## Equilibrium constants

**Equilibrium Constant – Introduction:**

Chemical equilibrium can be defined as the state f a reversible reaction when the two opposing reactions occur at the same rate and the concentrations of reactants and products do not change with time. A chemical equilibrium is dynamic, when the forward and reverse reactions are taking place at equilibrium but the concentrations remain unchanged.

Equilibrium constant of a reaction is a very important quantity to explain the speed of a forward and the reverse reaction.I like to share this Affinity Chromatography with you all through my article.

**Equilibrium Constants:Equilibrium Law:**

To explain equilbrm constant law, let us consider a general equation:

And [A], [B], [C] and [D] represent the molar concentrations of A, B, C and D at the equilibrium point.

According to the law of mass action,

Rate of forward reaction ∞ [A] [B] = k1 [A] [B]

Rate of reverse reaction ∞ [C] [D] = k2 [C] [D]

Where k1 and k2 are rate constants for the forward and reverse reactions.

Therefore,

k1 [A] [B] = k2 [C] [D]

or,

At any specific temperature k1/k2 is constant since both k1 and k2 are constants.

The ratio, k1/k2 is called equilbrm constant and is represented by the symbol Kcor, just K.

The subscript ‘c’ indicates that the value is in terms of concentrations of reactants and products.

**Equilibrium Constant Expression:**

The equilbrm constant expression may be written as:

The above expression is called as Equilbrm constant expression or equilibrium law.

**Equilibrium Constant Equation-**

Consider the reaction:

In the above equation, the forward reaction is dependent on the collisions of each of two A molecules only. Therefore for writing the equilibrium expression, each molecule is regarded as a separate entity:

Then equilbrm constant expression is:

As a general rule, if there are two or more molecules of the same substance in the chemical equation, its concentration is raised to the power equal to the numerical coefficient of the substance in the equation.

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**Calculate Equilibrium Constant-**

The general reaction may be written as:

Where a, b, c and d are numerical quotients of the substance A, B, C and D respectively. The equilbrm constant expression is:

Kc is the equilbrm constant. The general definition of the equilbrm constant may thus, be stated as:

“The product of the equilibrium concentrations of the products divided by the product of the equilibrium concentrations of the reactants. With each concentration term ‘raised to a power’ equal to the coefficient of the substance in the balanced equation.”

Example:

To write the equilibrium expression for:

The equilbrm constant expression for the above reaction can be written as.