Equilibrium in Physical Processes

The equilibrium that occurs between distinct phases or physical qualities is referred to as physical equilibrium. There is no alteration in chemical structure in these operations. It depicts the same material existing in two different physical states. 

• Phase equilibrium is a type of physical equilibrium.
• Equilibrium of solutes and solids
• Liquid-gas equilibrium

Equilibrium refers to the consistency of content and composition (as assessed by color, pressure, or temperature) of a system’s item of interest across time. The forward response rate is equal to the backward rate of the reaction in the Equilibrium state.

Some instances of equilibrium are a book on a table, liquid in a confined container, saturated solution, ionic compounds in polar solvents, and the production of ammonia.

Types of Physical Equilibrium

Equilibrium in Phase:

At 0°C, the amount of water molecules that turn into ice equals the number of water molecules that melt into liquid water. The pace at which water freezes is the same as the rate at which ice melts. As a result, there is a balance between solid ice and liquid.

Ice (s) ⇌ Water (l)

In a closed container, the number of molecules that become vapour is equal to the number of molecules that condense into liquid. The pace at which liquid water evaporates is equal to the rate at which water vapour condenses. The liquid phase and its own vapour phase are in balance.

Water (l) ⇌ Water (g)

Equilibrium of Solutes and Solids:

When a saturated solution comes into contact with a non – soluble solute, the number of molecules leaving the solution (depositing) equals the number of molecules entering the liquid (dissolving) from the solid. As a result, the undissolved solid and the solute in a solution are in equilibrium.

Equilibrium of Gases and Liquids:

Gases which do not react with liquids may dissolve in direct proportion to the liquid’s pressure. The air within the liquid and the gas present just above the liquid are in balance in a closed container. The carbon dioxide gas in soft drinks, for example, is in balance with the gas in the container’s empty space.

Gas (solution) ⇌ Gas (g)

Chemical Processes and Equilibrium

Reversible Reaction

These responses occur in both forward and backward directions, as the name implies. The concentrations of the reactants and the products stay constant in such reactions when the forward and reverse reaction rates are identical. The reaction seems to have been in chemical equilibrium at this point.

This equilibrium, however, is believed to be dynamic in nature. This is due to the fact that it has a forward reaction in which the reactants react to produce products and a reverse reaction in which the products might combine to produce the original reactants. Let’s have a look at these chemical reactions in more detail.

Dynamic Nature of Chemical Processes in Equilibrium

A + B ⇌ C + D

The products (C and D) increase with time while the reactants (A and B) diminish. As a result, the rate of forward response decreases while the rate of reverse reaction increases. Both reactions eventually occur at the same moment, resulting in a state of equilibrium. This balance can be achieved in either way.

Chemical equilibrium is Bidirectional

Equilibrium can be achieved on both sides of a reaction, whether we start with reactants or products. Let’s have a look at the next reaction.

H2(g) + I2(g) ⇌ 2HI(g)

If we catalyze the reaction with constant concentration of H2 and I2, the reaction will proceed in the forward direction, with the percentages of H2 and I2 decreasing while the concentration of HI increasing until equilibrium is reached.

If we reverse the foregoing reaction, the concentration of HI falls while the concentrations of H2 and I2 grow until equilibrium is reached. As a result, if the total number of atoms of the element in a given volume is the same, the equilibrium mixture will be the same whether we start with reactants or products.

(i) When a liquid and a gas are in equilibrium, the liquid’s vapour pressure remains constant at a specific temperature.

(ii) At a given temperature, the concentration of the solute in solutions becomes constant when solid = solution equilibrium occurs.

(iii) At a certain temperature, the pressure of the gas just above liquid becomes constant in the situation of gas = solution equilibrium.

(iv) When two phases are in equilibrium, there is only one temperature (melting point) at which they may coexist, i.e. temperature remains constant at a given pressure.

Physical Equilibrium has the following characteristics:

1. Only in the situation of a closed system can equilibrium be achieved.

2. The equilibrium is dynamic in character, meaning that the process does not come to a halt once equilibrium is reached, but the rate of forward response equals the rate of backward reaction.

3. Because the concentration of the chemicals remains constant, the system’s quantifiable characteristics such as melting point, boiling temperature, vapour pressure, and solubility remain constant.

4. When equilibrium is reached, an expression involving the concentration of reacting components takes on a constant value at a specific temperature.

5. When a gas dissolves in a liquid, increasing the pressure always improves the gas’s solubility in the liquid.

Conclusion:

Equilibrium is the state of a process in which parameters such as temperature, pressure, and flow are in balance, pressure, with the flow of time, the system’s concentration does not alter. The equilibrium is termed physical equilibrium when the opposing processes simply involve physical changes. The equilibrium is termed chemical equilibrium when the opposing processes entail chemical changes, i.e. when the opposing processes are chemical reactions. At equilibrium, both opposing processes occur at the same time. Equilibrium mixture refers to a mixture of reactants and the products that is in a state of equilibrium.

The following is an example of a chemical reaction:

aA + bB ⇌ xX + yY

The reactants are A and B, while the products are X and Y. The double arrow between reactants and products indicates that the reaction is occurring in both directions at the same time.