Chemical - Ionic Equilibrium

Equilibrium state

In thermodynamics, an equilibrium state is a state of a system in which there are no net changes in the macroscopic properties of the system over time. It is a state of balance between opposing processes, such as opposing chemical reactions, or opposing physical processes, such as evaporation and condensation.

In a chemical equilibrium state, the forward and backward reactions are occurring at the same rate, and the concentrations of reactants and products remain constant. This means that the system is in a state of balance, and there is no net change in the concentrations of the reactants and products over time.

The concept of equilibrium is important in thermodynamics because it allows us to predict the behavior of systems under different conditions. By understanding the conditions under which a system can reach equilibrium, we can predict whether a given reaction will occur, and how much of each product will be formed. This knowledge is critical in many fields, including chemistry, biology, and engineering, where understanding the behavior of complex systems is essential for designing new materials and processes.

 Equilibrium mixture

An equilibrium mixture is a mixture of reactants and products that exists in a chemical reaction when the rate of the forward reaction is equal to the rate of the reverse reaction. In other words, it is a state of dynamic balance where the concentrations of reactants and products remain constant over time. At equilibrium, the forward and reverse reactions occur simultaneously, resulting in no net change in the concentrations of the reactants and products. The composition of the equilibrium mixture depends on the nature of the reactants and the conditions under which the reaction is taking place. The equilibrium mixture is a crucial concept in chemical thermodynamics as it allows us to predict the behavior of chemical systems at equilibrium and to understand the factors that influence the position of equilibrium.


Equilibrium in physical processes refers to a state in which there is no net change in the macroscopic properties of a system with time. In other words, the system is in a stable state and there is no observable change in its overall properties.

Some examples of physical processes in which equilibrium occurs are:

1.     Phase Equilibrium: When two or more phases of matter are present in a system, they can exist in equilibrium with one another. For example, water and ice can coexist in equilibrium at the melting point of ice.

2.     Chemical Equilibrium: This type of equilibrium occurs when the rate of forward and reverse reactions become equal, and there is no further net change in the concentration of reactants and products. This type of equilibrium is discussed in chemical thermodynamics.

3.     Thermal Equilibrium: When two or more bodies at different temperatures are in contact with each other, they tend to exchange heat until they reach a common temperature. This state is called thermal equilibrium.

4.     Mechanical Equilibrium: When the net force on an object is zero, it is in a state of mechanical equilibrium. This occurs when there is a balance of forces acting on the object.

In summary, equilibrium in physical processes occurs when there is no further net change in the macroscopic properties of a system with time. This state is characterized by the stability of the system, and the absence of observable changes in its properties.

Solid-Liquid Equilibrium

Solid-liquid equilibrium is a state of balance between a solid and liquid phase of a substance. At this equilibrium, the rate of melting (solid to liquid) and the rate of freezing (liquid to solid) are equal, resulting in a constant concentration of the solid and liquid phases in the system. This state is reached when the substance is heated to its melting point and kept under constant conditions of temperature and pressure.

The equilibrium is governed by the equilibrium constant, which is the ratio of the concentrations of the solid and liquid phases at the equilibrium state. This constant is specific to each substance and is affected by changes in temperature and pressure. In general, an increase in temperature or a decrease in pressure will shift the equilibrium towards the liquid phase, while a decrease in temperature or an increase in pressure will shift it towards the solid phase.

The concept of solid-liquid equilibrium is important in many areas of science and technology, including materials science, chemical engineering, and food science. It is used to understand the behavior of substances during melting and freezing processes, as well as to design and optimize processes for the production of materials and products.

Liquid-Vapour Equilibrium

Liquid-Vapour Equilibrium is a type of phase equilibrium in which a liquid and its vapour are in equilibrium with each other. This occurs when the rate of evaporation of the liquid is equal to the rate of condensation of the vapour.

At a given temperature and pressure, every liquid has a characteristic vapour pressure, which is the pressure exerted by the vapour in equilibrium with the liquid. If the vapour pressure of the liquid is equal to the external pressure, the liquid will boil and convert into vapour. Conversely, if the external pressure is reduced, the vapour pressure of the liquid will become higher than the external pressure and the liquid will evaporate.

The point at which the liquid and vapour phases are in equilibrium is called the boiling point of the liquid. At the boiling point, the vapour pressure of the liquid is equal to the external pressure and the liquid undergoes a phase change from liquid to vapour. The boiling point depends on the temperature and external pressure.

The liquid-vapour equilibrium is an important concept in many practical applications, such as in the design of distillation columns used in the separation of different components in a mixture.

Solid – Vapour Equilibrium

Let us now consider the systems where solids sublime to vapour phase. If we place solid iodine in a closed vessel, after sometime the vessel gets filled up with violet vapour and the intensity of colour increases with time. After certain time the intensity of colour becomes constant and at this stage equilibrium is attained. Hence solid iodine sublimes to give iodine vapour and the iodine vapour condenses to give solid iodine. The equilibrium can be represented as,

I2(solid) I2 (vapour)

Other examples showing this kind of equilibrium are,

Camphor (solid) Camphor (vapour)

NH4Cl (solid) NH4Cl (vapour)

Equilibrium Involving Dissolution of solids or gases in liquids

Solids in liquids

  • A solution is said to be saturated when it cannot dissolve any more solute at a given temperature.
  • The concentration of the solute in a saturated solution is dependent on temperature.
  • In a saturated solution, there is a dynamic equilibrium between the solute molecules in the solid state and in the solution.
  • The process of dissolution of the solute in the solution and the process of crystallization of the solute from the solution occur at equal rates.
  • The equality of the two rates and the dynamic nature of equilibrium has been confirmed using radioactive sugar.
  • When radioactive sugar is dropped into a saturated solution of non-radioactive sugar, after some time, radioactivity is observed in both the solution and the solid sugar.
  • Due to the dynamic nature of equilibrium, there is an exchange between the radioactive and non-radioactive sugar molecules between the two phases.
  • The ratio of the radioactive to non-radioactive molecules in the solution increases until it attains a constant value.

Gases in liquids

  • The solubility of gases in liquids is influenced by pressure and temperature.
  • Henry's Law states that the mass of a gas dissolved in a given mass of a solvent at any temperature is proportional to the pressure of the gas above the solvent.
  • The solubility of a gas in a liquid decreases with increasing temperature.
  • The equilibrium between the gas phase and the dissolved phase is governed by Henry's Law.
  • When a soda water bottle is opened, the pressure over the solution decreases, causing some of the dissolved gas to escape until a new equilibrium is reached.
  • For solid-liquid equilibrium, there is only one temperature at which the two phases can coexist.
  • For liquid-vapour equilibrium, the vapour pressure is constant at a given temperature.
  • For dissolution of solids in liquids, the solubility is constant at a given temperature.
  • For dissolution of gases in liquids, the concentration of a gas in liquid is proportional to the pressure of the gas over the liquid.

General Characteristics of Equilibria involving physical processess

For the physical processes discussed above, following characteristics are common to the system at equilibrium:

(i) Equilibrium is possible only in a closed system at a given temperature.

(ii) Both the opposing processes occur at the same rate and there is a dynamic but stable condition.

(iii) All measurable properties of the system remain constant.

(iv) When equilibrium is attained for a physical process, it is characterised by constant value of one of its parameters at a given temperature.

(v) The magnitude of such quantities at any stage indicates the extent to which the physical process has proceeded before reaching equilibrium.