Physical Equilibrium
Phase Transformation Processes
$$ce{solid <=> liquid}$$
$$ce{liquid <=> gas}$$
$$ce{solid <=> gas}$$
Solid – Liquid Equilibrium
When water is placed in a perfectly insulated thermos flask (an isolated system with no exchange of heat between it and its surroundings) at a constant temperature 273K (freezing point of water) and under one atmospheric pressure, there is formation of ice in it. There is also conversion of ice into water but at smaller rate. As a result, the mass of ice increases with time while mass of water decreases. But after sometime, the masses of ice and water both become constant. Though, the masses of the two remain constant, the processes of conversion of water into ice (freezing) and conversion of ice into water (melting) at the boundary of the ice and water continue to occur. The rates of the freezing and the melting remain same. This is the equilibrium condition (ce{solid <=> liquid}).
Here, the system is in dynamic equilibrium due to simultaneous occurrence of opposing processes, melting and freezing.
NOTES:
For a solid – liquid equilibrium, (ce{solid <=> liquid}) at the melting point of a pure substance under one atmospheric pressure,
- there is a dynamism in equilibrium conditions as two opposing processes conversion of water into ice (freezing) and conversion of ice into water (melting) occur simultaneously.
- both the processes, melting and freezing, occur at the same rate so that the amount of solid and liquid at the equilibrium condition remain same.
MELTING POINT: The normal melting point of a pure substance or its normal freezing point under one atmospheric pressure is the temperature at which the solid and liquid phases of the substances are at equilibrium. Melting point is fixed at constant atmospheric pressure.
Liquid-Vapour Equilibrium
When water in a watch glass is placed in a transparent box fitted with a manometer and dried using drying agent like anhydrous calcium chloride or phosphorus pentaoxide, the manometer indicates a rise in pressure inside the box. The increase in pressure is due to conversion of water into vapour. The vapour also converts into water as the vapour particles collide with water surface and are trapped in it. But its rate of conversion is low. As a result, the vapour pressure inside the box increases with time while amount of water in watch glass decreases. But after sometime, the vapour pressure and amount of water both become constant. Despite these being constant, the processes of conversion of water into vapour (evaporation) and conversion of vapour into water (condensation) at the boundary of the vapour and water continue to occur. The rates of the evaporation and the condensation remain same. This is the equilibrium condition (ce{H2O(l)<=> H2O(vap)}) [(ce{liquid <=> vapour})].
Here, the system is in dynamic equilibrium due to simultaneous occurrence of opposing processes, evaporation and condensation.
At the equilibrium, the pressure exerted by water molecules in gaseous phase at given temperature remains constant and is called as equilibrium vapour pressure of water or just vapour pressure of water. The vapour pressure of a liquid depends on the inter-molecular forces and the temperature. Liquids with stronger inter-molecular interaction have lower vapour pressure as compared to liquids with weaker inter-molecular interactions. Liquids having higher vapour pressure at given temperature are called as more volatile liquids. Increase in temperature increases the vapour pressure of the liquid.
Boiling is a special case of evaporation when vapour pressure of a liquid is equal to atmospheric pressure (1.013 bar). Normal boiling point of a pure substance is the temperature at which its liquid and vapour phases are at equilibrium under one atmospheric pressure. For example, boiling point of water is 373K. Boiling point depends on atmospheric pressure. With increase in atmospheric pressure, boiling point of liquid increases and vice versa. This is the reason that with increase in altitude, the boiling point of a liquid decreases.
NOTES:
For a Liquid – Vapour equilibrium, (ce{Liquid <=> Vapour }) at a given temperature and under one atmospheric pressure,
- there is a dynamism in equilibrium conditions as two opposing processes conversion of water into vapour (evaporation) and conversion of vapour into water (condensation) occur simultaneously,
- both the processes, evaporation and condensation, occur at the same rate so that the amount of liquid and pressure of its vapour at the equilibrium condition remain same.
EQUILIBRIUM VAPOUR PRESSURE at a given temperature and under one atmospheric pressure is defined as the pressure exerted by the molecules of a pure substance on the surface of its liquid phase when the vapour and liquid phases are at equilibrium. This is also called as vapour pressure of the liquid. Its value depends on the nature of the substance (its inter-molecular forces) and the temperature of the equilibrium conditions. Stronger inter-molecular forces lesser vapour pressure. If temperature is high, vapour pressure will be high too.
Equilibrium vapour pressure of a liquid is constant at given temperature.
