In many chemical reactions it is observed that they do not proceed to completion when they are carried out in a closed container. That means in such reactions the reactants are not completely converted into products.
Initially the concentration of the reactants decreases. After sometime the concentration of the reactants stop decreasing and it appears that reaction is stopped. This state of system in which no further net change occurs is called the state of equilibrium.
There are two types of equilibria. a) Physical equilibrium and b) Chemical equilibrium.
The equilibrium attained in physical processes is called physical equilibrium. e.g. Equilibrium achieved in physical processes like the dissolution of salt or evaporation of water etc.
The equilibrium attained in chemical processes is called chemical equilibrium. e.g. Equilibrium achieved in chemical processes like decomposition of calcium carbonate, the reaction between hydrogen and iodine etc.
Types of Physical Equilibrium:
The change of a substance from one phase to the another phase is called a physical process.The equilibrium attained in physical processes is called physical equilibrium.
Solid – Liquid Equilibrium
Example: Ice(s) ⇌ Water(l) . It is a physical equilibrium because no chemical reaction is involved.
For any pure substance at atmospheric pressure, the temperature at which the solid and liquid can coexist is called as a normal melting point of the solid.
For any pure substance at atmospheric pressure, the temperature at which the solid and liquid can coexist is called as a normal freezing point of the liquid.
When a pure solid is heated it starts transforming into a liquid at a certain temperature (melting point of the solid). The process is called the melting of solid. At this temperature, both the solid and the liquid state of the substance coexist under the given conditions of pressure.
At this temperature, the solid state of a substance is in equilibrium with the liquid state of a substance. If this mixture is taken in a well-insulated container then this constitutes a system in which solid is in dynamic equilibrium with the liquid.
At such point, the interconversion between solid state and the liquid state takes place continuously. It is not stopped. actually, the number of molecules of solid getting converted into the liquid are equal to the number of molecules of liquid getting converted into solid. Thus the mass of the solid and mass of the liquid in the system remains constant. This represents the dynamic equilibrium between the solid and liquid.
At this stage, The rate of melting = The rate of freezing
Liquid – Vapour Equilibrium
Example: water(l) ⇌ Steam(g) It is a physical equilibrium because no chemical reaction is involved.
Let us consider evaporation of water in a closed vessel fitted with a mercury pressure gauge. At room temperature the evaporation of water starts, gradually the quantity of vapours in the vessel increases and pressure called a vapour pressure builds up. Due to this gradual increase in the pressure is indicated by the manometer.
A stage is reached when the manometer shows a constant reading of the vapour pressure. Showing no more evaporation of the water in the vessel. But it is not the case. Actually, the rate of evaporation of water is equal to the rate of condensation of the vapours. It shows there is an equilibrium between the two states.
At this stage, the rate of evaporation = The rate of condensation
The equilibrium between the vapours and the liquid is attained only in a closed vessel. If the vessel is open, the vapours leave the vessel and get dispersed. As the result, the rate of condensation can never become equal to the rate of evaporation.
The vapour formed has more volume to occupy due to increase in the volume. Hence initially vapour pressure decreases due to increase in the volume. Due to more availability of the volume, the rate of evaporation increases and that of condensation decreases. The vapour pressure does not depend upon the size of the vessel containing it. It is constant at a given temperature. Thus at equilibrium, the starting vapour pressure is restored. Similarly, at the equilibrium, the rate of evaporation of the liquid is equal to the rate of condensation of its vapours.
Solid – Gas (Vapours) Equilibrium:
Example: NH4Cl(s) NH4Cl(g)
This type of equilibrium is observed in sublimable substances.
Other Examples of Physical Equilibrium:
Dissolution of solids in liquids:
It is not possible to dissolve just any amount of a solute in a given amount of solvent. A stage will be reached when no more salt can be dissolved.
A solution in which no more solute can be dissolved is called a saturated solution.
The amount of solute required to prepare a saturated solution in a given quantity of a solvent at given temperature is known as the solubility of the solute at that temperature.
The saturated solution corresponds to the state of physical equilibrium.
When a solid (say sugar) is dissolved in a liquid (say water) then due to molecular vibrations the molecules on the surface of the crystal leaves, the crystal and start moving in the solvent freely. At the same time the molecule which already left the crystal return back to the crystal. Initially, the rate of leaving of the molecule from the crystal is much greater than their rate of returning to the crystal.
As the number of molecules in the solution increases the rate leaving of molecules from crystal decreases while the rate of returning of molecules to the crystal increases. A stage is reached when the rate of leaving of the molecules from the crystal surface (the rate of dissolution) is equal to the rate of returning of the molecules to the crystal surface (the rate of precipitation). Thus equilibrium state is attained.
