Equilibrium involving in physical change

Substances exist in three states: solids, liquids and gases. The following

types of equilibriums exist in three states:

Solid-liquid equillibrium

When a solid is heated it starts melting at a certain fixed temperature (melting point). At this stage even when the heating is continued, the temperature does not change until the whole of solid is converted into liquid. The state when solid and liquid phases of a substance coexist is called solid-liquid equilibrium. Solid-liquid equilibrium is described as,

If no heat is exchanged with the surroundings, then the temperature and the mass of the two phases (solid and liquid) remain constant.

For example, if we place ice and water at 273 K (0°C) under normal atmospheric pressure in a perfectly insulated thermos flask, since the flask is insulated, there will be no exchange of heat between its contents and the surroundings. We notice that

• The temperature of both the phases remains constant, i.e., temperature of the system does not change.

• Mass of each phase (ice and water) does not change with time.

It appears as if nothing is happening in the system. But, if we could observe the individual molecules of ice and water, we would notice that there is a considerable activity in the system. Some molecules of the liquid are joining on to the ice and at the same time, some molecules of ice convert into liquid. These two processes act in opposite directions. As there is no change in mass of ice and water, the number of molecules going from water to ice is the same as the number of molecules going from ice to water. Hence the rate of transfer of molecules from ice to water and from water to ice, must be equal, i.e.

Rate of transfer of molecules from ice to water = Rate of transfer of molecules from water to ice

or

Rate of melting of ice = Rate of freezing of water

Therefore, the system shows constancy in concentration and is termed

as equilibrium state.

In terms of free energy change, we can say that G becomes zero at equilibrium state.

• At 273 K and 760 mm of Hg pressure G = 0 for ice and water system and they are in equilibrium with each other.

• At temperature higher than 273 K, and 760 mm of Hg pressure G <>

• At temperatures less than 273 K, and 760 mm of Hg pressure G > 0. Thus, the reverse reaction will become favourable, and more ice will be formed from liquid water.

From these it is concluded that, ice and water are at equilibrium only at a particular temperature. For any pure substance at 760 mm of Hg, 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.

is also a dynamic equilibrium. From the above, we infer that in any system at dynamic equilibrium,

• Free energy change, G = 0.

• Two opposite changes occur at the same time.

• These two changes occur at the same rate so that the mass of both the phases (solid and liquid) remain constant.

Liquid-gas equillibrium

When a liquid is placed in an open container it disappears completely after some time. However, when the same liquid is placed in a closed container, even after a long period only a part of the liquid disappears. The obvious difference is that the vapours of the liquid held in an open container can escape to the atmosphere, while the vapours in the closed container are confined to the space above the liquid.

When liquid water is placed in a closed vessel at room temperature, it starts evaporating. As the process continues, more and more water molecules escape from liquid to the vapour state and the pressure starts increasing. The change of pressure can easily be measured with the help of a manometer attached to the vessel. In the beginning, the mercury level in the two limbs of the manometer will be the same. As evaporation continues, pressure goes on increasing and the level of mercury in the right hand limb starts rising.

After some time the pressure becomes constant indicating that no more water is evaporating even though some liquid water is still present. This state of constant pressure can be attained for any period of time provided the temperature remains constant.

Fig: 7.1 - Liquid gas equilibrium

This represents an equilibrium state and can be represented as:

This process may be examined from molecular point of view. As evaporation progresses the number of gaseous molecules in the vapour phase increases gradually. The molecules in the vapour phase move about at random in limited space. During their random movement some of the molecules strike the surface of liquid and get condensed. This process of condensation acts in opposite direction to the process of vaporization. Initially, the rate of condensation is less than the rate of evaporation. But as evaporation proceeds, the concentration of molecules in the vapour phase increases and therefore, the rate of condensation also increases. Finally, the rate of evaporation becomes equal to the rate of condensation. At this stage, the system exhibits constancy in its pressure. Thus, at equilibrium,

Rate of evaporation = Rate of condensation

It may be noted that equilibrium cannot be attained if the process is carried out in an open vessel. Because in the open vessel, the reverse process, i.e., condensation of vapours back to water will not take place. The conditions necessary for liquid gas equilibrium are:

• The system must be a closed system because the amount of matter must remain constant.

• The system must be at constant temperature.

• The visible properties of the system must not change with time.

The pressure exerted by the vapours in equilibrium with the liquid at a particular temperature is called vapour pressure of the liquid. This is constant at a particular temperature but varies with temperature. With the increase in temperature, the vapour pressure increases, however, it is independent of the amount of water in the vessel.

Solid-gas equillibrium

When substances get sublimated,the solid gets converted into vapour without passing through the liquid phase.Sublimation thus involves solid vapour equilibrium. On cooling the vapours, the solid is given back. This equilibrium is obtained in closed systems only. Examples of solid-vapour equilibrium are, camphor, iodine, ammonium chloride etc.