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CBSE Class 11th Chemistry Notes for Chapter 7 Equilibrium

                       UNIT 7 EQUILIBRIUM
Equilibrium is a state where the rate of evaporation is equal to the rate of condensation.
The mixture of reactants and products in the equilibrium state is called an equilibrium mixture. When there is no change in the concentration s of either of the reactants or the products, the stage is called dynamic equilibrium. At dynamic equilibrium, the rates of forward and backward reaction become equal.
For physical processes, the following characteristics are common to the system at equilibrium:

  • Equilibrium is possible only in a closed system at a given temperature.
  • Both the opposing processes occur at the same rate and there is a dynamic but stable condition.
  • All measurable properties of the system remain constant.
  • When equilibrium is attained for a physical process, it is characterised by a constant value of one of its parameters at a given temperature.
  • The magnitude of such quantities at any stage indicates the extent to which the reaction has proceeded before reaching equilibrium.

At a given temperature, the product of concentrations of the reaction products raised to the respective stoichiometric coefficient in the balanced chemical equation divided by the product of concentrations of the reactants raised to their individual stoichiometric coefficients has a constant value. This is known as the Law of chemical equilibrium or Equilibrium law.
The important features of equilibrium constants are:

  • Equilibrium constant is applicable only when concentrations of the reactants and products have attained their equilibrium state.
  • The value of equilibrium constant is independent of initial concentrations of the reactants and products.
  • Equilibrium constant is temperature dependent having one unique value for a particular reaction represented by a balanced equation at a given temperature.
  • The equilibrium constant for the reverse reaction is equal to the inverse of the equilibrium constant for the forward reaction.
  • The equilibrium constant K for a reaction is related to the equilibrium constant of the corresponding reaction, whose equation is obtained by multiplying or dividing the equation for the original reaction by a small integer.

The following are the applications of equilibrium constant:

  • To predict the extent of a reaction on the basis of its magnitude.
  • To predict the direction of the reaction.
  • To calculate equilibrium concentration.


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