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Out of the many chemical reactions certain reactions are reversible reaction i.e. they proceed in forward as well as backward direction i.e. reactants combine to give products and products gives back the reactants. In such reactions after some time of commencement of reaction system a stage is reached, when the properties of the system remain constant with time, then the reaction system is said to be in equilibrium. There are many kinds of equilibrium but we will focus on Dynamic equilibrium only, then we will study the important laws governing equilibrium, Le Chatelier’s Principle & Law of Mass Actions. Then we will see some important constants associated with the equilibrium & relationship between them. Then we will end with some special features of Equilibrium.

OBJECTIVE

The stage of equilibrium can be reached starting from either reactants or products only or with any amounts of reactants and products taken together. At equilibrium, the processes of conversion of reactant(s) into product(s) and vice versa do not cease but both of them occur simultaneously at equal rates. Thus an equilibrium is actually a dynamic equilibrium. Our aim in this chapter is to learn about Chemical Equilibrium & its properties so that we can manipulate the reactants or product concentrations along with the reaction conditions in order to get a desired effect.

PRE-REQUISITE

The knowledge of the following will help you understand the chapter easily & effectively.

Þ            Gaseous State and Gas laws.

Þ            Stoichiometry

Þ            Mole concept.

Core Concept

Certain reactions are reversible reaction i.e. they proceed in forward as well as backward direction i.e. reactants combine to give products and products give back reactants. In such reactions after some time of commencement of reaction a stage is reached, when the properties of the system remain constant with time, then the reaction system is said to be in equilibrium. The stage of equilibrium can be reached starting from either reactants or products only or with any amounts of reactants and products taken together. At equilibrium, the processes of conversion of reactant(s) into product(s) and vice versa do not cease but both of them occur simultaneously at equal rates. Thus an equilibrium is actually a dynamic equilibrium. Our aim in this chapter is to learn about Chemical Equilibrium.

The concept of Equilibrium:

It is a common observation that clothes dry quicker when there is a breeze or when we keep shaking it. Have you ever thought why does this happen? Well, we can get an answer to this query if we know what an equilibrium is.

In the study of any chemical reaction, two questions are of vital importance viz. How far a reaction would go and how fast will it reach the goal.

Well, equilibrium is that state of a system in which it has minimum energy. If we disturb the equilibrium state (by changing T, p, volume, concentration of reactants, products or adding an inert gas), the system always tries to come back to an equilibrium state. This tendency can be exploited for getting maximum production in industry.

Equilibrium could be of two types

Physical Equilibrium 

It is found between two different states of a substance, e.g., water at its boiling point has got two states, water (liquid) and water vapour (gas). Both of these are in equilibrium with each other.

Chemical Equilibrium

When a reaction starts, reactants are present at some definite concentrations. As reaction proceeds, concentration of reagents decreases and that of products increases. Sooner or later, however, concentration levels off and becomes constant. A state in which concentration no longer changes with time (this state which persists as long as the system is free of external perturbations) is known as the state of chemical equilibrium

We use the symbol to denote equilibrium in contrast to the symbol → where the reaction goes to completion.

The reaction in which products recombine to give back reactants or the reaction which proceeds in forward as well as backward direction is termed as reversible reaction or in other words it is a dynamic state.