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NEET Chemistry 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.Error converting from MathML to accessible text.
  • Half-life: The time taken for a reaction when half of the starting material has reacted is called half-life of the reaction.
    Error converting from MathML to accessible text.

Second order reaction:

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

    begin mathsize 12px style dx over dt equals straight k left square bracket straight A right square bracket to the power of 1 left square bracket straight B right square bracket to the power of 1 end style

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.
    begin mathsize 12px style CH subscript 3 COOC subscript 2 straight H subscript 5 rightwards harpoon over leftwards harpoon with straight H to the power of plus on top CH subscript 3 COOH plus straight C subscript 2 straight H subscript 5 OH end style

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.

    begin mathsize 12px style dx over dt equals straight k left square bracket straight A right square bracket to the power of straight x left square bracket straight B right square bracket to the power of straight y end style            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

  • 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.
    begin mathsize 12px style dN over dt αN end style

  • 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.

  • Radioactive Series:

     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 Equilibrium:
    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.