Law of Chemical Equilibrium (Law of Mass Action)

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11th Chemistry NCERT notes

At a constant temperature, the rate of a chemical reaction is directly proportional to the product of the molar concentrations of the reactants each raised to a power equal to the corresponding stoichiometric coefficients as represented by the balanced chemical equation. Let us consider the reaction,

aA + bB +… ⟶ products

Rate of reaction α [A]a[B]b……

By the law, rate of reaction = k[A]a[B]b

Here a and b are stoichiometric coefficients. K is the rate constant.

Let us consider a general reversible reaction

A + B ⇌ C + D

Applying Law of Mass Action,

Rate of the forward reaction α[A][B] = Kf[A][B]

When Kf is a constant of proportionality and is called velocity constant for the forward reaction.

Rate of the backward reaction α[C][D] = Kb[C][D]

At equilibrium,

Rate of the forward reaction = Rate of the backward reaction

Law of Chemical Equilibrium (Law of Mass Action)

At constant temperatures K is also constant and is called Equilibrium constant.

Now let us consider a more general reversible reaction in a state of equilibrium. By applying Law of mass action.

Law of Chemical Equilibrium (Law of Mass Action)

Equilibrium Constant for the reaction

Law of Chemical Equilibrium (Law of Mass Action)

Relationship between Equilibrium constant K, reaction Quotient Q and Gibbs energy G.

A mathematical expression of thermodynamic view of equilibrium can be described by time equation.

Relationship between Equilibrium constant K, reaction Quotient Q and Gibbs energy G.

where G is standard Gibbs energy.

At equilibrium when ∆G = 0

Relationship between Equilibrium constant K, reaction Quotient Q and Gibbs energy G.

Taking antilog on both sides

Relationship between Equilibrium constant K, reaction Quotient Q and Gibbs energy G.

 

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