Physical Equilibrium:

Some Definitions:

The melting point of a solid:

  • 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.

The freezing point of a liquid:

  • 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.

Vapour Pressure:

  • 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.

Characteristics of Vapour Pressure:

  1. The vapour pressure of a liquid is constant at a given temperature.
  2. It does not depend upon the amount of liquid or size of the vessel containing it.
  3. The vapor pressure of liquid increases with the increase in the temperature.
  4. It depends on nature of liquid. A liquid having weaker interparticulate (adhesive) forces (Or more volatile) has higher vapour pressure. e.g. acetone, petrol have greater vapour pressures.
  5. Liquids having higher vapour pressure have lower boiling points, because for these liquids vapour pressure becomes equal to atmosphereric pressure at lower temperature.

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)
  • 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)
  • 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 Equilibria:

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 leave, 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.

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