In a system, the temperature-dependent amount of heat that cannot be utilised to benefit the system is its entropy. As molecular motion is the source of work, entropy is also a measure of disorder or randomness. The heat absorbed when one mole of a substance melts at its melting point reversibly is the molar heat of fusion. ∆S = ∆Hf / Tf is the entropy change formula. Tf is the melting temperature, while ∆Hf is the molar heat of fusion. Because the value of entropy or Entropy Change depends on the substance present in a thermodynamic system, entropy is indicated by the letter ‘S’. Entropy is a fascinating concept since it questions the notion of complete heat transfer. It aids in the reinterpretation of the second law of thermodynamics.

Entropy characteristics

Below given are some of the essential features of a thermodynamic system’s entropy:

• The tendency of the cosmos to move towards disorder or unpredictability is known as entropy.
• Entropy is a function of enthalpy or the amount of heat that can be transformed into work.
• The mass of a thermodynamic system affects entropy. It is a broad quality since it is independent of heat exchange or heat conversion path.
• The entropy of the universe continues to rise.
• The adiabatic process has constant entropy because the change in entropy is zero.

Entropy interpretations

• Von Neumann used a density matrix to extend the concept of entropy to the quantum domain in quantum statistical mechanics.
• It refers to a measure of how efficiently a system transmits a signal or the quantity of information lost in a transmitted signal in the context of information theory.
• In dynamic systems, entropy represents the increasing complexity of a system. It also calculates the average rate of information flow per unit of time.
• According to sociology, entropy is the social deterioration or natural decay of structure in a social system (such as law, organisation, and convention).
• The tendency of the cosmos to approach a state of maximum homogeneity is referred to as entropy in cosmology. A constant temperature implies a constant entropy.

Entropy of fusion

• Solids melt into liquids as their entropy increases. Phase change increases entropy by increasing the freedom of movement of molecules.
• The entropy of fusion is calculated by dividing the enthalpy of fusion by the melting point (fusion temperature).

∆fusS=∆fusH / Tf

• A natural process (such as fusion) will occur when the Gibbs free energy changes negatively.
• Most of the time, ∆fusS is positive.

Exception

• The entropy of fusion is negative for helium-3 at temperatures below 0.3 K. Entropy of fusion is also negative for helium-4 at temperatures below 0.8 K.

Importance

The entropy of fusion is usually the change in the randomness accompanied when a solid substance melts or fuses into a liquid. Entropy increases during the fusion process. Entropy measures the thermal energy per temperature of any physical process. The entropy of fusion can be observed in daily life physical changes like melting sugar, melting salt etc. The entropy of this process tells us whether the reaction is spontaneous or non-spontaneous. During the entropy of fusion, solid particles that are closely arranged move apart by gaining thermal energy, moving far apart, and becoming more random. The thermal energy absorbed breaks the intermolecular forces between the solid particles. The entropy of fusion is different for different substances. With entropy of fusion values, we can understand how easily a substance can be converted from the solid phase to the liquid phase. The entropy of fusion is not always spontaneous but can be non-spontaneous sometimes.

Example: The melting of ice or dry ice is a spontaneous process that occurs naturally.

Melting of gold, iron etc., needs a certain amount of energy that is approximately equal to their melting points. 

Conclusion

The enthalpy of fusion of a substance, also known as (latent) heat of fusion, is the change in enthalpy that ensues when energy, usually heat, is provided to a specified quantity of the substance to change its state from a solid to a liquid under constant pressure.