Heating and Cooling Curve of Lauric Acid

OBJECTIVES Examine the effects of the heating and cooling of a pure substance through phase change. Construct a heating and cooling curve of a pure substance using experimental data. Determine the freezing point and melting point temperatures of a pure substance.

MATERIALSA test tube containing lauric acid, C12H24O2 Water 250-mL beaker 2 thermometersRing stand assembly Hot Plate

PRE-LABORATORY QUESTIONS(Some research may be required to complete the following)

1. Sketch a basic, hypothesized Heating Curve of Lauric Acid.2. Sketch a basic, hypothesized Cooling Curve of Lauric Acid3. Sketch a diagram of the set-up for this laboratory.4. If you spill Lauric Acid on your skin what is the procedure for cleansing it.

PART I. THE COOLING CURVE1. Fill a 250-mL beaker ¾ full with cold tap water.2. Obtain a stoppered test tube containing a sample of lauric acid form your teacher. Clamp

the test tube to a ring stand and assemble the hot water bath as shown by your teacher.3. Remove the stopper. Heat the sample very gently starting on a low setting and gently

raising the temperature. 4. As soon as the sample starts to melt, remove the heat and place a clean thermometer in

the sample. Using the thermometer to stir the sample, begin heating the test tube gently again. When the sample is almost completely melted, take away the heat and stir the sample well. The temperature should be around 65oC. DO NOT OVERHEAT. The substance conducts heat well and will warm to over 100oC if not monitored closely. The final temperature of the melted sample should be no higher than 75oC. If it is more than 75oC, allow the sample to cool down before going to the next step.

5. Now, with one student watching the clock, read the temperature of the melted sample and record this as the temperature at t = 0. Immediately lower the test tube into the 250-mL beaker of water so that the sample is below the surface of the water.

6. With one partner constantly stirring the sample with the thermometer, the other partner needs to monitor the clock. A temperature should be recorded every 30 seconds. As the temperature drops, the sample will begin to turn into a solid. Continue stirring until the thermometer can no longer be moved. Allow the thermometer to be “frozen” in the sample and continue measuring and recording the temperature until the sample reaches 25oC.

7. Remove the test tube from the water, leaving the thermometer in the sample, for use in the next part of the experiment.

PART II. THE HEATING CURVE8. Set up a ring stand assembly to heat the beaker of cold water using the Hot Plate.9. Get another thermometer and place it in the beaker of water. Heat the water until the

temperature reaches 75oC. Turn off the heat and place the beaker on the table and remove the thermometer.

10. Read the temperature of the solidified sample in the test tube and record this as temperature at t = 0.

11. Now, lower the test tube into the beaker of hot water. With one partner reading the thermometer, the other partner needs to monitor the clock. A temperature should be recorded every 30 seconds. As the sample warms it will begin to melt and thermometer should be used to stir the sample continuously. Measure and record the temperature until the sample reaches about 60oC.

Heating and Cooling Curve of Lauric Acid

12. Remove the test tube from the water and quickly remove the thermometer from the sample. Quickly wipe off the thermometer with a paper towel to remove any residue. Replace the stopper on the test tube and return it to the teacher.

Sample Data tableDATA TABLE

TIME (sec)

Part I. The Cooling Curve

TEMP (oC)

Part II. The Heating Curve

TEMP (oC)0

30

60

… … …

POST-LAB ANALYSIS

1. Using the data obtained in the experiment, construct a graph with time as the independent variable (x-axis) and temperature as the dependent variable (y-axis). The time should range from 0 sec to possibly 900 sec. The temperature should range from 20oC to 80oC. Use as much of the graph paper as possible. Label the axes. You will plot the data for the cooling curve and the heating curve on the same graph paper. You will need to use different colors or indicators to identify each curve. Connect the points for each set of data.

2. Observe your graph for the cooling of lauric acid. a. What is physically happening to the lauric acid during this cooling process?b. There should be a noticeable flat part to your graph. This is called the plateau. What do

you think the plateau represents?

3. On the heating curve the plateau is less visible. However, look at where the heating curve intersects the cooling curve. During this heating process, what physical change is happening to the lauric acid?

4. Referring to the graph for the cooling curve, determine the freezing point temperature of the lauric acid.

5. According to the graph, what is the melting point temperature of the lauric acid?

6. Notice the diagonal parts of the heating and cooling curve. a. What is happening to the kinetic energy of the particles of lauric acid during the cooling

curve? Is this an endothermic or exothermic change?b. What is happening to the kinetic energy of the particles of lauric acid during the heating

curve? Is this an endothermic or exothermic change?

7. At the plateau of the cooling curve, the temperature of the lauric acid does not change, even though the sample is still being cooled in the water bath. Propose what might be happening to the particles of lauric acid during this time.