Law of mass action/Law of chemical equilibrium

Principle of Chemical equilibrium:

  • This theory was proposed by Goldberg and Waage in 1884.
  • According to this law ,rate of reaction at a given temperature is directly proportional to the product of active masses of the reactants, raised to the power equal to stoichiometric co- efficient of the balanced chemical equation at a particular instant of reaction.
  • It explains and predicts the behaviour of solution in dynamic equilibrium.
  • Specifically it implies that for a chemical reaction reaction mixture in equilibrium, the ratio between concentration of reactants and products is constant.

Derivation of equilibrium constant in terms of concentration: 

        Consider a hypothetical reversible reaction i.e.
               aA(g) + bB(g) ⇔ cC(g) + dD(g)
Applying law of mass action here, 
  • Rate of forward reaction would be k[A]a[B]b
  • Rate of backward reaction would be k[C]c[D]d 

       [A], [B], [C] and [D] being the active masses  
        kf and kb are rate constants of forward and backward reactions,  
        a, b, c, d are the stoichiometric coefficients related to A, B, C and D respectively. However, at the equilibrium ,
         Rate of forward reaction = Rate of backward

         Equilibrium Constant

         Equilibrium Constant
         Equilibrium Constant
          Equilibrium Constantor,

         Keq  = Kf / K

Where K is known as equilibrium constant(if concentration is given).
Session Quiz:
Q. For the reaction  Br2(g)  + BF2(g) → 2BrF2(g) the equilibrium constant at 2000K and 0.1 bar is 5.25.When the pressure is increased by 8 fold, what is the change in equilibrium constant? 

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