Equilibrium Constant - Practice Questions & MCQ

It is the ratio of rate of forward and backward reaction at a particular temperature or it is the ratio of active masses of the reactants to that of active masses of products at a particular temperature raised to their stoichiometric cofficients. It is denoted by K_c or K_p. The distinction between K_eq and K_c is that expression of K_eq involves all the species (whether they are pure solids, pure liquids, gases, solvents or solutions) while the expression K_c involves only those species whose concentration is a variable (gases and solution). It means K_c is a devoid of pure components (like pure solids and pure liquids) and solvents.

Let us look at the definition of Equilibrium constant in terms of concentration and partial pressure

(1) Equilibrium constant in terms of Concentration

For a reaction:

We know that at equilibrium

The above expression gives us the value of Kc as the activity or the active mass is expressed in terms of the concentrations (c) or the molarity

(2) Equilibrium constant in terms of Partial pressure

In this case, the equilibrium constant is known as K_p. It is applicable only for gaseous systems.
For the reaction:

At equilibrium

The above expression gives us the value of K_P as the activity is expressed in terms of the partial pressures.

Note: This is generally used for gaseous systems or systems where gases are in equilibrium with liquids or solids