IntroductionEquilibrium is a general term which applies to physical processes and chemical reactions. It represents the state of a process in which the properties like temperature, pressure, concentration of the system do not show any change with passage of time.
When the opposing processes involve only physical changes, the equilibrium is called physical equilibrium.
Equilibrium of ice and water at 0°C and atmospheric pressure is an example of physical equilibrium.
Equilibria in Physical Processes
The common examples of equilibria involving physical process are the phase transformations. These include:
(1) Solid -liquid equilibrium (Melting of ice)
Solid ↔ Liquid
When ice and water kept in a perfectly insulated thermos flask so that there is no exchange of heat between its contents and the surroundings at 0°C and the atmospheric pressure are in equilibrium state. Under these conditions, it is observed that the mass of ice and water do not change with time and the temperature remains constant.
For any pure substance at atmospheric pressure, the temperature at which the solid and liquid phases are at equilibrium is called the normal melting point or normal freezing point of the substance.
(2) Liquid - Gas equilibrium (Evaporation of water in a closed vessel)
When a watch glass containing water is kept in a box (in dry atmosphere), the mercury level in the right limb of the manometer slowly increases and finally attains a constant value, that is, the pressure inside the box increases and reaches a constant value. Also the volume of water in the watch glass decreases.
This implies that the more and more water molecules change into vapour till the equilibrium is attained. This process is called evaporation. Ultimately when the amount of water vapours become constant, the same water vapour changes back to liquid water called condensation. At equilibrium:
Rate of evaporation = Rate of condensation
The equilibrium is represented as
H2O(liquid) ↔ H2O (vapour)
3. Solid - Gas equilibrium
Solid ↔ Gas
Example: When solid iodine is kept in a closed vessel, violet vapours appear in the vessel whose intensity increases with time and ultimately becomes constant.
I2(solid) ↔ I2(vapour)
Camphor (solid) ↔ Camphor (vapour)
NH4Cl (solid) ↔ NH4Cl (vapour)
4. Solid - solution equilibrium (Dissolution of sugar in water)
Solid ↔ Solution
Example: Dissolution of sugar in water
Rate of dissolution of sugar = Rate of precipitation of sugar
Sugar (solid) ↔ Sugar (in solution)
5. Gas - Solution equilibrium (Dissolution of a gas in a liquid under pressure in a closed vessel)
Example: Equilibrium in a soda bottle
CO2 (gas) CO2 (in solution)
This equilibrium is governed by Henry’s law, which states that the mass of a gas dissolved in a given mass of a solvent at any temperature is proportional to the pressure of the gas above the solvent.
General Characteristics of Equilibria in Physical Processes
For the physical processes, following characteristics are common to the system at equilibrium:
- Equilibrium involving gas is possible only in a closed system.
- Equilibrium is dynamic in nature.
- All measurable properties of the system remain constant.
- At equilibrium, the concentrations of different substances become constant at a given temperature.
- The magnitude of such quantities at any stage indicates the extent to which the reaction has proceeded.
Why do we say equilibrium is dynamic in nature?Try to answer. Still need help? Want to know more about it? Click here to schedule live help from a certified tutor!
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