Equilibrium Constant Formula

In chemistry student have studied various kinds of chemical equations. Many of them are irreversible in nature. It means it is not possible to reversely produce the reactants with the help of products. On the other hand many of them or reversible in nature. A reversible reaction can proceed in both directions always. Most reactions are ideally reversible in a closed system. In this article will learn about the chemical equilibrium state of such a reversible chemical equation. Thus the student will see here the equilibrium constant formula with examples. Let us begin it!

Equilibrium Constant Formula

What is Chemical equilibrium?

All reactions tend towards the state of chemical equilibrium. It is the point at which both the forward process and the reverse process are taking place at the same rate. Since the forward and reverse rates are equal, hence the concentrations of the reactants and products will be constant at the equilibrium state. Here, it is important to remember that even though the concentrations are constant at equilibrium. The reaction is still happening in this state. Therefore, this state is also sometimes referred to as dynamic equilibrium.

The double half-arrow sign we use for writing the reversible reaction equations. It is represented as:

\(\rightleftharpoons\)

One example of the reversible reaction is the formation of nitrogen dioxide, given below:

\(N_2 O_4 (g) \rightleftharpoons  2 NO_2 (g)\)

The formula for Equilibrium Constant:

For a certain chemical reaction, the equilibrium constant is defined as the ratio between the amount of reactant and the amount of product that is used to determine chemical behavior.

At equilibrium, Rate of the forward reaction = Rate of the backward reaction

For a reversible reaction:

\(aA + bB \rightarrow cC + dD\)

The equilibrium constant, \(K_c\), will be:

\(K_c\) = (\frac {[C]^c \cdot [D]^d }{[A]^a \cdot [B]^b}\)

Where

[A]	equilibrium concentration of A
[B]	equilibrium concentration of B
[C]	the equilibrium concentration of C
[D]	equilibrium concentration of D
K_C	Equilibrium Constant

There are many different types of equilibrium constants. Some of these are binding constants, association constants, dissociation constants, stability constants, and formation constants, etc. The factors which may affect the equilibrium constant include temperature, ionic strength, and choice of solvent.

Some important points:

There are some important things to remember when calculating \(K_c\):

• \(K_c\) is a constant for a specific reaction at a specific temperature. If we change the temperature of a reaction, then it also changes.
• Pure solids and pure liquids, including solvents, are not included in this equilibrium expression.
• \(K_c\) is often written without units.
• The reaction must be balanced with the coefficients written as the lowest possible integer values, to get the correct value for \(K_c\).
• The value of the equilibrium constant is useful to determine the concentration or pressure of all the reactants and products at the equilibrium.

Solved examples:

Q.1: Take the following chemical equation. Determine the equilibrium constant for this reversible reaction.

\(2SO_2 (g) + O_2 (g) \rightleftharpoons  2SO_3 (g)\)

It is given that equilibrium concentration of the substances are as

\(SO_2 : 0.62 M\)

\(O_2 : 0.45 M\)

\(SO_3 : 0.92 M\)

Solution: We have:

\([SO_2] = 0.62 M; [O_2] = 0.45 M; [SO_3]\) = 0.92 M

Therefore:

\(K_c\) = \(\frac {[0.92]^2 } {[0.62]^2 [0.45]} \)

\(K_c\) = 4.893

Thus, equilibrium constant will be 4.893.