Liquid nitrogen experiment to verify the heat capacities of LN2 and a metal between 77K and room temperature

• Before lab, get hold of a maple file for your metal sample (iron, aluminum, or copper). This will be under the resources page on my web site. This file will contain heat capacity data for your metal, and a functional form with two adjustable parameters. You are to adjust the parameters until the form is a good fit to the experimental data over the range 77 K to room temperature 295 K. Then have maple integrate the form from 77 to 295 K and see what the heat capacity is in cal/mole. This is the heat lost per mole of sample as it cools from room temp to LN2 temp. I will collect and grade your stuff (on your laptop or on a printout) when you come to class on Friday.
• Program ser.exe should already be loaded on a PC in the lab, and hooked up to a BIB box. A strain gauge will be set up on the base of a long metal arm (a cantilever) and plugged into the BIB. Two styrofoam cups will be hanging from the end of the metal arm.
• Select 'Hdware Check', and 'Check the A/D'. This will show you what value is being read from the strain gauge in volts. Add several grams (15 or more grams at a time, by placing them in the cup) and record the voltage readings. Calculate how many volts per gram it takes during each addition. This should be fairly constant.

Wear eye protection (glasses of some sort) when in the proximity of liquid nitrogen (LN2).

• Under 'Set Analog Parameters, set the software for 300 data points, then set the low frequency data rate by skipping 18 ticks. This will give you a sampling rate of just over 1 hz, so that the 300 samples will take about 300 sec.
• Under 'File I/O' set the software data directory to be c:\lab
• Determine the mass of your samples on the balance (before or after freezing them in LN2)
• When your cup has been filled with LN2, start taking data, and wait 2 minutes before dunking your metal sample, then place the sample in the liquid in the center of the container, holding it by a thread. It will take one minute or less for the furious boiling of the LN2 to stop, indicating the sample has reached 77 K Then remove the sample when it has been thoroughly cooled, and the data will continue for another 2 minutes or so.

The graph should show a steady decline in weight for 2 minutes, then jumpy stuff while the sample is cooling. When the sample has been removed, there will be another 2 minutes of gradual decline in weight.

Under File I/O, save the data to a file, for example oct17a.dat. You get 80 characters to make comments, and you should say what the metal is, and its weight. This file will be saved in c:\lab, and then you can transfer it via a floppy to your own computer.

Analysis.

Draw straight lines through the gradual portions of the weight loss, and estimate the weight loss of LN2 at the mid-point of the sample cooling. The heat of vaporization of LN2 is right around 1325 cal/mole (remember a mole is 28 g), so you will be able to calcuate the heat gained by the LN2.

Compare this to the theoretical heat lost by your sample based on your fit of the heat capacity data.