The major chemical concept is the freezing point depression due to a dissolved solute.
General high school chemistry
Expected Student Background
The major prerequisite for this activity is the ability to read and
follow instructions, make and record
observations, plot graphs, and safely handle hazardous materials. Related concepts necessary for understanding
this exercise include: atomic structure, formula writing, equations, and concentration calculations which are a
direct part of the exercise. Students should know and understand the concept of colligative properties.Time
Completing the activity and answering questions requires one 50-min class period.
The laboratory can be completed in two 45-min periods with Part I completed
on the first day and Part II on another day without dismantling the apparatus.
Teacher preparation time is about one hour. A great timesaving step is
to supply the hot water baths at the proper temperature. The hot water
from the tap may be hot enough,
though further heating may be necessary in some schools.
Read the Safety Considerations in the Student Version. Students should
not heat the test-tube of lauric acid directly in the flame as this can
cause it to vaporize. It is also flammable. Care should be exercised in
using the thermometer, particularly if mercury thermometers are used. Have
sulfur or other mercury clean-up materials
ready and immediately available in case of accidental breakage if mercury thermometers are used.
At least one balance with 0.1 g sensitivity should be available to the class.
24 Ring stands with iron rings and wire gauze
24 Thermometer clamps
12 Test-tube clamps
24 Beakers, 600-mL
12 Test-tubes, 25 x 200-mm
24 Copper heavy gauge wires with loops for stirring
Lauric acid, 300 g
Benzoic acid, 30 g
It is important that you do the laboratory yourself beforehand. Time may be saved by pre-filling the test-tubes with about 25 g of lauric acid and having the hot and warm water baths ready. [NOTE: The mass of the lauric acid in the test-tubes must be determined to the nearest gram.]
Make sure students understand the safety instructions. It is important that you demonstrate the proper placement of the thermometer, clamp and stirrer. Caution students not to attempt to remove or even move the thermometer or stirrer from the solidified lauric acid. A brief discussion of the reason for freezing point depression of solutions as explained in Answers to Implication and Applications (Question 4) would be quite useful, too.
One of the most common problems encountered in this procedure is the fact that the molten acid will warm the 30 °C water so that it no longer cools, or the reverse, that warming of the solid acid will by hampered by the warm water cooling off. If you have sufficient water in the beakers, this problem is avoided. During the session, make sure students do not use the thermometers as stirrers, and that they do not attempt to remove the thermometers after the lauric acid has solidified.
Anticipated Student Results
1. The melting point of lauric acid is 44 °C.
2. The average freezing point depression observed is around 2 °C. This translates into a K f of about 2.5
Answers to Data Analysis
1. The graphs should consist of a downward slope
followed by a flat plateau and then another downward
slope after the lauric acid has solidified. In the case of the solution of benzoic acid in lauric acid, the plateau
following the initial downward slope is not perfectly level (as discussed in Answers to Implications and
Applications, Question 4).
2. The freezing point of lauric acid is 44 °C, based on the plateau observed at this temperature on the graph.
3. The freezing point of the lauric acid/benzoic acid mixture is 42 °C, based on the start of the plateau
observed on the graph at this temperature.
4. Moles of benzoic acid used: 122ÊgÊmol –1 = 0.020 mol benzoic acid
5. Molality of the solution: 0.025ÊkgÊlauricÊacid = 0.80 m
6. Freezing point depression: 44 °C - 42 °C = 2 °C.
7. K f = 0.80Êm = 2.5 °C/m 2.5 g 0.020 mol benzoic acid 2 °C
Answers to Implications and Applications
1. The freezing point and the melting point of all
pure substances are the same. (Naturally, superheating or
supercooling can provide apparent differences.) There cannot be a “forbidden region” where a substance
exists as neither a liquid nor a solid.
2. As we cool the liquid lauric acid, the energy we remove initially comes from the slowing down of the
molecules as the temperature drops. The plateau is the point where the energy is lost in the phase change.
The lauric acid molecules are forming a solid (crystallizing) and releasing energy as they do so. During the
phase change no temperature changes are observed. As we cool the solid lauric acid further the energy we
remove comes from the slowing down of the molecules of the solid and the temperature continues to drop.
3. This is the point of the phase change; i.e., the molecules become an ordered solid. Although the kinetic
energy of the molecules remains constant, as the liquid solidifies, the potential energy decreases while the
molecules become part of an ordered state. (In the opposite direction, when melting occurs, the added
energy is absorbed as potential energy as the molecules move into the liquid phase.)
4. The graph of the pure lauric acid shows an abrupt change to a level plateau at the freezing point. In the
solution, the lauric acid solidifies, though the molecules of benzoic acid partially disrupt the process and
cause solidification to happen at a lower temperature. When the lauric acid solidifies, the benzoic acid tends
to stay in the liquid phase and further decrease the freezing point somewhat (i.e., the higher molality of the
remaining liquid phase has an even lower freezing point). Thus the plateau is not flat in the solution cooling
5. The lauric acid is crystallizing into an ordered solid state as in the case of the pure acid, though the benzoic
acid eventually also precipitates when its solubility in lauric acid is exceeded.
1. Repeat the procedure with a larger amount (e.g.,
5.0 g) of benzoic acid.
2. Repeat the procedure with a soluble ionic solute to see the particle relationship that exists. A feasible
system is water and CaCl 2 if you can obtain dry ice from the grocery store to use as a coolant. A dry
ice/acetone slurry should be an efficient cooling bath.
Assessing Laboratory Learning
1. Laboratory Practical. A laboratory exercise requiring
the determination of the freezing point depression of
some other material may be used as assessment. Water is one such solvent.
2. Written Examination. Questions about the molecular level changes and questions requiring calculation of
freezing point depression may be used.
This activity is an extension
of a phase-transition experiment by Wilbraham et al. (1987).
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