Thus at equilibrium, Sugar(in solution) ⇌ Sugar(solid)
Dynamic Nature of Equilibrium of Dissolution of Solid in Liquid:
The dynamic nature of equilibrium of dissolution of solid in the liquid can be experimentally demonstrated by dissolving radioactive sugar (containing radioactive carbon) into a saturated solution of non-radioactive sugar.
After some time it is observed that the solution becomes radioactive, while the quantity of non-dissolved sugar remains the same.
It clearly indicates that in saturated solution radioactive sugar is getting dissolved into solution at the same time non-radioactive sugar is getting precipitated out.
Thus even if the process of dissolution seems to be stopped, actually both dissolution and precipitations are going on such that their rates are equal. This experiment demonstrates the dynamic nature of equilibrium of dissolution of solid in a liquid.
Effect Of Increase in Temperature on Solubility of Solid in Liquid:
Solid molecules are at fixed positions in their crystal lattice.
When solid is dissolved in liquid, the molecules of the solid acquire randomness.
Thus the kinetic energy of the molecules of the solid after dissolving in liquid increases.
This increase in kinetic energy is due to absorption of heat from the system. Thus dissolution of a solid in a liquid is an endothermic process.
By Le-Chatelier’s principle, if we increase the temperature, then the reaction proceeds in a direction to decrease the temperature. Hence the forward reaction is favoured. Hence the increase in the temperature of the endothermic reaction increases the rate of reaction.
Hence the solubility of a solid in liquid increases with increase in temperature
Dissolution of Gases in Liquids:
The best example of this type of equilibrium is in soda water. When the bottle is opened the carbon dioxide gas dissolved in it fizzles out rapidly.
By Henry’s law “The mass of a gas dissolved in a given mass of a solvent, at a given temperature, is directly proportional to the pressure of the gas above the solvent”. In sealed soda water bottle the carbon dioxide gas is filled with high pressure. Due to the high-pressure appreciable amount of the gas is dissolved in water. When the cap is opened the gas pressure above the solution decreases and the gas dissolved under pressure fizzes out of the solution to attain new equilibrium state. In the case of gas in liquid solution, the solubility of the gas in liquid decreases with increase in the temperature.
The pressure exerted by the vapors in equilibrium with liquid at a particular temperature is called a vapor pressure of the liquid at that temperature.
This law explains the effect of pressure on the solubility of a gas in a liquid.
It states that “The mass of a gas dissolved in a given mass of a solvent, at a given temperature, is directly proportional to the pressure of the gas above the solvent”.
Explanation: If ‘m’ is the mass of the gas dissolved in the solvent and ‘p’ is the pressure of the gas above the solvent then m α p i.e. m = kP
Effect Of Increase in Temperature on Solubility of Gas (Carbon dioxide) in Liquid (Water):
Gas molecules are always in the state of random motion. When gas is dissolved in water, the randomness of molecules decreases.
Thus the kinetic energy of the molecules of the gas after dissolving in liquid decreases.
This decrease in kinetic energy results in the evolution of heat. Thus dissolution of a gas in a liquid is an exothermic process.
By Le-Chatelier’s principle, if we increase the temperature, then the reaction proceeds in a direction to decrease the temperature. Hence the backward reaction is favoured. Hence the increase in the temperature of exothermic reaction decreases the rate of reaction.
Hence the solubility of a gas in water decreases with increase in temperature
Characteristics of Physical Equilibrium:
In the case of Liquid Gas equilibrium, the vapour pressure of the liquid is constant at equilibrium at a particular temperature.
For Solid Liquid equilibrium, there is only one temperature at which the two phases can co-exist at a particular pressure. This temperature is known as the melting point of the solid or freezing point of the liquid.
For dissolution of solids in liquids, the solubility is constant at a given temperature.
For dissolution of gases in liquids, the concentration of a gas in a liquid, at a given temperature is directly proportional to the pressure of the gas over the liquid.
At physical equilibrium, the measurable properties of the system become constant.
At physical equilibrium, there is a dynamic balance between the two opposite processes.
The equilibrium is attained only in a system which cannot gain matter from the surroundings or lose matter to the surroundings. i.e. the system should be a closed system.
When the equilibrium is attained, there exists an expression involving the concentration of substances involved in equilibrium which reaches a constant value at a given temperature. For dissolution of carbon dioxide in water, following expression has a constant value.
The magnitude of the constant value of the concentration-related expression indicates the extent to which the process proceeds before reaching equilibrium. In above expression if the value of the ratio is higher then the numerator should be greater, hence more carbon dioxide gas is dissolved in water.