Physical Equilibrium – 02:
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.
- The equilibrium between the carbon dioxide gas (solute) and water (solvent) may be represented by
CO2(g) ⇌ CO2(in solution)
- 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.
- 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”.
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
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
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.
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