Thermodynamics:
Entropy
Thermodynamics Gateway Page
In this module:
Introduction
Disorder in Atoms
Disorder in Energy
Measuring Entropy
Entropy of Phase Changes
Patterns in the Entropies of Substances
Entropy in Thermochemical Equations
The Second Law of Thermodynamics
The Effect of Temperature
Predicting How Reactions will Go
Two Examples

Entropy of Phase Changes

Sometimes calculating entropy changes using equation (1) is not so complicated. If the temperature stays constant as heat is added or removed, equation (1) is easy to use.

During phase changes, as discussed in Enthalpy 4 and 5, the temperature stays constant while the substance accepts or gives up heat, until the phase change is complete. Thus, equation (1) can be applied directly.

Predict the entropy change when 100.0 g of liquid benzene (C6H6) is converted to vapor at its boiling point, 80.1 ºC. The DHºvap for benzene is 394 J/g.

Correct! See the complete solution below.
Remember to convert the temperature to Kelvins.
The solution is shown below.
That is incorrect. Try again.

DSº = q/T = mDºvap/T
DSº = 100 g • 394 J/g/(80.1 + 273.15)K = 112 J/K

Calculate the entropy change when 50.0 g of liquid water freezes to ice at its freezing point, 0.00 ºC. The DHºfus of water is 333 J/g.

Correct! See the complete solution below.
Remember to convert the temperature to Kelvins.
Since the system is losing energy, the sign
of its enthalpy change must be negative.
The solution is shown below.
That is incorrect. Try again.

DSº = q/T = -mDºfus/T
DSº = -50 g • 333 J/g/(0 + 273.15)K = -61.0 J/K

Entropy of Phase Changes