Chemical kinetics class 12 notes

Chemical kinetics class 12 notes Most important notes for Revision

  • Chemical kinetics: It is the branch of chemistry which deals with the study of reaction rates and their mechanisms.
  • Rate of a reaction: The rate of a reaction can be defined as the change in concentration of a reactant or a product in unit time. For the reaction,

chemical kinetics class 12 notes

  • Units of rate: Concentration time–1 i.e., molL–1s–1 or atms–1 for gaseous reactions.
  • Average rate of reaction: It is the average value during large time interval.

chemical kinetics class 12 notes
  • Instantaneous rate of reaction: It is the rate of a reaction at a particular instant of time i.e., when ∆t approaches zero.
Chemical kinetics class 12 notes

  • Factors influencing rate of a reaction: Chemical kinetics class 12 notes
  • Concentration: Greater the concentrations of the reactants, faster is the rate of reaction.
  • Physical state of reactants: Reactions involving gaseous reactants are faster than reactions containing solid and liquid reactants.
  • Temperature: The rate of reaction increases with increase of temperature. For most of the reactions, rate of reaction becomes almost double with 10°C rise of temperature.
  • Presence of catalyst: A catalyst generally increases the speed of a reaction.
  • Surface are of reactants: For a reaction involving a solid reactant or catalyst, the greater is the surface are, the faster is the reaction.
  • Presence of light: Photochemical reactions take place in the presence of light only.
  • Activation energy: Lower the activation energy faster in the reaction.
  • Rate law and rate constant: The equation that correlates the rate of reaction with concentration of reactants is known as rate law.
  • For a simple reaction,  A+B→C+D Rate = k[A][B] where k is the rate constant which is equal to the rate of reaction when concentration of each of the reactant is unity.
  • Order and molecularity:

Chemical kinetics class 12 notes table

  • Half-life of reaction: The time in which the concentration of a reactant is reduced to one half of its initial concentration is called half-life of the reaction.
Rate law, integrated rate law, half-life, units of rate constant and graph for the reactions of different orders: Chemical kinetics class 12 notes
  • Pseudo first order reactions: Those reactions which are not truly of the first order but under certain conditions become reactions of the first order are called pseudo first order reactions. e.g.
  • Acid hydrolysis of ethyl acetate:
chemical kinetics class 12 notes
  • Acid catalysed inversion of cane sugar:
chemical kinetics class 12 notes
  • Effect of temperature on rate of reaction:
  • For a chemical reaction with rise in temperature by 10° C, the rate constant is nearly doubled.
  • Arrhenius equation:

    Chemical kinetics class 12 notes

chemical kinetics class 12 notes
chemical kinetics class 12 notes
where, k = Rate constant, A = Pre-exponential factor (frequency factor),

Ea Activation energy, T = Temperature,

  • Activation energy: The minimum amount of energy required by reactant molecules to participate in a reaction is called activation energy (Ea).
  • Activation energy =  Threshold energy – Average kinetic energy of reacting molecules Activation energy ( Ea ) = E(activated complex) –E(ground state) ∆H=Activation energy of forward reaction – Activation energy of backward reaction

Collision theory of chemical reactions:

  • Reaction occurs due to collision of molecules.
  • All collisions are not effective.
  • Effective collisions are those collisions in which molecules collide with sufficient kinetic energy (called threshold energy which is equal to activation energy + energy possessed by reacting species) and proper orientation.
  • Rate = PZABe–Ea/RT where, P = probability or steric factor which takes into account the  fact that in a collision, molecules must be properly oriented and ZAB = collision frequency (i.e., the no. of collisions per second per unit volume of the reaction mixture) of reactants, A and B.

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