Introduction
When a chemical reaction gets started the reactants are present in the initial stage, but as the reaction proceeds, the concentration of the reactants decreases and that of products increases. Finally, a stage is reached when no further change in concentration of reactants and products is observed. This is a common observation that most of the reactions when carried out in closed vessels do not go to completion under given set of conditions of temperature and pressure. Such reactions in which only a part of the total amount of the reactants is converted into products are reversible reactions.
Equilibrium and its Dynamic Nature
The state of reversible reaction at which the concentrations of reactants and products do not change with time is called a state of chemical equilibrium. This state can be recognized by the constancy of certain measurable properties such as pressure, density, colour, concentration, etc. At equilibrium, both the forward and backward reactions are still taking place, but the rates of the forward and backward reactions become equal. The concentration of each species become constant and the system is said to be at dynamic equilibrium. It is dynamic because the movement of mass is at the microscopic level although no apparent change is observed. The equlilibrium may be established in physical process (physical equilibrium) and in chemical process (chemical equilibrium).
Equilibrium Involving Physical Changes
Substances exist in three states: solids, liquids and gases. The following types of equilibriums exist in three states:
Solid-Liquid Equilibrium
When a solid is heated it starts melting at a certain fixed temperature (melting point). At this stage even when the heating is continued, the temperature does not change until the whole of solid is converted into liquid. The state when solid and liquid phases of a substance coexist is called solid-liquid equilibrium. Solid-liquid equilibrium is described as,
Liquid-Gas (vapor) Equilibrium
When a liquid is placed in an open container it disappears completely after some time. However, when the same liquid is placed in a closed container, even after a long period only a part of the liquid disappears. The obvious difference is that the vapours of the liquid held in an open container can escape to the atmosphere, while the vapours in the closed container are confined to the space above the liquid.
Solid-Vapor Equilibrium
When substances get sublimated,the solid gets converted into vapour without passing through the liquid phase.Sublimation thus involves solid 
vapour equilibrium. On cooling the vapours, the solid is given back. This equilibrium is obtained in closed systems only. Examples of solid-vapour equilibrium are, camphor, iodine, ammonium chloride etc.
Equilibrium Between a Solid and its Solution
The extent to which a solute dissolves in any solvent is termed as its solubility. Different substances have different solubility, some dissolve more and some less. A solution containing the maximum amount of solute at a fixed temperature and pressure is called a saturated solution. A saturated solution cannot dissolve more solute and the added quantity of solute merely settles down in the container. At this condition of saturation a dynamic equilibrium exists between the solid and the solution phases.
Solid substance
solution of the substance
Equilibrium Between a Gas and its Solution
Gases can be dissolved in suitable liquids. The solubility of a gas in any liquid depends upon:
* Nature of the gas and the liquid.
* Temperature of the liquid.
* Pressure of the gas over the surface of the solution.
General Characteristics of Physical Equilibrium
From the physical equilibrium studied above, we have noticed that at equilibrium, some of the measurable properties of the system become constant.
Equilibrium Involving Chemical Systems
In reversible reactions chemical reactions take place in both the forward and backward directions. For example, the reaction between gaseous hydrogen and iodine vapours to give gaseous hydrogen iodide is a reversible reaction and may be expressed as:
Characteristics of Chemical Equilibrium
Chemical equilibrium is dynamic in nature. The constancy of observable property in an equilibrium system does not mean that the reaction has stopped altogether. After the attainment of equilibrium the reaction does not stop although, it appears to have been stopped. The rates of two opposing reactions become equal. This means that if products are formed from the reactants, exactly equivalent amount of reactants being formed from the products.
Law of Mass Action and Equilibrium Constant
The law of mass action correlates the rate of a chemical reaction and the concentration of the reactants. C.M. Guldberg and P. Waage (1864-67) suggested this relationship. This law states that, at a constant temperature and pressure, the rate of a chemical reaction is directly proportional to the product of the molar concentration of the reactants each raised to a power equal to the corresponding stoichiometric coefficient, which appears in the balanced chemical equation.
Rules for Writing Equilibrium Constant Expressions
Expression for the equilibrium constant of a reaction is written in the form of a ratio. The numerator consists of the molar concentration (or partial pressure) terms of the products each raised to a power equal to its stoichiometric coefficient in the balanced chemical equation, and the denominator consists of the molar concentration (or partial pressure) terms of the reactants each raised to a power equal to the stoichiometric coefficient in the balanced chemical equation.
Units of Equilibrium Constant
In mathematical sense, equilibrium constant is not a constant. It is called constant because at a fixed temperature and pressure, the value of the equilibrium constant for a reaction is constant. The unit of equilibrium constant (K) of a reaction depends upon the number of moles of the reactants and products involved in the reaction.
Characteristics of Equilibrium Constant
The value of the equilibrium constant of a reaction is the same, at constant 'T' and 'P'. However, if either temperature 'T' or pressure 'P' or both are changed, the value of the equilibrium constant may also change. For example, the value of Kp (equilibrium constant in terms of partial pressures) for the reaction,
at 700 K and 800 K are 1.5 x 10-4 and 1.4 x 10-5 respectively.
Reaction Quotient and the Equilibrium Constant
The concentration ratio, i.e., the ratio of the product of concentration of the products to that of the reactants at any time (t) is known as the concentration quotient 'Q' of the reaction at time (t). The value of Qc or Qp will be different from the value of Kc or Kp if the reaction is not at equilibrium.
Predicting the Direction and Extent of Reaction
The magnitude of the equilibrium constant, K of a reaction indicates how far a reaction can go in the direction as written. The larger the value of K, the greater will be the equilibrium concentration of the components on the right hand side of the reaction (products) relative to those on the left hand side (reactants).
Le Chatelier's Principle
The state of equilibrium in any system depends upon factors present in the system, such as temperature, pressure and concentration of various species. These factors are called reaction variables or parameters. A change in any one of the parameters may affect the position of the equilibrium. The general rule that can explain the effect of changes in these parameters on the state of equilibrium was formulated by H. Le Chatelier (1885), and F. Braun (1886) and is commonly called as Le Chatelier's principle.
Applications of the Le Chatelier's Principle
The Le Chatelier's principle has a great practical significance for all physical and chemical systems.
