Intermolecular Forces—A CBL/Microtime Analysis

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Procedure

1. Set up three temperature probes with a small piece of filter paper secured with a band made from rubber tubing. Put them in channels 1, 2, and 3 of a CBL*, CBL2, or Lab Pro.
2. Turn on CBL and TI-83+ calculator. Go to APPS, CHEMBIO, and follow the instructions on the screen for setting up 3 temperature probes. Use the stored calibrations in each case.
3. Collect data. Use a time graph. Time between samples = 2 seconds. 40 samples. Y_min=10, Y_max=30, Y_scl=1.
4. You have three vials labeled 1, 2, and 3. Place probe from each channel in the appropriate bottle. The bottles contain n-hexane (C₆H₁₄), acetone (CH₃COCH₃), and 2-propanol (CH₃CHOHCH₃), but not in that order.
5. Simultaneously remove the probes from all three bottles and begin to collect data. The graph will display in real time.

Analysis

While the three substances have approximately the same molecular weight, they obviously do not have the same intermolecular forces. Draw structural formulas for all three compounds below.

Quit the program. Open STAT EDIT. Time is in L₁, while the temperatures for the three probes are in the next three lists in order. Go to STAT PLOT and turn on PLOTS 1, 2, and 3. Let plot 1 be L₁, L₂ with a square point protector. Let plot 2 be L₁, L₃ with a cross point protector, and let plot 3 be L₁, L₄ with a dot marker. Go to ZOOM9 to display. Now you can tell which channel made each graph.

Identify the three substances, and explain with both words and diagrams the differences in the evaporation rates of each.

* For the CBL, use the brass temp probes. For CBL2 or Lab Pros, use the stainless steel temp probes.

Teachers' notes: Since all of the molecules have approximately the same dispersive forces, the major difference will lie in dipole-dipole attractions or with hydrogen bonding. Students should show with diagrams the H-bonding with the 2-propanol. Another version of this same lab uses methanol, ethanol, and 2-propanol. All of these substances will hydrogen bond, but they will have difference dispersive (London) forces. Be certain that students understand the London forces are due to polarizable electron clouds, and not due to mass of the molecule.

Structures of molecules shown below:

Adapted by Carol B. Brown, Saint Mary's Hall, San Antonio, TX from Chemistry with Calculators by Dan Holmquist and Donald Volz.