Law of mass action

     
CHEMICAL EQUILIBRIUM
ACTIVE MASS
Concentration of a substance expressed in mole/dm3 or in molar unit is called ACTIVE MASS. Active mass in mole/dm3 is represented by a square bracket [ ].
LAW OF MASS ACTION
According to the law of mass action.
“The rate at which a substance reacts is directly proportional to its active mass ”
Rate µ [Reactant]
The rate of reaction is directly proportional to the product of the active masses of reactants.
Consider a general reversible reaction
A + B C + D
According to the law of mass action:
Rate of reactionµ [A][B]
Determination ofequilibrium constant by using equilibrium law
Consider a general reaction
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aA + bB cC + dD
According to the law of mass action
Rate of forward reaction µ [A]a[B]b  
 Rate of forward reaction = Kf [A]a[B]b
Similarly,
   Rate of backward reaction µ [C]c[D]d   
    Rate of backward reaction = Kb [C]c[D]d
Where
Kf = rate constant for forward reaction
Kb = rate constant for backward reaction
a, b, c, d = number of moles
At equilibrium rate of forward reaction becomes equal to the rate of backward reaction, thus,
Rate of forward reaction = Rate of backward reaction
Kf [A]a[B]b = Kb [C]c[D]d
Kf / Kb =[C]c[D]d/[A]a[B]b
Let     Kf/Kb = Kc
Kc = [C]c[D]d/[A]a[B]b
This is the expression of equilibrium constant where c represents concentration.
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