Chemistry 102 Laboratory

1. If you want a pot of soup to cook faster, can you do this by turning up the temperature?

2. What is the maximum temperature which can be reached in a pure water bath at 1 atm. pressure?

3. The temperatures inside of a pressure cooker can be greater than that of the last question. Why?

Laboratory

    The colligative properties of molecules and solutions concern: (1) freezing point depression, (2) boiling point elevation, and (3) osmotic pressure. In order to understand the first of these, freezing point depression, this laboratory explores the melting and freezing of an organic acid (lauric acid). The melting and/or freezing of any substance is an example of a phase transition of that substance. This means that the physical state of the substance changes during the course of the transition. As such, this is not a chemical reaction or change, but does involve either the absorption or the release of heat. The amount of heat required to cause the transition depends upoon the nature of substance ( its heat capacity, the external pressure and its amount. A specific measure of point at which the transition takes place is its melting or freezing temperature. Surprisingly, for some materials, the two may be experimentally observed to occur at different temperatures. This can result from two, different effects. The first effect is that of kinetics. In this instance, the rate of heating or cooling influences the observed melting or freezing point. As such, this is an experimental error. The second reason that different melting and freezing points are observed occurs when more than two different phases (states) of the substance can be present. A simple example of the first is the melting and freezing of sucrose. Rapid freezing creates a solid known as a glass while slow freezing creates crystalline sugar. The freezing points for the two are not the same.

In this experiment, you will explore the melting and freezing of lauric acid. The chemical formula of lauric acid is CH₃(CH₂)₁₀CO₂H.  It consists of 12 carbons in a linear chain, the last of which is in the of the form of a carboxylic acid. The phase transition will be measured by means of a thermal probe, and will be recorded by computer. A graph of the temperature vs. the time will indicate the melting and freezing of the acid. Ideally, this graph should be as follows:


   In this figure, the x-axis represents time and the y-axis the measured temperature. As the time increases from left to right, the acid first melts (the temperature increase). As it is melting, there is a significant change in temperature. While both solid and liquid are present, the temperature remains constant (this is the melting temperature). As the liquid cools, there is a sudden decrease in temperature. The rapid increases or decreases in temperature are indications of the phase changes of the lauric acid. The total time required for the transitions (and hence how sharp they appear) depends upon: (1) how much lauric acid is present and (2) how fast the heating/ cooling bath changes in temperature.

Heating Lauric Acid

1. Using Science Workshop, set up a temperature probe with a table and graph, and under sampling options, set the time for every five seconds.

2. Construct a warm water bath as shown in the figure below. Use a 400 mL beaker with roughly 300 mL of water in it. Record the amount of water in the bath. Heat the water bath to about 70o C.

(Click to enlarge)

3. Place approximately 6 grams of lauric acid in a Pyrex test tube while the water bath is heating. Record the amount of the lauric acid which you place in the test tube.

4. When the water bath is about 70 - 75o, turn the burner off.

5. On the spreadsheet, note the time and temperature of the sample, then clamp the test tube to the ring stand and make sure the sample is submerged (not totally submerged) in the water bath and that the tube is not touching the bottom or the sides of the beaker.

6. Make sure the thermal probe is immersed in the sample, but don't let it touch the test tube.

7. Record the temperature of the sample every five seconds.

8. When the sample begins to melt along the sides, mix the sample with the probe.

9. Continue this procedure until the sample reaches approximately 60o C.

10. While heating the sample, occasionally check the temperature of the water bath. If the temperature falls below 60o C. gently heat to approximately 70o C.

11. Have one person set up a ring stand holding a beaker of room temperature water for the next part of the experiment.

Cooling the Lauric Acid

Graphing the data

Post-Lab Questions

1. What where the melting and freezing points of lauric acid (in degrees C)?

2. How many seconds did it take for the lauric acid to melt? To freeze? (Determine these times from the difference in time between when rapid temperature changes begin and end.)

3. Using your answer from #2 above, which process (melting or freezing) would you expect would yield the most accurate measure of the solid <--> liquid transition for lauric acid?  Justify your answer with a short (!) explaination.