Physical Equilibrium

Equilibrium is the state in which the system and its macroscopic properties are in a stable state and hence don’t change. For example, the water enclosed in some container is in the equilibrium state. As here, the number of molecules escaping the water surface is equal to the number of water molecules entering into the surface. Equilibrium is also possible in many physical processes but the physical phase will be static. The liquid in equilibrium is also common. Based on this we can see its categorization.

Types of Equilibrium:

Mainly equilibrium can be of two types:

• Physical Equilibrium.

• Chemical Equilibrium.

Physical Equilibrium is the equilibrium that can be seen in the physical processes also. It is the equilibrium between the same chemical species in the different phases. Physical equilibrium can be defined as the equilibrium which exists between different phases or physical properties. In such processes, chemical composition and properties are not changed. It also represents the existence of the same substance in two or more different physical states. Further, we can divide the physical equilibrium as follows:

• Phase equilibrium.

• Solute - Solid Equilibrium.

• Gas - Liquid Equilibrium.

Type of Physical Equilibrium:

Equilibrium shows the constancy of content as well as the composition of an item of interest in a system. It does not depend on the time period. Also, the equilibrium state always has an equal rate of the forward reaction and rate of the backward reaction. We can observe in our surroundings to see many examples such as a book on a table, saturated solution, and ionic substances in polar solvents etc.

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Now, for better understanding let us discuss the different types of physical equilibrium in detail.

Phase Equilibrium:

At 0°C, the number of water molecules becoming ice will be equal to the water molecules. This is because ice is melting to form the liquid water. Here, the rate of freezing of water will be equal to the rate of melting of the ice. Therefore, an equilibrium will exist between the solid ice and liquid water. Thus,

Ice (s) ⇌ Water (l)

Phase equilibrium chemistry is a very happening equilibrium. The number of molecules of a liquid converting into vapour will be equal to the number of molecules which are condensing into the liquid in a closed container. So, we can say that the rate of evaporation of liquid water is equal to the rate of condensation of the water vapour. In this way, the liquid phase is in equilibrium with its vapour phase. Thus,

Water (l) ⇌ Water (g)

Solute-Solid Equilibrium:

When a solute in any saturated solution comes in contact with the undissolved solute, then the number of molecules depositing out of the solution is equal to the number of molecules dissolving from the solid into the liquid. Therefore, the solute in a solution will be in equilibrium with the undissolved solid. Thus,

Solute (aq) ⇌ Solute (s)

Gas-Liquid Equilibrium:

Gases which cannot react with liquid, but can dissolve directly related to the pressure in the liquid. In a closed container, there is an equilibrium between the gas inside the liquid and the gas above it. For example, in soft drinks, carbon dioxide gas in the liquid will be in the equilibrium with the gas which is present in the empty part of the container. Thus,

Gas (solution) ⇌ Gas (g)

Some Examples:

1. Solid-Liquid Equilibria:

Take the example of ice and water in a perfectly insulated thermos flask at 0¬¬¬¬0C. Quantity and level of water will not be changed, the rate of transfer of molecules from water to ice will be the same as the rate of transfer of molecules from ice to water. So, we may conclude that this system is in a steady state. This can be represented as:

The rate of melting = Rate of freezing.

2. Liquid-Gas Equilibria:

Take distilled water in some closed containers. Then heat it, to convert the water into vapour. After some time, you will see that the level of water becomes constant implying that there is no more conversion of water to vapour and vice-versa.

We can say that the rate of evaporation will be equal to the rate of condensation to achieve a steady-state. Thus,

The rate of evaporation = Rate of condensation

3. Solid-Vapour Equilibria:

This type of equilibrium exists only in the case of sublimates when solid directly converts to vapour. Take solid iodine in a closed container, and then heat it, then slowly the vessel is filled with violet-coloured vapour. You will see that after some time its intensity and colour do not change with time. This concludes the steady-state where the rate of sublimation of solid iodine is equal to the rate of deposition of iodine vapour. Thus,

The rate of sublimation = Rate of deposition.

Fun Fact:

• At equilibrium, measurable properties become constant at equilibrium measurable.

• Open container is a must for such an equilibrium establishment.

• At the equilibrium state, the opposing forces become equal.

• The equilibrium is dynamic always.

• In the evaporation process, the material goes from the liquid phase to the gas phase.

• At equilibrium, the concentration also becomes constant.

• In the condensation process, the substance goes from the gas phase to the liquid phase.

• In a reversible reaction, the chemical reaction can proceed in both the forward and reverse directions.

• The magnitude of the equilibrium value indicates the extent of the reaction.

• There will be a dynamic equilibrium in the reversible reaction.