Chem 2 - The Second Law of Thermodynamics: Entropy and Heat IV

1. The Second Law of
Thermodynamics:
Entropy and Heat, plus
Examples
(Pt 4)
By Shawn P. Shields, Ph.D.
This work is licensed by Shawn P. Shields-Maxwell under a Creative Commons
Attribution-NonCommercial-ShareAlike 4.0 International License.

2. Recall: The Second Law of Thermodynamics
The total entropy change for any
spontaneous process is positive
Positional disorder- The distribution of
molecules (particles) in space (related to
volume)
Thermal disorder- The distribution of
energy states among the particles
(related to temperature)

3. Entropy and Heat
Heat flows occurs spontaneously
from hot objects to colder ones.
Objects in contact
will end up at the
same temperature.
“Heatflow" from
http://www.chem1.com/acad/webtext/thermeq/TE1.html

4. Predicting the Sign of S for Changes in
Temperature
Entropy increases with increasing
temperature.
As heat is added to the system,
the number of accessible energy
states increases.

5. Example: Entropy of Fusion (Sfus)
The entropy associated with fusion
(melting) is
∆Sfus =
qrev
T
=
∆Hfus
T
qrev is the heat associated with a
reversible process.

6. Heating and
Entropy
“A Generalized Plot of Entropy versus
Temperature for a Single Substance
Absolute” from
http://2012books.lardbucket.org/books/principle
s-of-general-chemistry-v1.0m/s22-04-entropy-
changes-and-the-third-.html
Entropy increases
steadily with T.
Large jumps are
observed during
phase transitions.

7. S for Phase Transitions
Fusion (melting):
S increases as a substance melts.
In the solid, particles are essentially
fixed into place.
As the substance melts, a larger
number of positions are accessible to
the molecules (in the liquid state)

8. Predict the Sign of S for Vaporization
What is the sign of S for
vaporization?

9. Predict the Sign of S for Vaporization
What is the sign of S for
vaporization?
As the liquid vaporizes, more
positions are available to the gas
molecules.
S is positive.

10. Next up…
The Third Law and Standard
Molar Entropy (S) (Pt 5)