Freezing Point Of Lauric Acid

LABORATORY
Action:
NOTES
Teacher
Activeness two: The Freezing Indicate Depression of Lauric Acid
Major Chemical Concept

The major chemical concept is the freezing point depression due to a dissolved solute.

Level

General high school chemistry

Expected Student Groundwork

The major prerequisite for this action is the ability to read and follow instructions, make and tape
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 office of the exercise. Students should know and understand the concept of colligative backdrop.Fourth dimension
Completing the action and answering questions requires i 50-min class period.

Time

The laboratory tin can be completed in two 45-min periods with Part I completed on the first day and Part Ii on another mean solar day without dismantling the appliance. Teacher training time is nearly one hour. A keen timesaving step is to supply the hot water baths at the proper temperature. The hot water from the tap may exist hot enough,
though further heating may exist necessary in some schools.

Safety

Read the Condom Considerations in the Educatee Version. Students should non heat the test-tube of lauric acid directly in the flame as this tin can cause it to vaporize. It is too flammable. Care should exist exercised in using the thermometer, peculiarly if mercury thermometers are used. Have sulfur or other mercury clean-up materials
fix and immediately bachelor in case of accidental breakage if mercury thermometers are used.

Materials

At least one balance with 0.one g sensitivity should be bachelor to the class.

Nonconsumables

24 Ring stands with atomic number 26 rings and wire gauze

24 Thermometer clamps

12 Test-tube clamps

24 Beakers, 600-mL

24 Thermometers

12 Burners

12 Test-tubes, 25 ten 200-mm

24 Copper heavy gauge wires with loops for stirring

Consumables

Lauric acrid, 300 g

Benzoic acid, xxx g

Paper towels

Advance Preparation

It is important that you do the laboratory yourself beforehand. Fourth dimension may be saved by pre-filling the test-tubes with about 25 chiliad of lauric acid and having the hot and warm h2o baths ready. [NOTE: The mass of the lauric acid in the test-tubes must be determined to the nearest gram.]

Pre-Laboratory Discussion
Brand sure students sympathize the prophylactic instructions. Information technology is important that you demonstrate the proper placement of the thermometer, clamp and stirrer. Circumspection students not to effort to remove or even move the thermometer or stirrer from the solidified lauric acrid. A brief discussion of the reason for freezing point low of solutions as explained in Answers to Implication and Applications (Question four) would be quite useful, too.

Teacher-Educatee Interaction

One of the nigh common problems encountered in this process is the fact that the molten acrid will warm the 30 °C h2o so that it no longer cools, or the reverse, that warming of the solid acid will past hampered by the warm h2o cooling off. If you accept sufficient water in the beakers, this problem is avoided. During the session, make sure students do non use the thermometers as stirrers, and that they practice not endeavor to remove the thermometers subsequently the lauric acrid has solidified.

Anticipated Student Results

    i. The melting point of lauric acrid is 44 °C.
    2. The boilerplate freezing point depression observed is around ii °C. This translates into a K _f of about 2.five
        °C/molal.Data Analysis

Answers to Data Analysis

    ane. The graphs should consist of a downwards gradient followed by a apartment plateau and then some other down
        slope after the lauric acid has solidified. In the case of the solution of benzoic acid in lauric acid, the plateau
        following the initial down slope is not perfectly level (as discussed in Answers to Implications and
        Applications, Question four).
    2. The freezing betoken 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 �one = 0.020 mol benzoic acid
    5. Molality of the solution: 0.025ÊkgÊlauricÊacid = 0.80 m
    six. Freezing point depression: 44 °C - 42 °C = 2 °C.
    7. 1000 f = 0.80Êm = 2.5 °C/one thousand ii.five g 0.020 mol benzoic acid 2 °C

Answers to Implications and Applications

    1. The freezing signal and the melting point of all pure substances are the same. (Naturally, superheating or
        supercooling tin can provide apparent differences.) At that place cannot be a �forbidden region� where a substance
        exists equally neither a liquid nor a solid.
    two. As we cool the liquid lauric acrid, the energy nosotros remove initially comes from the slowing down of the
        molecules equally the temperature drops. The plateau is the point where the energy is lost in the stage change.
        The lauric acid molecules are forming a solid (crystallizing) and releasing energy as they exercise and then. During the
        phase alter no temperature changes are observed. As we absurd 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.
    iii. This is the signal of the phase modify; i.east., the molecules become an ordered solid. Although the kinetic
        energy of the molecules remains constant, equally the liquid solidifies, the potential energy decreases while the
        molecules go function of an ordered state. (In the reverse direction, when melting occurs, the added
        energy is absorbed every bit potential energy as the molecules movement into the liquid phase.)
    iv. The graph of the pure lauric acid shows an sharp alter 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 stage and further decrease the freezing point somewhat (i.e., the college molality of the
        remaining liquid stage has an even lower freezing point). Thus the plateau is not flat in the solution cooling
        bend.
    5. The lauric acrid is crystallizing into an ordered solid state every bit in the case of the pure acid, though the benzoic
        acid somewhen also precipitates when its solubility in lauric acid is exceeded.

Extensions

    1. Echo the procedure with a larger amount (e.thousand., 5.0 g) of benzoic acrid.
    2. Repeat the process with a soluble ionic solute to see the particle relationship that exists. A feasible
        system is water and CaCl₂ if yous can obtain dry out ice from the grocery store to use as a coolant. A dry out
        ice/acetone slurry should be an efficient cooling bath.

Assessing Laboratory Learning

    1. Laboratory Applied. A laboratory do requiring the determination of the freezing point low of
        some other textile may exist used as assessment. Water is one such solvent.
    2. Written Examination. Questions about the molecular level changes and questions requiring calculation of
        freezing betoken depression may exist used.

Reference

This activity is an extension of a phase-transition experiment by Wilbraham et al. (1987).