFECTING RATE OF REACTION-

1. Concentration of reactant

2. Surface area

3. Temperature

4. Nature of reactant

5. Presence of catalyst

6. Radiation

RATE CONSTANT (k)- It is equal to the rate of reaction when molecular concentration of reactant is at unity.

RATE LAW- The rate of reaction is directly proportional to the product of concentration of reactant and each concentration is raised to some power which may or may not be equal to stereochemistry experimentally.

For a reaction

aA+bB → cC+dD

Rate law = k[A]p[B]q                             Where powers P and Q are determined experimentally

MOLECULARITY – The total no. of reactants taking part in elementary chemical reaction is called molecularity.

ORDER OF REACTION- The sum of powers to which the concentrations terms are raised in a rate law expression is called order of reactions. For above case order = P+Q: orders of rn is determined experimentally

HALF-LIFE PERIOD- The time during which the concentration of the reactant is reduced to half of its initial concentration is called half-life period.

ACTIVATION ENERGY- The minimum extra amount of energy absorbed by reactant molecules so that their energy becomes equal to the threshold energy is called activation energy.

Activation energy = Threshold energy – kinetic energy

TEMPERATURE COEFFICIENT- The ratio of rate constant at two temperatures having difference of 100C is called temperature coefficient.

Temperature coefficient = Rate constant at T+100C/Rate constant at T0C

Arhenius Equation-

K= Ae-Ea/RT

K-rate constant

A-Arrhenius energy

Ea-Activation energy

R- Rate constant

T-Temperature

Log K = Log A- Eq / 2.303RT

Energy of activation can be evaluated as

1. Log [K2/K1] = Ea(1/T1-1/T2)/ 2.303RT

Log [K2/K1] = Ea(1/T1-1/T2)/ 19.15

3. Give one example of pseudo first order reaction.

Ans. Hydrolysis of an ester 

 Please click the link below to download pdf file for CBSE Class 12 Chemistry notes and questions for Chemical Kinetics Part B.