Law of chemical equilibrium is direct result of law of mass action applied to reversible chemical reaction.
Consider a general reversible reaction,
aA + bB ↔ cC + dD.
Applying law of mass action to both reactions i.e. forward reaction and backward reaction.
ü FOR FORWARD REACTION
aA + bB → cC + dD
\ Rate of forward reaction, rf µ [A]a[B]b
or rf = Kf [A]a[B]b ––––––– (i).
Where [A], [B] are molar conc. of A and B, and Kf is rate constant for forward reaction.
ü FOR BACKWARD REACTION
cC + dD → aA + bB
\ Rate of backward reaction, rb µ [C]c[D]d
or rb = Kb[C]c[D]d ––––––– (ii).
Where [C], [D] and Kb are molar concentrations of A, B and rate constant of backward reaction respectively.
ü AT EQUILIBRIUM
Rate of forward reaction, rf = Rate of backward reaction, rb
Using (i) and (ii) we get,
\ rf = rb
Kf [A]a[B]b = Kb[C]c[D]d
Kf / Kb = [C]c[D]d / [A]a[B]b
or KC = [C]c[D]d / [A]a[B]b, where Kc = Kf / Kb
‘KC’ is known as Equilibrium Constant in terms of concentrations.
From the above equation, Law of chemical equilibrium may be stated as, “At constant temperature, for a reversible chemical reaction in equilibrium, the ratio of product of molar concentrations of products to that of molar concentration of reactants each term is raised to a power of stoichiometric coefficient as represented by balanced chemical reaction.”
In a chemical reaction if reactants and products are in gaseous phase, then partial pressures of reactants and products are considered instead of molar concentrations. Under such conditions equilibrium constant is represented by ‘KP’ (i.e. equilibrium constant in terms of partial pressure), For example
aA (g) + bB (g) ↔ cC (g) + dD (g)
KP = [PC] c [PD] d / [PA] a [PB] b
Where PA, PB, PC and PD are partial pressures of A, B, C and D.
The concentration ratio
[C]c[D]d / [A]a[B]b is called Concentration Quotient or Reaction Quotient.
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