Boiling is a special case of evaporation. The normal BOILING Point of a pure substance is the temperature at which its liquid and vapour phases are at equilibrium under one atmospheric pressure. For example, boiling point of water is 373K. It increases with increase in atmospheric pressure and vice versa. It decreases with increase in the altitude as the atmospheric pressure decreases with increase in altitude.
Solid – Vapour Equilibrium
When solid iodine is placed in a closed transparent vessel, the vessel starts filling with violet vapour as the solid iodine sublimate. There is also deposition of iodine vapour into solid iodine but at smaller rate. As a result, the intensity of violet colour (iodine vapour) increases with time while amount of solid iodine decreases. But after sometime, the intensity of violet colour and amount of solid iodine both become constant. Though, the amount of the two remain constant, the processes of conversion of solid into vapour (sublimation) and conversion of vapour into solid(deposition) at the boundary of the solid and vapour interface continue to occur. The rates of the sublimation and the deposition remain same. This is the equilibrium condition, (ce{I2(solid) <=> I2(vapour)}), (ce{solid <=> vapour}).
Similar equilibrium is observed in, (ce{Camphor(solid) <=> Camphor(vapour)}) and (ce{NH4Cl(solid) <=> NH4Cl(vapour)}).
Here, the system is in dynamic equilibrium due to simultaneous occurrence of opposing processes, sublimation and deposition .
NOTES:
For a solid – vapour equilibrium, (ce{solid <=> vapour}) at a given temperature ,
- there is a dynamism in equilibrium conditions as two opposing processes conversion of solid into vapour (sublimation ) and conversion of vapour into solid (deposition ) occur simultaneously.
- both the processes, sublimation and deposition, occur at the same rate so that the amounts of solid and vapour at the equilibrium condition remain same.
Dissolution of solids/gases in liquids
$$ce{sugar(solid) <=> sugar(solution)}$$
$$ce{CO2(gas) <=> CO2(solution)}$$
Solids in Liquids
When sugar is dissolved in a given amount of water at room temperature, there are two processes that occur simultaneously, dissolution of sugar (solid) into water and crystallization of sugar(solution) out of water. At start, the rate of dissolution is faster than the rate of crystallization. The water turns sweeter with time. But after sometime, the sweetness of the water becomes constant and no more sugar can be dissolved in it. This is the saturated sugar solution. At this juncture, the amount of undissolved sugar (solid) and the dissolved sugar (solution) remain constant. However, the processes of conversion of sugar (solid) into sugar (solution) and vice versa continue to occur at the same rate. This is the equilibrium condition, (ce{sugar(solid) <=> sugar(solution)}).
Here, the system is in dynamic equilibrium due to simultaneous occurrence of opposing processes, dissolution and crystallization. The extent of equilibrium depends on the temperature of the equilibrium conditions. Higher temperature means more dissolution of solid into water.
NOTES:
For a (ce{sugar(solid) <=> sugar(solution)}) at a given temperature ,
- there is a dynamism in equilibrium conditions as two opposing processes dissolution of sugar (solid) into water and crystallization of sugar(solution) out of water.
- both the processes, dissolution and crystallization, occur at the same rate so that the amounts of undissolved sugar (solid) and the dissolved sugar (solution) at the equilibrium condition remain same.
Concentration of solute in solution is constant at given temperature.
Gases in Liquids
When a solvent is exposed to a gas in a closed vessel, the gas gets dissolved into it. In such a system, some molecules of gas get trapped into the solvent while some trapped gas molecules escape out of it. At equilibrium, rate of gases being trapped into solvent and rate of escape of molecules of gas out of solvent are same. The amount of the gas dissolved in the solvent at equilibrium condition is called as solubility. The solubility of the gas depends on the partial pressure of the gas over the solvent. The equilibrium condition is, (ce{CO2(gas) <=> CO2(solution)}).
Here, the system is in dynamic equilibrium. The extent of equilibrium depends on the partial pressure of the gas over the solvent in equilibrium conditions. Higher pressure means more solubility of gas into the solution.
NOTES:
For a (ce{CO2(gas) <=> CO2(solution)}) at a given temperature ,
- there is a dynamism in equilibrium conditions as two opposing processes dissolution of (ce{CO2})(gas) into water and release of (ce{CO2})(solution) out of water take place simultaneously.
- both the processes occur at the same rate so that the amounts of undissolved (ce{CO2})(gas) and the dissolved (ce{CO2})(solution) at the equilibrium condition remain same.
Solubility of a gas in a given amount of solvent at given temperature is directly proportional to the vapour pressure of the gas over the solvent .