Explain entropy change during phase transitions.

Short Answer:

Entropy change during phase transitions refers to the change in the disorder or randomness of a substance when it changes from one phase to another, such as from solid to liquid or liquid to gas. During these transitions, even though temperature remains constant, entropy increases because the molecules become more spread out and disordered.

For example, when ice melts into water, the molecular structure becomes more random, leading to an increase in entropy. Similarly, during boiling, liquid water turns into vapor, and the molecular freedom increases further, causing a larger entropy gain. This concept helps understand energy behavior during heating and cooling.

Detailed Explanation:

Entropy change during phase transitions

In thermodynamics, entropy (S) is a measure of disorder or randomness in a system. It also tells us about the quality of energy and how much energy becomes unavailable for doing useful work. When a substance changes its phase — like melting, boiling, or condensing — there is a significant reorganization of molecules, which leads to a change in entropy.

Phase transition means a change from one physical state to another — solid, liquid, or gas. These transitions happen at constant temperature and pressure, but even without a temperature change, there is a change in entropy because of the change in molecular arrangement.

How Entropy Changes in Different Phase Transitions

  1. Solid to Liquid (Melting / Fusion)

When a solid melts into a liquid:

  • The molecules move from a fixed, orderly structure to a more mobile and less ordered structure.
  • The entropy increases because the molecular motion increases.
  • Temperature remains constant during melting.
  • Example: Ice melting into water.
  1. Liquid to Gas (Boiling / Vaporization)

When a liquid becomes gas:

  • Molecules go from being close together to being far apart and highly disordered.
  • There is a large increase in entropy.
  • Boiling occurs at a constant temperature, but molecular randomness increases greatly.
  • Example: Water boiling to form steam.
  1. Solid to Gas (Sublimation)

When a solid directly becomes a gas:

  • There is a very high increase in entropy because the structure jumps from solid to freely moving molecules.
  • Example: Dry ice (solid CO₂) sublimating to gas.
  1. Gas to Liquid (Condensation)

When vapor condenses into liquid:

  • Molecules come closer together, forming a more organized phase.
  • Entropy decreases because the disorder reduces.
  • Example: Steam turning into water on a cold surface.
  1. Liquid to Solid (Freezing)

When a liquid becomes a solid:

  • Molecular motion decreases.
  • Structure becomes fixed and ordered.
  • Entropy decreases.
  • Example: Water freezing into ice.

Formula for Entropy Change During Phase Transition

For a reversible phase change, entropy change can be calculated using:

ΔS = Q_rev / T

Where:

  • ΔS = Entropy change
  • Q_rev = Heat added or removed during phase change (reversible)
  • T = Absolute temperature at which the phase change occurs (in Kelvin)

Since phase changes happen at constant temperature, we just divide the latent heat (like heat of fusion or vaporization) by the temperature.

Example:
If water absorbs 2260 kJ/kg of heat at 373 K during vaporization, the entropy change is:

ΔS = 2260 / 373 = 6.06 kJ/kg·K

This value tells how much entropy increases per kilogram of water during boiling.

Importance in Thermodynamics and Engineering

  • Helps understand energy efficiency during heating and cooling.
  • Important in designing refrigerators, boilers, steam turbines, and condensers.
  • Used to calculate available energy and evaluate entropy generation.
  • Helps in phase change calculations in thermal systems and in predicting spontaneity of phase changes.

By understanding entropy changes, engineers can better manage heat transfer and energy usage in various mechanical systems.

Conclusion

Entropy change during phase transitions reflects the change in molecular disorder when a substance changes its phase. During melting and vaporization, entropy increases because molecules become more disordered. In contrast, during freezing and condensation, entropy decreases as molecules become more organized. Even though temperature remains constant during these transitions, entropy plays a major role in understanding how energy is transferred and used. This concept is vital in analyzing real-world thermal systems and improving energy efficiency.