Equilibrium – Session 1

A closed system is a thermodynamic system that allows the transferring of energy across its boundaries but does not allow the exchange of matter with its surroundings. In other words, the energy can flow into or out of the closed system, while the total mass of the system remains constant. An example of a closed system is a pot of boiling water with a lid. Heat can enter and exit the pot to boil the water. The amount of water inside the pot remains the same.

An open system allows both energy and matter to be exchanged with its surroundings. In other words, an open system has a free flow of matter and energy across its boundaries. These systems are more common in nature. They often involve interactions with the environment. A pot of boiling water without a lid is an example of an open system. In this case, the water vapors can escape, and fresh air can enter. So in addition to exchange of heat, the exchange of matter also occurs in this case.

An isolated system does not exchange energy or matter with its surroundings. It is entirely self contained. There is no transferring of heat, work, or mass, across its boundaries. An example of an isolated system could be an insulated, perfectly sealed, and rigid container with no interaction with the external environment. For an isolated system, the change in internal energy is zero. This is because there is no energy exchange with the surroundings.

A reversible reaction is a chemical reaction that can proceed in both the forward and reverse directions. This means that the reactants can form products, and at the same time, the products can react to form the original reactants. Reversible reactions are denoted using a double arrow. This indicates that the reaction can take place in both directions.

Reversible reactions are dynamic. This means that they are constantly shifting between reactants and products, depending on the conditions. The formation of water from hydrogen gas and oxygen gas is an example of the reversible reaction. Another example of the reversible reaction is the dissociation of ammonium chloride into the ammonia gas and hydrogen chloride gas.

Chemical equilibrium is a state in which the rates of the forward and the reverse reactions are equal. At equilibrium, the concentrations of reactants and products remain constant. The system does not exhibit any macroscopic changes over time.Let us consider an example of a chemical equilibrium involving the reaction between nitrogen dioxide and dinitrogen tetroxide. In this reversible reaction, the dinitrogen tetroxide is in equilibrium with the nitrogen dioxide gas.

This reaction can proceed in both forward and reverse directions. In the forward reaction, dinitrogen tetroxide decomposes to produce two molecules of nitrogen dioxide. In the reverse reaction, two molecules of nitrogen dioxide combine to form dinitrogen tetroxide.

There is an excess of dinitrogen tetroxide at the start of the reaction. As the reaction proceeds, some of the dinitrogen tetroxide molecules will decompose into the nitrogen dioxide molecules. At the same time, as the nitrogen dioxide molecules are formed, some of the nitrogen dioxide molecules will combine to produce dinitrogen tetroxide. This process continues until the rate of the forward reaction and the rate of the reverse reaction becomes equal. As a result, a state of the chemical equilibrium is established. Chemical equilibrium is also called the dynamic equilibrium. The term dynamic highlights the continuous nature of the reaction.

Let us draw a graph to demostrate the dynamic equilibrium for a reversible reaction. Rate is plotted on the y axis. Time is plotted on the x axis. We can see that initially, the rate of forward reaction is high and the rate of reverse reaction is almost zero. But as time passes, the rate of forward reaction starts to decrease. With the passage of time, the rate of reverse reaction increases. A point comes when the rate of forward reaction becomes equal to the rate of reverse reaction. From this point there is a straight-line parallel to the x axis. This line represents the state of dynamic equilibrium.

Liquid gas equilibria refers to the equilibrium state between a liquid phase of a substance and its corresponding gas phase in a closed system. This equilibrium occurs when a liquid and its vapor coexist in a closed container. There is a continuous exchange of molecules between the liquid and gas phases through evaporation and condensation.

Let us first understand evaporation and condensation. Evaporation is the process by which a liquid changes into a gas. During evaporation the molecules at the surface of the liquid gain enough kinetic energy to escape into the gas phase. Condensation is the process in which the gas molecules undergo loss of energy and return to the liquid phase.

At dynamic equilibrium, the following happens. The rate of evaporation of molecules is equal to the rate of condensation. An example of the liquid gas equilibria is boiling of water. The boiling point of water is 100 Celsius. At the boiling point, the liquid water coexists with water vapor. At this temperature and pressure, the rate of evaporation of water molecules becomes equal to the rate of condensation of water vapor. This is the state of dynamic equilibrium.

Solid gas equilibria refers to the dynamic equilibrium that occurs between the solid phase of a substance and its corresponding vapor phase. In this equilibrium, there is a continuous exchange of molecules between the solid phase and the gas phase through the sublimation and the deposition. Let us first understand sublimation and deposition.

Sublimation is the process in which a solid substance changes directly into a gas without undergoing a liquid phase. Deposition is the process in which a gas directly converts in to solid without passing through the liquid phase. At equilibrium the rate of sublimation of a substance is equal to the rate of its deposition.

Let us understand solid gas equilibria with an example. When the solid iodine is kept in a closed vessel, it changes directly into the gas phase. At the same time, the gaseous iodine converts again into the solid iodine. The rate of sublimation of the solid iodine and the rate of deposition of the gaseous iodine becomes equal. As a result, the dynamic equilibrium is established between the liquid phase and the gaseous phase of the iodine.