Chemical Equilibrium: Reversible and Irreversible Reaction, Condition Require for Equilibrium State (For CBSE, ICSE, IAS, NET, NRA 2022)

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Chemical Equilibrium

  • In chemical reaction, it is believed that all the reactants would be converted into products with the release or absorption of energy.
  • But many chemical reactions proceed only to a certain extent and stop, and the resulting mixture contains both the reactants and products.
  • This is because when reactants combine to form products, the products also start combining to give back the reactants.
  • When such opposing processes take place at equal rates, It is said that atate of chemical equilibrium has reached.

Reversible Reaction

  • A reaction is said to be reversible if under certain conditions of temperature and pressure, the forward and reverse reactions occur simultaneously.
  • For example, the reaction between ethanol and acetic acid. When mixed in the presence of dilute sulphuric acid they react and form ethyl acetate and water.

When ethyl acetate and water are mixed in the presence of dilute sulphuric acid the reverse reaction occurs.

The above reaction can be written as

A reversible reaction is said to be in the equilibrium state when the forward and backward reaction occur simultaneously at the same rate in a closed system and the concentrations of reactants and products do not change with time.

Reversible Reaction

The graph depicts that the rate of forward reaction gradually decreases while the rate of backward reaction increase till they become constant and equal to each other.

Irreversible Reaction

  • The reactions occur only in one direction. They are called irreversible reaction.
  • For example, when carbon is burnt in air to form carbon dioxide the reaction goes only in one direction i.e.. in the direction of formation of carbon dioxide.

  • In these reaction, the rate of reaction in one particular direction is extremely small compared to the other.
  • The reaction proceeds practically in one direction to near completion leaving a negligibly small amount of reactant at end.
  • Hydrochloric acid is mixed with sodium hydroxide, a base, in equimolar quantities, a neutralization reaction takes place, with the formation of salt and water.

This reaction proceeds to completion in the forward direction only.

Condition Require for Equilibrium State

  • The temperature of the system attains a constant value.
  • The pressure of the system attains a constant value.
  • The concentrations of all the reactants and products attain constant value.

Characteristics of Equilibrium State

Chemical Equilibrium is Dynamic in Nature

The chemical equilibrium is the result of two equal but opposite processes occurring in the forward and reverse directions and there is no “net” change occurring in the system .

Equilibrium Can be Attained from Either Side

The same state of equilibrium (characterized by its equilibrium constant) can be reached whether the reaction is started from the reactants or products side.

For example, the same equilibrium

is established whether we start the reaction with or .

Equilibrium Can be Attained Only in a Closed System

  • Equilibrium can be attained only if no substance among, reactants or products, is allowed to escape i.e.. the system is a closed one.
  • System consisting of gaseous phase or volatile liquids must be kept in a closed container, e. g.

System consisting of only non-volatile liquid and solid phases can be kept even in an open container because such substances have no tendency to escape, e. g.

A Catalyst Cannot Change the Equilibrium State

Addition of a catalyst speeds up the forward and reverse reactions by same extent and help in attaining the equilibrium faster. But the concentrations of reactants and products are not affected in any manner.

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