Prelab Report

by My Name

Student ID #

Diffusion of Gases

Teaching Assistant: TA Name

Chem 374-005

Group #6

11/14/96

Lab Partner: Their Name

Introduction & Theory

Diffusion is the tendency of liquids to spontaneously mix when brought into contact. In this experiment, the liquids are gasses. As molecules from one gas move into spaces where molecules from a different gas once were, the gases mix. As this mixing is based on the inherently random motion of molecules, it results in an increase in disorder and an increase of entropy. As a result, diffusion is spontaneous.

The rate at which diffusion occurs depends on an enormous variety of factors, many of which related to the identity of the gasses taking place in the diffusion. Among these factors are forces between like and dissimilar molecules, molecular sizes, spatial considerations, etc. As a result, diffusion rates are best considered to be empirical results to be measured by experimentation for the systems in question.

Goal

The goal of this experiment is to determine the diffusion coefficients of He-CO2 and Ar-CO2 gaseous mixtures using a modified Loschmidt apparatus.

Apparatus

A vacuum system with two taper joint attachments to its manifold; Loschmidt diffusion tube with stopcocks at the top and bottom and a large bore stopcock in the middle; two ball-socket joints to fit the Loschmidt tube; "T" joint; six lengths of rubber vacuum tubing, five long and one short; tightly coiled copper tubing; large Dewar flask; manometer (or other pressure gauge); 1 liter beaker; towel; stopwatch; meter stick; thermometer (appropriate for measuring ambient temperature).

Liquid nitrogen; compressed gaseous helium, argon, and carbon dioxide.





















Figure not available

Figure 1 - Experimental Apparatus

Experimental Observables

In this experiment, the pressure of the portion of the system containing the copper coil is the only variable to be measured. Other important constants are L, the length of one half of the Loschmidt tube, and the ambient temperature (which is assumed to be constant).

Experimental Method

Carbon dioxide, a heavier gas than helium or argon, is introduced to the lower portion of a Loschmidt tube positioned vertically. The upper half of the tube is then filled with helium or argon, and the two are allowed to diffuse for a length of time. Both halves of the tube are then isolated, and their contents are cooled to solidify the carbon dioxide content and remove the inert gas. The solid CO2 is then vaporized and measured via a pressure increase. The gases are moved to the desired locations using a vacuum system and pressurized sources.

The specifications for the experiment are approximately the following: a length L ( tube length) of 50 cm, and a diffusion constant of 0.6 cm2 s-1 for CO2-He and 0.15 cm2 s-1 for CO2-Ar (within an order of magnitude).

Experimental Procedure

Using a diffusion tube constructed such that the length of the upper section is the same as the lower section and the bore of stopcock A combined. All stopcocks should then be opened and, with the gas turned off, the apparatus should be evacuated for about 20 minutes. Then close stopcock F and check for leaks. Any increase of pressure of more than 5 Torr h-1 signifies a significant leak that must be corrected before continuing.

Evacuate again for a few minutes, then close stopcock D and release CO2 into the system until a pressure between 750 and 770 Torr is obtained. Record the pressure exactly after closing stopcock B. Close stopcock A and open stopcock F to pump away the CO2 in the rest of the system. After the CO2 is removed, close stopcock F and release He Ar into the system until the pressure is at least withing 1 Torr of the CO2 pressure. Take care to match the CO2 pressure as closely as possible.

Close stopcock C and evacuate the rest of the apparatus. Start a stopwatch at the same moment stopcock A is opened completely. Once the optimum time has elapsed, close stopcock A, stop the stopwatch and note the elapsed time.

Using a Dewar flask, cool the copper coil with liquid nitrogen until violent boiling ceases. After closing stopcock D open stopcock C. After 30 seconds, open stopcock D slowly until the pressure reading on the manometer just begins to decrease. Open stopcock D until the pressure lowers by approximately 20 Torr every 15 seconds. After the helium is evacuated, close stopcocks C and D. Allow the solidified CO2 to vaporize, releasing any trapped gas. Before the coil reaches room temperature, cool the coil with liquid nitrogen again. When completely cooled, open stopcock D for about one minute.

Close stopcock D and allow the majority of the trapped CO2 to vaporize. Next, place a beaker filled with room temperature water around the coil the hasten warming. After about a minute, remove the bath and dry the coil (to prevent evaporative cooling). After the coil has reached thermal equilibrium, measure the pressure p'1.

Evacuate the system above stopcock A. Measure the CO2 pressure in the lower half of the tube by repeating the above procedure after opening stopcock A.

Repeat this entire procedure for Ar, and again with either gas at atmospheric pressure (3805 Torr). Precisely measure length L and record the ambient temperature.

Experimental Precautions

Preliminary Calculations

The most important quantity to be calculated is the optimum time. The most precise measurement of diffusion rates will be obtained if the gas is allowed to diffuse for this amount of time, given by:

The length of the tube L is given in the text(1) to be approximately 50 cm, and approximate values of diffusion rates D are given as well. Using these values, optimum times for both the He-CO2 and Ar-CO2 systems can be calculated:


This optimum time serves as a basis for the length of the run, and the actual run should not differ from it by more than 30 percent. For He-CO2 this gives a window of 8.4 minutes, and for Ar-CO2 a window of 34 minutes.

If n1 is the number of moles of gas in the lower part and n'1 is the number of moles of gas in the upper part, a quantity f may be defined by:

In turn, f may be used to calculate D with the following relation:

Safety(2)

Mercury and mercury vapor are highly poisonous. Inhalation of vapor may lead to fever, nausea, vomiting, diarrhea, headache, chest pain, and possibly death. Skin contact may lead to a rash or allergic reaction, and if extensive may also cause the same effects as inhalation of vapor. Affected persons should be removed to fresh air and contaminated clothing removed. Necessary first aid techniques should be performed. Seek medical attention immediately.

Inhalation of carbon dioxide gas in low to medium concentrations can cause nausea, dizziness, headache, affect blood circulation, and acidify bodily fluids. High concentrations can lead to death, especially if oxygen is displaced. Persons affected should be removed to fresh air and necessary first aid techniques performed. Seek medical attention immediately.

Inhalation can lead to asphyxia with any, all, or none of the following: dizziness, tightness in the forehead, tingling in tongue, fingertips, or toes, loss of vocal abilities, loss of movement, reduced consciousness, loss of tactile sensations, and heightened mental activity. Persons affected should be removed to fresh air and necessary first aid techniques performed. Seek medical attention immediately.

Inhalation can lead to asphyxia with any, all, or none of the following: dizziness, tightness in the forehead, tingling in tongue, fingertips, or toes, loss of vocal abilities, loss of movement, reduced consciousness, loss of tactile sensations, and heightened mental activity. Persons affected should be removed to fresh air and necessary first aid techniques performed. Seek medical attention immediately.

Extremely cold. Avoid contact with skin and mucous membranes. May cause severe burns. Inhalation of vapor can lead to asphyxia with any, all, or none of the following: dizziness, tightness in the forehead, tingling in tongue, fingertips, or toes, loss of vocal abilities, loss of movement, reduced consciousness, loss of tactile sensations, and heightened mental activity. Persons affected should be removed to fresh air and necessary first aid techniques performed. Burns should be wrapped, use no salves or oils. If severe or covering more than small areas, seek medical attention immediately.

References

1. D. P. Shoemaker, C. W. Garland, J. W. Nibler, Experiments in Physical Chemistry, 6th ed., chap. 6, experiment 6, The McGraw-Hill Companies (1996).

2. Safety data taken from MSDS database on the Internet at "http://hazard.com/msds".