Equilibrium – Session 2

Imagine you are standing by a lake on a sunny day. You could see bubbles rising from the depths of the water. Have you ever wondered what these bubbles are and why they form? Actually the gas is dissolved in water. The bubbles are formed because the system tries to establish the dissolved gas-gas phase equilibria. But what is dissolved gas-gas phase equilibria? Let us understand this concept.

When a gas comes into near-proximity with a liquid, the interactions occur between gas molecules and the molecules of the liquid. The gas molecules have kinetic energy and move randomly. They collide with the the surface of the liquid. Due to these collisions, some gas molecules are captured by the attractive forces of the liquid molecules. As a result the gas molecules become incorporated into the liquid phase. This process is known as gas dissolution.

The process of gas dissolution in a liquid is dynamic. When the liquid and the gas first come into near-proximity, more gas molecules will dissolve in the liquid. This is because the concentration of gas molecules in the gas phase is higher. As the gas dissolves in the liquid, the concentration of gas molecules in the liquid increases. When the concentration of gas in the liquid increases, some of the dissolved gas molecules also escape again into the gas phase. This process is known as evaporation.

At the molecular level, the gas molecules continue to dissolve into the liquid. At the same time an equal number of gas molecules continue to escape from the liquid into the gas phase. However, there is no overall change in the concentrations of the gas in either phase. This is because the rate of dissolution of gas molecules becomes equal to the rate of escape of gas molecules. As a result a dynamic equilibrium is established between the dissolved gas molecules and the gas phase molecules. This is also called the dissolved gas-gas phase equilibria.

Have you ever observed the equilibrium between dissolved gas and the gas phase in real life? Well, you might have experienced it when opening a bottle of soda. When you open the soda bottle, you hear the sound of gas escaping from the bottle. This indicates that the gas is dissolved in the soda bottles. Let us understand how the equilibrium is established between the dissolved gas and the gas phase in a soda bottle.

In a sealed soda bottle, there is carbon dioxide gas dissolved in the liquid. During the manufacturing process, the soda undergoes the carbonation. Carbonation means it is infused with the carbon dioxide gas under pressure. This process results in a higher concentration of the carbon dioxide gas in the gas phase, above the liquid.

The pressure from the carbonation causes the carbon dioxide gas to dissolve in the liquid. Some of the dissolved carbon dioxide molecules also escape into the gas phase above liquid. At a particular point the rate of dissolution of the carbon dioxide molecules and the rate of escape of carbon dioxide molecules become equal. This is the state of dynamic equilibrium.

When you open the bottle of soda, the pressure inside the bottle decreases rapidly. This is due to the release of the carbon dioxide gas to the top of the bottle. As a result, the concentration of carbon dioxide gas in the gas phase above the liquid decreases. Equilibrium is also disturbed. The system tries to establish the equilibrium again. The carbon dioxide gas from the liquid escapes into the gas phase. Some carbon dioxide molecules from the top of the bottle dissolves again into the liquid. This process causes fizzing and bubbling. As a result the equilibrium is established again.

Immiscible liquids are the substances that do not mix or dissolve in each other. When you combine two immiscible liquids, they form separate layers with a distinct boundary between them. Oil and water are examples of immiscible liquids. When we mix the oil and the water, they form separate layers. One layer is of oil and other layer is of water.

Consider a system containing two immiscible liquids and a solute. These immiscible liquids are carbon tetrachloride and water. The solute is the iodine. When we mix these two immiscible liquids, they form separate layers.

After that we add the iodine. Initially, the iodine will dissolve in the carbon tetrachloride layer. This is because the carbon tetrachloride and the iodine both are non polar. That is why the iodine is mostly soluble in the carbon tetrachloride. As the concentration of iodine increases in the carbon tetrachloride layer, some of the iodine molecules will start moving into the water layer. This happens only when the concentration of the iodine molecules is very high in the carbon tetrachloride layer.

At high concentration of the iodine, only a small amount of the iodine molecules will dissolve in water. This is because the iodine is sparingly soluble in water. Some of the iodine molecules will move from aqueous layer into the carbon tertachloride layer. Some will move from the carbon tetrachloride layer into the aqueous layer. At this point dynamic equilibrium is established. This equilibrium is specifically called immiscible liquid solute equilibria.

Ionic equilibrium refers to a situation in which ions in a solution are in a dynamic balancing between the dissociation and the recombination. When a compound is dissolved in water, it can break apart into its constituent ions. This breaking of compound into its constituents ions is called the dissociation. For example, when hydrogen chloride is dissolved in the water, it undergoes dissociation. It dissociates into hydrogen ions and chloride ions.

As the dissociation continues, some of the dissolved ions can recombine to form the original compound. This process is called the recombination. The dissociated hydrogen ions and the chloride ions can recombine to form hydrogen chloride.

The whole process can be represented as a reversible reaction. In this reaction, the dissociation and the recombination takes place simultaneously. Ionic equilibrium is established when the rate of dissociation of hydrogen chloride becomes equal to the rate of recombination of the hydrogen ions and the chloride ions. Ionic equilibrium is essential for understanding the behavior of solutions containing ions.