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# Chemical Kinetics

## Chemical Kinetics PDF Notes, Important Questions and Synopsis

SYNOPSIS

Rate of chemical reaction:

• The rate of chemical reaction is the change in concentration over the change in time.

Types of rates of chemical reaction:

• Average rate: The rate of reaction measured over a long time interval is called the average rate of reaction.
• Instantaneous rate: It is the rate of reaction when the average rate is taken over a very small interval of time.
Rate law:
• Rate = K (conc.)order − differential rate equation or rate expression,
where K = rate constant = specific reaction rate = rate of reaction when concentration is unity
• unit of K = (conc)1- order time-1
Molecularity:
• Total number of atoms, ions or molecules of the reactants involved in the reaction is termed its molecularity.
Order of reaction:
• m1A + m2B→products
• R ∝ [A]P [B]q, where p may or may not be equal to m1 and the similarly q may or may not be equal to m2.
• p is the order of reaction with respect to reactant A, q is the order of reaction with respect to reactant B and (p + q) is the overall order of the reaction.
Zero order reaction:
• The rate of reaction does not change with the concentration of the reactants.
• Rate = k [conc.]° = constant

First order reaction:
• The reaction in which the rate of reaction is directly proportional to the concentration of a reacting substance.
• Half-life: The time taken for a reaction when half of the starting material has reacted is called half-life of the reaction.

Second order reaction:

• The reaction in which the sum of powers of concentration terms in rate law or rate equation is two.

Pseudo first order reaction:

• The reaction which is bimolecular, but the order is one is called a pseudo first order reaction; for example, acidic hydrolysis of ester.

Third order reaction:

• The reaction in which the sum of the powers of concentration terms in rate law or rate equation is equal to three.

where x + y = 3

Specific rate constant (k):

• It is equal to the rate of reaction when the molar concentration of the reactant is unity.

Activation energy:

• The minimum amount of energy that is required to activate atoms or molecules to start a reaction.

Initial rate:

• The rate at the beginning of the reaction when the concentrations have not changed appreciably.
Factors affecting the rate of reaction:
• Greater the surface area, more will be the rate of reaction.
• Rate of reaction increases with the increase in concentration in general except in zero order reaction.
• When we increase the temperature, the number of molecules possessing activation energy increases because average kinetic energy of molecules increases. So, increasing the temperature increases the reaction rates.

• A catalyst increases the rate of reaction by lowering the activation energy.

Collision Theory:

• It is based on the kinetic theory of gases. A chemical reaction takes place as a result of reacting collisions.
• Collision frequency (Z): The number of collisions which takes place per second per volume of the reaction mixture is called collision frequency.
• Effective collision: Collisions which lead to the formation of product molecules are called effective collisions.
• The rate of reaction depends on the number of effective collisions.

• Radioactivity is the spontaneous emission of penetrating rays in the form of particles or high-energy photons resulting from a nuclear reaction.
• These penetrating rays are classified in three categories—alpha, beta, gamma.
• Law of Radioactive Decay (Rutherford and Soddy Law):
According to this law, the activity of radioactive nuclei is directly proportional to the number of radioactive nuclei present at any instant.

• Law of Radioactive Displacement Law (Group Displacement Law):
On the emission of an alpha particle, the new element lies two columns left in the periodic table and the mass number decreases by 4 points.
On the emission of a beta particle, the new element lies one column right in the periodic table and the mass number remains the same.

 Series Parent element End element 4n series Th-232 Pb-208 4n+1     series Pu-241 Bi-209 4n+2 series U-238 Pb-206 4n+3 series U-235 Pb-207
Radioactive change is an irreversible process, but it shows equilibrium when a daughter element disintegrates at the same rate at which it is formed from parent element.

• Nuclear Fission:
The phenomenon of splitting up of a heavy nucleus on bombardment with slow speed neutrons is known as nuclear fission.

• Nuclear Fusion:
The phenomenon of joining of two lighter nuclei into a heavier nucleus is called nuclear fusion.

• Dating:
Radioactive dating means determining the age of a mineral specimen by determining the relative amounts present in certain radioactive elements.

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