Colligative Properties

Colligative Properties

Description

The freezing temperatures of distilled water, a solution of a nonelectrolyte such as sucrose, and a solution of an electrolyte such as calcium chloride are determined. From the resulting data, regularities in the freezing properties of solutions are discovered.

Hazards

Thermometer bulbs are fragile.

Precautions

Handle thermometers with great care.

Procedure

Place crushed ice in a 600-mL beaker. Add salt and stir. Wait until the temperature is below -10 °C.
Into each of three test tubes, place each of the following: 5 mL distilled water; 5 mL CaCl₂ solution; and 5 mL sucrose solution.
Prepare a stirrer by rolling the end of a piece of heavy wire into a circular shape whose diameter is larger than that of the thermometer but smaller than that of the test tube. Bend the circle at a right angle to the rest of the wire, so that the circle becomes perpendicular to the wire. Place this stirrer in the distilled water test tube. Place a thermometer very carefully in the tube. Place the tube in the ice bath. Stir gently by moving the wire up and down until a slush is formed.
Record the temperature of the slush.
Remove the thermometer and stirrer. Clean them and wipe dry. Repeat for one of the remaining tubes. Finally, repeat for the last tube.

Handout Makeup

Name ___________________________ Class ________

Teacher__________________________

DoChem 066 Colligative Properties

Watch the movie, and answer the questions.

Solutes cause a solvent to lower its freezing temperature; the temperature at which pure solvent freezes out of a solution is lower than the temperature at which the pure solvent alone freezes. This is expressed quantitatively by the equation:

F.P. solution = F.P. solvent - Zm (kf)
Use the formula above to see how close the experimental freezing point comes to the theoretical freezing point; Z represents the number of different types of particles dissolved; m represents the molality (moles of solute/1000g of solvent) of the solution; and (kf) represents the freezing point depression constant for a given solvent. For water, (kf) is 1.86 °C.

Use this sample data for your calculations:

	FP, °C	ΔTf,°C
Distilled H₂O	-0.2 °C	0.0 °C
1 m urea	-1.7 °C	-1.5 °C
1 m CaCl₂	-5.1 °C	-4.9 °C

Why was the temperature recorded for the freezing point of water less than 0.0°C? Measurements were made carefully to the slush point.
How many particles does each molecule of urea produce in a water solution? (Show calculation.) Write an equation of the solution of urea?
How many particles does each molecule of CaCl₂ produce in a water solution? (Show calculation.) Write an equation of the solution of CaCl₂ ?
How would the outcome have been changed if more salt had been used in the ice-salt bath?

Teachers Guide

Purpose

To measure the freezing point lowering of electrolytes and nonelectrolytes in solution.

Materials

5 400-mL beaker
5 thermometer
15 test tube, 25 x 150 mm
5 test tube rack
1 gallon ice
1 kg sodium chloride (technical grade)
distilled water
50 mL 1.0 m CaCl₂ (1.1 g CaCl₂ / 10 mL water)
50 mL 1.0 m sucrose (3.42 g sucrose/ 10 mL water)
5 40-cm length heavy copper wire (for stirrer)
5 heavy gauge ice bath stirrer (plastic or wood preferred)

Lab Hints

Be sure students take the temperature of the slush. The freezing point is the temperature of the equilibrium mixture of liquid and solid, that is the slush.
Urea may be used in place of sucrose.
Be sure to prepare the solutions using molal concentrations, moles of solute per kilogram of solvent.
Calcium chloride dissociates to give 3 ions in solution for every mole dissolved; therefore Z = 3. In a concentrated solution, the apparent value of Z will be less than 3.00. The actual value of Z depends on the degree of ionization of the solute, which in turn is a function of its concentration.
The experimental value for Z may be used to calculate the percent of dissociation of the solute.
If several experiments are performed concurrently, thermometer calibration is required.

Time

Teacher preparation: 30 minutes

Class Time: 30-40 minutes

Disposal

These materials may be disposed of at the sink.

Presentation?

Presentation Question:

How would the outcome have been changed if more salt had been used in the ice-salt bath?
- Adding more salt would lower the temperature of the ice bath, and perhaps cause faster heat transfer. There would be no effect on the actual outcome other than the speed of the processes.

Closure

Solutes cause a solvent to lower its freezing temperature; the temperature at which pure solvent freezes out of a solution is lower than the temperature at which the pure solvent alone freezes. This is expressed quantitatively by the equation:
F.P. solution = F.P. solvent - Zm (kf)
Use the formula above to see how close the experimental freezing point comes to the theoretical freezing point; Z represents the number of different types of particles dissolved; m represents the molality of the solution; and (kf) represents the freezing point depression constant for a given solvent. For water, (kf) is 1.86 °C.
For nonelectrolytes, Z = 1. For electrolytes, dissociation leads to more moles of particles. Thus, one mole of NaCl gives 2 moles of particles, and will depress the freezing temperature twice as much as would a mole of a nonelectrolyte such as sucrose.

Applications

Salt water estuaries freeze at lower temperatures than do rivers and streams because of the presence of salt. Antifreeze is added as a solute to lower the freezing temperature inside an automobile radiator and protect it from cold temperatures.

Makeup Ans.

Thermometer may be calibrated incorrectly. (ΔT will be more accurate than the temperature.)
(-1.7 +0.2)/(-1.86) = 0.81

or about 1 particle/molecule urea
urea (s) urea (aq)
( -5.1+0.2)/(-1.86) = 2.64

or about 3 particle/molecule CaCl₂
CaCl₂(s) Ca²⁺(aq) + 2 Cl^-(aq)
Adding more salt would lower the temperature of the ice bath, and perhaps cause faster heat transfer. There would be no effect on the actual outcome other than the speed of the processes.

Key Words

freezing
freezing temperature
colligative property
molality
solvent
solute