Observable properties such as colour, temperature, pressure, concentration, and so on do not change during the course of a process when an equilibrium is reached. The word equilibrium literally means ‘balance,’ and it refers to the fact that a chemical reaction represents a state of equilibrium between the reactants and products that are involved in the reaction. Certain physical processes, such as the melting point of ice at 0°C, are observed to be in an equilibrium state, where both ice and water are present at the same time.

USE OF EQUILIBRIUM

Physical equilibrium is used to describe the state of affairs in physical processes such as the melting of solids, the dissolution of salt in water, and so on, whereas chemical equilibrium is used to describe the state of affairs in chemical reactions.This equilibrium is associated with the physical process that is taking place in the world. 

Physical changes are in a state of equilibrium.

(i) Equilibrium between solids and liquids.For example, H2O(s) → H2O(l); the rate of melting of ice equals the rate of freezing of ice; etc.

(ii) The equilibrium between liquid and gas. For example, H2O(l) → H2O (g)

(iii) The equilibrium between solids and gases.For example, I2(s) → I2 (vapour)

Equilibrium in Chemical changes

The state of a reversible reaction in which both forward and backward reactions occur at the same rate is referred to as the chemical equilibrium state.

The equilibrium state of a reversible reaction is the stage of the reaction during which the concentrations of the reactants and products do not change with time.

Or,

According to the definition, an equilibrium state is defined as one in which the system’s measurable properties (such as pressure or density) don’t undergo any further noticeable changes with time when subjected to a given set of conditions.

Characteristics of Equilibrium states

(i)Equilibrium states can only be achieved if a reversible reaction is carried out in a closed space, as described above.

(ii)In chemical equilibrium at a given temperature, the consistency of certain properties such as pressure, concentration, density, or colour is characterised.

(iii) At equilibrium, the concentrations of each reactant and product are fixed, and this is true regardless of whether the reaction is started with the reactants or with the products as the starting point. 

(iv) Equilibrium is achieved in a shorter period of time through the use of a positive catalyst.

(v) It is a dynamic force in the natural world. It appears that the reaction has come to a halt, however, because the concentrations of the reactants and products do not appear to have changed.

Ionic equilibrium

Chemical reactions can also take place in a solution, in which the majority of the time ions are involved. The substance that reacts with an ion in solution is referred to as an electrolyte. Ionic Equilibrium is defined as the state of equilibrium that exists between the unionized molecules of a particular substance and the ion that has formed in a solution.

Acids, bases, and salts are the most common types of ionic compounds. As a result, when they are dissolved in water or any other solvent, the ionic equilibrium is maintained in them. The equilibrium constant is related to the strength of these electrolytes, i.e. the number of ions that they supply in solution, because all of them are electrolytes.

Strong and Weak electrolytes

The strength of an electrolyte is measured in terms of the degree of ionization (or ionisation).

In the equation: 

Degree of ionisation (alpha) = Number of molecules of electrolyte that split into ions/Total number of molecules of electrolyte

Strong electrolytes are those electrolytes that have been ionised almost completely, whereas weakening electrolytes are those electrolytes that have only been ionised to a lesser extent.

When it comes to strong electrolytes, it is equal to 1.

For weak electrolytes, the value of is less than one.

There is no equilibrium in strong electrolytes because they ionise completely when dissolved in a solvent, so there is no equilibrium in strong electrolytes. However, only a small amount of ionisation occurs in weak electrolytes. The equilibrium between unionised electrolytes and ions formed in solution can be achieved as a result of this process.

COOH + H2O → CH3COOH

NH4OH + H2O—- NH4+ + OH–

Related topic on ionic equilibrium

The equilibrium state is one in which there is no net change in the concentrations of reactants and products. Despite the fact that there is no apparent change at equilibrium, this does not mean that all chemical reaction has ceased. Nothing could be further from the truth; at equilibrium, the forward and reverse reactions continue, but at identical rates, thereby leaving the net concentrations of reactants and products undisturbed.

CONCLUSION

The equilibrium state is one in which there is no net change in the concentrations of reactants and products. Despite the fact that there is no apparent change at equilibrium, this does not mean that all chemical reaction has ceased. Nothing could be further from the truth; at equilibrium, the forward and reverse reactions continue, but at identical rates, thereby leaving the net concentrations of reactants and products undisturbed.We’ve discussed some very important concepts and principles in the past two chapters related to the studies of reaction rates and chemical equilibria. In this chapter, we began with the law of mass action and the significance of the equilibrium state of a chemical reaction. With our understanding of the significance of Keq and Q, we are able to predict the direction that a reaction will go in response to various conditions—concentration, pressure, or temperature changes—that might be applied to a system.