Experiment 17
VAPORIZATION OF LIQUID NITROGEN

PROCEDURE


1.  Place the dewar flask (thermos bottle) on the pan balance. Suspend the heater in the dewar in such a way that the heater is just above the bottom of the dewar. The heater must not touch the dewar in order that it not interfere with weighing the dewar and its contents. The 0 - 30 V, 1 A power supply will be used as a source of direct current for the heater. Connect the heater to the power supply, voltmeter, and ammeter as shown below.


Figure 17-1: Circuit for Heat of Vaporization Measurements


2.  The next step is NOT a data-taking step, but only a circuit check. Use the 1.0 amp scale on the ammeter and the 30 volt scale on the voltmeter. Observe the indicated polarity (+ or -) with the meter and supply. Make sure the voltage knob is turned completely counter-clock-wise. Turn on the supply and increase the voltage to make sure that both meters are working correctly. Don't let the current exceed 0.5 A! When you have confirmed that both meters work, Turn the power off.

3.  Open the Worksheet and fill in the header information. You will be recording readings of mass from the balance and elapsed time in three intervals: heat off, heat on, and heat off again.

4.  Fill the dewar with liquid nitrogen and wait for 3 to 5 minutes while the nitrogen cools the dewar and heater. Refill the dewar. After the boiling has settled somewhat, measure the mass of the dewar and nitrogen and record it in cell B15 on the Worksheet. In cell A15, record a time of 0 s. Start timing with the stopwatch immediately.

5.  Fill in the spreadsheet in cells A16:B36 as follows: for each 2 gram decrease in mass, record the mass and record the elapsed time. Record mass vs. time on the Worksheet for about 10 minutes with the HEATER POWER OFF.

6.  TURN ON the heater and adjust the power supply to set the current at 0.5 A. DO NOT ALLOW THE CURRENT TO EXCEED THIS VALUE. Be sure to record both the current and the voltage in cells F39:F40 and to check them frequently to make sure they are steady. Record mass vs. elapsed time for about 10 minutes for every 2 gram decrease in mass in cells A46:B67 on the Worksheet.

7.  Finally, TURN OFF the heater and continue the process of measuring mass vs. time and recording the values in cells A74:B95 on the Worksheet for 10 more minutes.