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 […]

### 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
where,
[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 or, or, or, or,

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?