Entropy measures the state of randomness or a measure of disorder. It is an important scientific concept that helps calculate the state of disorder or uncertainty—the entropy changes during phase transition increase when a phase transitions towards higher internal energy. However, entropy changes during phase transition decrease when the change lowers the internal energy. When there is a change in the phase, it leads to a change in the randomness or organisation of the molecules and hence impacts the overall entropy. During the phase transition, the temperature remaining constant impacts the entropy. The heat transfer is reversible at a constant phase transition temperature. The topic includes comprehensive information and entropy changes during phase transition notes. 

Why do entropy changes during phase transition occur?

The molecules exist in solid, liquid, and gaseous states. Each state has its entropy, and it varies based on the type of phase, and the level of entropy changes based on the phase. 

• Solid has a regular arrangement of the molecules, and hence it is tightly packed. The level of disorder in the solid is low and therefore has the lowest entropy.
• The liquid is free-flowing but does not readily disperse. However, as it is not tightly packed like a solid, it has a higher level of entropy than a solid.
• Gases readily disperse and hence have a high level of randomness or disorganisation of molecules. As the level of randomness is higher, it has the highest entropy compared to all the types of molecules.

Hence, it is found that there are entropy changes during the phase transition from solid to gaseous and vice versa. As there is a change in the state from solid to gaseous, it gradually increases the level of entropy. However, when there is a change in the state from gas to solid, it decreases the level of entropy.

Entropy in terms of Thermodynamics 

Entropy can be explained thermodynamically and is associated majorly with three laws of thermodynamics. Thermodynamics is an important branch of physics that helps show the relation between heat and various forms of energy. It helps establish the relationship between all forms of energy and how it is transferred from one form to another. The entropy changes during phase transition are thermodynamically shown as energy is transferred from one phase to another due to an increase in the random disorder.

• According to the first law of thermodynamics, energy cannot be destroyed nor created, but it can be changed from one form to another. Hence, where there is a phase transition, it leads to a surge in the entropy. This is because the number of moles of gaseous products rises compared to the reactants.
• According to the second law of thermodynamics, heat transfer into work takes place only at the expense of energy; hence, there is a continuous surging of entropy level due to the transfer of heat into work.
• As per the third law of thermodynamics, when the temperature of solid crystalline is at its absolute temperature, it increases the entropy to zero. Hence, the entropy will move toward zero when a phase is in perfect order.

Entropy changes during phase transition occur, and free energy changes

• As there is an increase in the temperature, it increases the random movement of the molecules and hence increases the rate of entropy.
• The entropy changes during phase transition increase when there is melting of the phase as it leads to higher internal energy change. However, there is a decrease in the system’s entropy if the phase is towards lower internal energy, and it leads to the transition of the phase from gaseous or liquid to solid.
• At thermodynamic equilibrium, when the phase transition occurs quasi–statistically, the total entropy change of the system and surroundings is zero.

Free energy is the internal energy of a thermodynamic system that helps perform work. Gibbs free energy is the energy that is converted into the system at constant pressure and temperature. Hence, it is shown by the following equation:

G = H – TS

Where G is Gibbs free energy

H is the enthalpy

T is the temperature

S is the entropy

When there is a phase transition, the system releases or absorbs a large amount of energy per volume change. Hence, when there is an increase in heat and temperature, it increases the disorder of the molecules and increases the entropy. As the heat energy increases, it increases the kinetic energy, which breaks the intermolecular binding forces.

Conclusion

The entropy changes during phase transition notes provide comprehensive information about how entropy changes when there is a change in the phase. As there is a phase change from solid to gaseous, it leads to an increase in entropy. The solid has a regular arrangement and is compact; hence it has a low disorder of molecules. Therefore, the entropy is low. The liquid has higher entropy than solid but lower entropy than gaseous. The entropy changes during phase transition; it increases the rate of entropy when a solid is converted into a gaseous phase. Entropy is a concept that can be explained thermodynamically and how its transformation from one form of energy to another impacts entropy.