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Overview of Chemical Kinetics

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Estimated time: 15 minutes
Maharashtra State Board: Class 12

Definition: Chemical kinetics

The branch of chemistry which deals with the rate of chemical reactions and the factors affecting them is called chemical kinetics.

Maharashtra State Board: Class 12

Definition: Rate of reaction

The rate which describes how rapidly reactants are consumed or products are formed is called rate of reaction.

Maharashtra State Board: Class 12

Definition: Average rate of reaction

The change in concentration of reactant or product divided by time interval is called average rate of reaction.

Maharashtra State Board: Class 12

Definition: Instantaneous rate of reaction

The rate of reaction at a particular instant of time is called instantaneous rate of reaction.

Maharashtra State Board: Class 12

Definition: Order of reaction

The sum of powers of concentration terms in the rate law is called order of reaction.

Maharashtra State Board: Class 12

Definition: Zero order reaction

The reaction whose rate is independent of concentration of reactant is called zero order reaction.

Maharashtra State Board: Class 12

Definition: Pseudo-first order reaction

The reaction which is expected to be of higher order but follows first order kinetics due to large excess of one reactant is called pseudo-first order reaction.

Maharashtra State Board: Class 12

Key Points: Molecularity of Elementary Reactions

  1. Elementary reaction is a reaction that occurs in a single step and cannot be further divided into simpler reactions.
    Example:
    O₃(g) → O₂(g) + O(g)
  2. Molecularity is the number of reactant molecules taking part in an elementary reaction. It is always a whole number (1, 2, 3) and never zero or fractional.
  3. If one molecule is involved, the reaction is unimolecular (molecularity = 1).
    Example:
    C₂H₅I(g) → C₂H₄(g) + HI(g)
  4. If two molecules are involved, the reaction is bimolecular (molecularity = 2).
    Example:
    2NO₂(g) → 2NO(g) + O₂(g)
  5. Order and molecularity of elementary reactions:
    For an elementary reaction, the order of reaction is equal to its molecularity.
    Example:
    2NO₂(g) → 2NO(g) + O₂(g)
    Rate = k[NO₂]²
    Order = 2 = Molecularity
  6. In complex reactions, the reaction occurs in several elementary steps. The slowest step is called the rate determining step, and it decides the rate of the overall reaction.
    Example:
    NO₂Cl(g) → NO₂(g) + Cl(g) (slow)
    Rate = k[NO₂Cl]
 
Maharashtra State Board: Class 12

Theory: Collision Theory of Bimolecular Reactions

  1. Chemical reactions occur due to collisions between reactant molecules.
  2. Only those collisions are effective in which molecules possess minimum energy called activation energy (Ea).
  3. Molecules must have proper orientation during collision for reaction to occur.
  4. During collision, a temporary unstable species called activated complex is formed:
    A + B–C → A···B···C → A–B + C
  5. The energy barrier that must be overcome is called activation energy (Ea), and the fraction of molecules having sufficient energy is: f = e−Ea/RT
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