Entropy Changes During Phase Transition

During a phase transition, such as melting or vaporization, there is a change in the entropy of the system as follows:

Entropy of Fusion

The entropy of fusion is the increase in entropy when melting a solid substance. It is almost always positive since the degree of disorder increases in the transition from a solid to a liquid state. The entropy of fusion is denoted as ΔS_fus and is typically expressed in joules per mole-kelvin (J/(mol·K))

The entropy of fusion is related to the heat of fusion and the melting point. The entropy of fusion can be calculated using the equation:

ΔS_fus = Q_fus/T_m

Where ΔS_fus is the entropy of fusion, Q_fus is the heat of fusion, and T_m is the melting point.

Key Points on Entropy of Fusion:

• The entropy of fusion is always positive for most substances, as the liquid state has a higher disorder than the solid state
• Helium has a negative entropy of fusion at temperatures below 0.3 K, while helium-4 has a very slightly negative entropy of fusion below 0.8 K.
• At temperatures well above their melting points, the entropy of fusion becomes positive, and the entropy approaches zero as the temperature approaches the melting point.

Entropy of Vaporisation

Entropy of Vaporization is the increase in entropy when a liquid substance evaporates. It is always positive since the degree of disorder increases in the transition from a liquid in a relatively small volume to a gas in a larger volume. The entropy of vaporization is denoted as ΔS_vap and is typically expressed in joules per mole-kelvin (J/(mol·K))

ΔS_vap = Q_vap/T_b

where,

• ΔS_vap is Entropy of Vaporisation
• Q_vap is the heat of Vaporisation
• T_b is boiling point

Key Points on Entropy of Vaporization:

• The entropy of vaporization is related to the heat of vaporization and the boiling point. According to Trouton’s rule, the entropy of vaporization (at standard pressure) of most liquids has similar values, typically around 85 J/(mol·K), 88 J/(mol·K), or 90 J/(mol·K)

• This rule is based on the observation that the entropy of a gas is considerably larger than that of a liquid, and the entropy of the initial state (e.g., the liquid) is negligible in determining the overall entropy change during vaporization.

• Hydrogen-bonded liquids have somewhat higher values of entropy of vaporization, while ordered gases and some other substances deviate from Trouton’s rule.

• Entropy of vaporization provides insight into the degree of disorder in the vapor and liquid phases and helps explain why specific processes, such as vaporization, occur spontaneously.

Entropy means the amount of disorder or randomness of a system. It is a measure of thermal energy per unit of the system which is unavailable for doing work. The concept of entropy can be applied in various contexts and stages, including cosmology, economics, and thermodynamics. Entropy is a concept that essentially discusses the spontaneous changes that take place in ordinary phenomena or the Universe’s inclination towards disorder.

In this article, we will learn what is meaning of Entropy, the entropy change formula, and how it is associated with the laws of thermodynamics.