Key Notes On Types Of Physical Equilibrium

Equilibrium is the process in which the temperature, pressure, and all the macroscopic processes remain the same and do not change throughout the process of reaction; in an equilibrium state, the forward reaction is equal to the backward reaction. The Equilibrium reaction is of two types –

1. Chemical Equilibrium 
2. Physical Equilibrium

Chemical Equilibrium

Chemical Equilibrium is a state of equilibrium in which the forward reaction and reverse reaction takes place at a similar pace or same rate. The other name of Chemical Equilibrium is also known as dynamic equilibrium. It is called dynamic equilibrium because the reaction does not stop even after reaching an equilibrium position. The individual molecule in a Chemical Equilibrium continuously reacts and there is no change in the actual amount of product and reactants, which simply means that the concentration of reactant and product remains the same before and after the reaction is in an equilibrium state.

Example of reversible reaction – 

2SO2 (Sulphur Dioxide) + O2 ((Oxygen)   450°C and 200 atm↔ 2SO3 (Sulphur Trioxide)

The catalyst used in this reversible reaction is V2O5

The dual direction of the arrow in the reaction shows that the reaction is a reversible reaction i.e. the reactant gets converted into the product (forward reaction) and the product again can get converted to the reactant (backward reaction). 

The two types of Chemical Equilibrium are as follows:-

• Homogeneous Equilibrium
• Heterogeneous Equilibrium

Physical Equilibrium

Changing a substance from one state or phase to another state or phase is called a physical process. For example – the state of changing water to solid, gas, and again to liquid.

The equilibrium attained between two phases of the same substance when the rates of forwarding and reverse reaction changes are equal is called physical equilibrium. 

H2O (solid) heat↔ H2O (liquid)

The state of solid i.e. ice can be changed from that state to liquid i.e. water by heating the ice and vice versa. It is a reversible reaction. The arrow between the reaction shows that the reaction is reversible. The physical equilibrium can be classified into the following types –

Phase Equilibria 

The measurement of molecules of water that turns into ice at 0℃ or solid-state is similar to or equal to the measurement of water molecules in a liquid state. This occurs due to the condition that ice which is the solid-state of water is melting to form the liquid state that is water itself. The rate of freezing of water i.e. the solid is similar to or equal to the rate of melting of ice that is liquid. As a consequence of this, an equilibrium state between the ice and water i.e. between the solid and liquid state will form.

Therefore,

Ice (solid) heat↔ Water (liquid)

One of the most active equilibriums is the Phase Equilibria of Chemistry.  A container whose four sides are closed will have the molecules turning into vapor equal to the measurement of molecules that are condensing to form a liquid. As a consequence of this, it can be said that the rate of evaporation of the liquid water by heating is equivalent to the rate of condensation of water vapor by cooling. 

Therefore,

Water (liquid) heat↔ Water (gas)

Solid-Liquid Equilibrium

The rate of transfer of molecules from solid i.e. ice to liquid i.e. water is equivalent to the rate of transfer of molecules from liquid i.e. water to solid i.e. ice; that simply defines the equilibrium between the solid and the liquid.  The rate of transfer of solid i.e. ice to liquid i.e. water is equivalent to the rate of transfer of liquid i.e. water to solid i.e. ice.

Gas-Liquid Equilibrium 

The rate of transfer of molecules from liquid i.e. water to vapor i.e. the gas is equivalent to the rate of water molecules that condenses to form liquid again.

H2O (liquid) heat↔ H2O (gas)

H2O (gas) cooling↔ H2O (liquid)

Phase Equilibria Questions

• At equilibrium the distribution between the two or more phases is determined by –

1. Gibbs free energy’s application
2. Entropy’s application
3. Newton’s Laws of motion’s application
4. Application of force

Answer: Option 1

• Write the ratio of phase equilibrium.

1. At equilibrium in two phases the temperature ratio.
2. At equilibrium in two phases the species’ equilibrium concentration ratio.
3. At equilibrium in two phases the species’ mole fraction ratio.
4. At equilibrium in two phases the species’ heat ratio.

Answer: Option 3

Conclusion

It is to conclude that the state of equilibrium is a condition in which the systematic and macroscopic processes remain as it is throughout the process of reaction. It simply means that from reactants as products are formed in a reaction, in equilibrium reactions from products the reactant can be formed i.e. the forward reaction is equivalent to the backward reaction.