Wednesday, Dec. 5th: “A” Day
Thursday, Dec. 6th: “B” Day
Agenda
 Homework Questions/Problems?
 Sec. 11.2 Quiz: “Intermolecular Forces”
 Section 11.3: “Energy of State Changes”
Enthalpy, Entropy, and Changes of Sate, Gibbs Energy and
Changes of State, Calculating Melting and Boiling Points
 Lab write-up: “Energy and Entropy: Phase Changes”
 Homework
Sec 11.3 review, pg. 398: #1-3, 6-9
“What Would Life Be Like?” Paper
Concept Review: “Energy of State Changes”
Homework
pg. 392: #6-12
Problems?
Questions?
Section 11.2 Quiz
“Intermolecular Forces”
You may use your notes
and your book to
complete the quiz on
your own…
Good Luck!
Enthalpy, Entropy, and Changes of
State
Enthalpy is the total energy of a system.
Entropy measures a system’s disorder.
The energy added during melting or removed
during freezing is called the enthalpy of fusion.
Particle motion is more random in the liquid
state, so as a solid melts, the entropy of its
particles increases. This increase is called the
entropy of fusion.
Enthalpy, Entropy, and Changes of
State
As a liquid evaporates, a lot of energy is
needed to separate the particles. This energy is
called the enthalpy of vaporization.
Particle motion is much more random in a gas
than in a liquid. A substance’s entropy of
vaporization is much larger than its entropy of
fusion.
Enthalpy and Entropy Changes for
Melting and Evaporation
Enthalpy and entropy both change as energy (in
the form of heat) is added to a substance.
The energy added as ice melts at its melting
point is the molar enthalpy of fusion (∆Hfus).
∆Hfus is the difference in enthalpy between solid
and liquid water at 273 K (O˚C).
∆Hfus = H(liquid at melting point)  H(solid at melting point)
Enthalpy and Entropy Changes for
Melting and Evaporation
Enthalpy and Entropy Changes for
Melting and Evaporation
The energy added as liquid evaporates at its
boiling point is the molar enthalpy of
vaporization (∆Hvap).
∆Hvap is the difference in enthalpy between liquid
and gaseous water at 373 K (100˚C).
∆Hvap = H(vapor at boiling point)  H(liquid at boiling point)
Enthalpy and Entropy Changes for
Melting and Evaporation
Because intermolecular forces are not significant in
the gaseous state, most substances have similar
values for molar entropy of vaporization, ∆Svap.
Gibbs Energy and State Changes
The relative values of H and S determine whether
any process, including a state change, will take
place.
A change in Gibbs energy is defined as:
∆G = ∆H  T∆S
A process is spontaneous if ∆G is negative
OR if ∆S is positive.
If ∆G is positive, then a process will not take place
unless there is an outside source of energy.
Gibbs Energy and State Changes
If ∆G is zero, then the system is at equilibrium.
At equilibrium, the forward and reverse
processes are happening at the same rate.
For example, when solid ice and liquid water
are at equilibrium, ice melts at the same rate
that water freezes.
Determining Melting and Boiling
Points
For a system at the melting point, a solid and a
liquid are in equilibrium, so ∆G is zero.
Remember, ∆G = ∆H - T∆S and if ∆G is zero,
then ∆H = T∆S.
Rearranging the equation, you get the
following equation for the melting point of a
substance:
Determining Melting and Boiling
Points
For a system at the boiling point, a gas and a
liquid are in equilibrium.
Rearranging the equation as before, you get
the following equation for boiling point :
When ∆Hvap > T∆Svap, the liquid state is favored.
When ∆Hvap < T∆Svap, the gaseous state is favored.
Pressure Can Affect
Change-of-State Processes
Boiling points are pressure dependent
because pressure has a large effect on the
entropy of a gas.
When a gas expands (pressure is decreased)
its entropy increases because the degree of
disorder of the molecules increases.
Thus, boiling points change as atmospheric
pressure changes due to changes in
elevation.
Pressure Can Affect
Change-of-State Processes
Liquids and solids are almost incompressible.
Therefore, changes in atmospheric pressure
have little effect on the entropy of substances
in liquid or solid states.
Also, ordinary changes in pressure have
essentially no effect on melting and freezing.
Thus, melting and freezing points hardly
change at all with changes in elevation.
Sample Problem A
Pg. 397
The enthalpy of fusion of mercury is 11.42 J/g, and
the molar entropy of fusion is 9.79 J/K∙mol. The
enthalpy of vaporization at the boiling point is
294.7 J/g, and the molar entropy of vaporization
is 93.8 J/K∙mol. Use the molar mass of mercury,
200.59 g/mol to calculate the melting point and
the boiling point.
Practice Problem #1
Pg. 397
For ethyl alcohol, C2H5OH, the enthalpy of fusion is
108.9 J/g, and the entropy of fusion is
31.6 J/K∙mol. The enthalpy of vaporization at the
boiling point is 837 J/g, and the molar entropy of
vaporization is 109.9 J/K∙mol. Calculate the
freezing and boiling points for each substance.
(Hint: you need the molar mass of ethyl alcohol)
“Energy and Entropy: Phase Changes”
Lab Discussion
As heat energy flows from a liquid, its
temperature drops.
The entropy (degree of disorder) also decreases.
The continuous flow of energy from a liquid will
cause the liquid to eventually undergo a phase
change to a solid.
“Energy and Entropy: Phase Changes”
Lab Discussion
During the time needed for the phase change to
be completed, the temperature of a pure
substance will not change.
Entropy, however, will continue to decrease.
Once the phase change is complete, the
temperature of the solid will decrease as energy
continues to be removed.
“Energy and Entropy: Phase Changes”
Purpose
 In this lab, you will monitor the temperature of
sodium thiosulfate pentahydrate (Na2S2O3∙ 5 H2O)
as it is cooled to several degrees below its freezing
temperature and then warmed to several degrees
above its melting temperature.
 The results of the lab will allow you to determine
the freezing and melting temperatures of sodium
thiosulfate pentahydrate and to interpret changes
in energy and entropy.
“Energy and Entropy: Phase Changes”
Materials
The materials for this lab are as listed under
“Apparatus” and “Materials” section of the
“Experiment 5” handout.
You will be using a bunsen burner, NOT a hot
plate.
Listen for other changes….
“Energy and Entropy: Phase Changes”
Procedure
For this lab, you will follow the procedure
from the “Experiment 5” handout but
complete the data table, analysis, conclusion,
and extension questions from the
“Construction a Heating/Cooling Curve”
handout.
Homework
Section 11.3 review, pg. 398: #1-3, 6-9
Finish lab write-up: “Energy and Entropy: Phase
Changes”
Concept Review: “Energy of State Changes”
*Remember*
Your “What Would Life Be Like?” paper is
due next week!