Depression of the Freezing Point of a Solvent We need two pieces of information to calculate the depression of the freezing point of the solvent in a solution containing a nonvolatile nonelectrolyte: • The molal concentration, m, of the solute in the solution (Sometimes it may be necessary to calculate this concentration from other information.) • The freezing point depression constant, Kf, for the solvent. We use the following equation to calculate a freezing point depression. (Note: This equation assumes the freezing point depression constant has a positive value.)

1. Freezing Point Depression
BY: Dr. Robert D. Craig, Ph.D.

2. Concept of phase diagram

3. Concept of phase diagram

4. For carbon dioxide

5. A supercritical fluid
• A supercritical fluid is any substance at
a temperature and pressure above its
critical point, where distinct liquid and gas
phases do not exist. It can effuse through
solids like a gas, and dissolve materials
like a liquid. In addition, close to the
critical point, small changes in pressure or
temperature result in large changes in
density, allowing many properties of a
supercritical fluid to be "fine-tuned

6. To extract caffiene-

7. Freezing Point Depression
• Colligative properties include: relative
lowering of vapor pressure;
elevation of boiling point;
depression of freezing point and
osmotic pressure. Measurements of these
properties for a dilute aqueous solution of
a non-ionized solute such as urea or
glucose can lead to accurate
determinations of relative molecular
masses

8. Patients with diabetes

9. Depression of the Freezing
Point of a Solvent
• We need two pieces of information to calculate the
depression of the freezing point of the solvent in a
solution containing a nonvolatile nonelectrolyte:
• The molal concentration, m, of the solute in the solution
(Sometimes it may be necessary to calculate this
concentration from other information.)
• The freezing point depression constant, Kf, for the
solvent.
• We use the following equation to calculate a freezing
point depression. (Note: This equation assumes the
freezing point depression constant has a positive value.)

10. .
• As is demonstrated by the phase diagram
above, adding a solute to a solvent lowers the
freezing point and raises the boiling point; it also
lowers the vapor pressure.
• The new freezing point of a solution can be
determined using the colligative property law:
• ΔTf = kf m

11. .
• The change The change in freezing point
is equal to the molal freezing-point
constant times the molality of the solution.
The molal freezing-point constant used is
the constant for the solvent, not the solute.

12. .
• In this experiment, the molar mass of sulfur will be
determined using the colligative property law. The
freezing point of naphthalene will be determined
• experimentally; then a controlled solution of naphthalene
and sulfur will be made, and the freezing point of that
solution will be determined. The difference in freezing
point can be used in the colligative property law to
determine the experimental molality of the solution,
leading to a calculation

13. .
• The freezing temperature is difficult to ascertain
by direct visual observation because of a
phenomenon called supercooling and also
because solidification of solutions usually occurs
over a broad temperature range. Temperature-
time graphs, called cooling curves, reveal
freezing temperatures rather clearly.

14. .
• The cooling curve will look like the
• one below in figure 19.2:n of molecular
weight.

15. .
• In order to minimize supercooling, the
solution will be stirred while freezing.
• To determine the molar mass of a
substance, one must simply divide the
grams of substance by the number of
moles of substance present. All of these
values will be determined experimentally.

16. ,
• Procedure:
• Part A – Cooling Curve for Pure Naphthalene
• 1. A large test tube was weighed to the nearest .
01 g using a standard laboratory balance.
• Approximately 15 to 20 grams of naphthalene
was added to the test tube. The test tube
• was weighed again using the standard balance.

17. .
• 3. The 600-mL beaker was nearly filled with water. It was heated to
about 85°C. The test
• tube was clamped in the water bath as shown in Figure 19.3 above.
When most of the
• naphthalene had melted, the stopper containing the thermometer
and stirrer was placed
• into the test tube. The thermometer was not allowed to touch the
bottom or sides of the
• test tube

19. .
• 4. When all of the naphthalene had
melted, the test tube was removed from
the beaker of boiling water. The test tube
was placed into a wide-mouthed bottle
with some paper towels at the bottom. The
temperature reading from the
thermometer was recorded every 30
seconds.

20. • The naphthalene was stirred using the
wire stirrer to ensure even freezing.
• When the temperature remained constant
for several readings, the naphthalene was
• allowed to cool without further temperature
readings.

21. freezing point depression
• We use the following equation to calculate
a freezing point depression. (Note: This
equation assumes the freezing point
depression constant has a positive value.)
• Freezing Pointsolution = Freezing Pointsolvent - ΔTf

22. freezing point depression
• Freezing Pointsolution = Freezing Pointsolvent - ΔTf
• where
• ΔTf = molality * Kf * i, (Kf =
cryoscopic constant, which is 1.86°C
kg/mol for the freezing point of water,; i =
Van 't Hoff factor)

23. Who is Van Hoff?

24. the enthalpy change

25. the enthalpy change
• If the enthalpy change of reaction is
assumed to be constant with temperature,
the definite integral of this differential
equation between temperatures T1 and T2
is given by his equation

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27. You should be getting know
them . .

28. Freezing Point of a Solution
• The freezing point of the solvent in a solution
containing a nonvolatile nonelectrolyte,
Tsolution, may be found from the following
information:
• ΔTF = KF · m · i
The freezing point of the pure solvent,
• Tpure solvent.
• The freezing point depression, T.
• Note: The freezing point of a solution will always
be lower than the freezing point of the pure
solvent.

29. Ideal solution-like ideal gas
• ΔTF =TF KF · m · I
• ΔTF, the freezing point depression, is defined as TF (pure solvent) -
TF (solution).
• KF, the cryoscopic constant, which is dependent on the
properties of the solvent, not the solute. Note: When
conducting experiments, a higher KF value makes it
easier to observe larger drops in the freezing point. For
water, KF = 1.853 K·kg/mol.[4]
• m is the molality (mol solute per kg of solvent)

30. van 't Hoff factor
• i is the van 't Hoff factor (number of solute
particles per mol, e.g. i = 2 for NaCl).

31. Depression of Napthalene

32. Depression of Napthalene
When a pure liquid cools its temperature decreases
Steadily until the freezing point is reached. Then its
Temperature remains constant as the liquid solidifies.
•
At constant pressure, the pure liquid and pure solid can be
in equilibrium at only one temperature , the freezing
point of the compound. When all of the liquid has
frozen, the temperature of the solid begins to
decrease.
•