DIFFUSION OF GASES

Advance Preparation

* Prep Time Needed: 20 minutes

* Safety Precautions:

* HCl and NH₃ fumes are unpleasant, avoid inhalation. Avoid skin contact with these two chemicals. Rinse spills with lots of water.

* Suggestions

* Use a plastic reaction tube of about 60 cm in length and 1 cm in diameter. A local plastic shop should be able to supply these tubes relatively inexpensively.

* The formation of NH₄Cl "smoke" can be minimized if the reagents are placed in small dropper bottles and plugged when not in use.

* Large beakers of tap water should be placed on the reagent table in which to deposit used swabs.

Introduction

The movement of the particles of one material through the particles of some other material (a medium) is called diffusion. Diffusion has many important applications in both chemistry and biology. For example,

* Diffusion of water and nutrients through a cell membrane is very important to keeping the cell alive.

* The kidneys and bladder in the human body use diffusion to filter waste and recycle clean fluids.

* The diffusion of a liquid through a packed column of beads allows chemists to separate liquids using a process called column chromatography.

In this lab we will deal with the diffusion of gasses. The rates of diffusion for different gases in the same medium are different. This difference is due mostly to the mass of the gas. We make the following assumptions:

* At the same temperature and pressure all gas samples have the same kinetic energy. Kinetic energy is energy of motion.

* Since all of the gases have the same kinetic energy, the smaller molecules must be moving faster. As an analogy: A bowling ball will do more damage to your foot than a ping-pong ball if both hit at the same speed. This is because the bowling ball has more mass.

The formula for the kinetic energy of any object is: KE = 1/2 mv²

Let's look at two gas molecules "a" and "b" at the same temperature and pressure.

Since "a" and "b" are at the same temperature and pressure, they must have the same kinetic energy. So:

KE_a = KE_b

KE_a = 1/2 m_av_a^{2 =} KE_b = 1/2 m_bv_b² or

1/2 m_av_a^{2 =} 1/2 m_bv_b²

2 *(1/2 m_av_a^{2) =} 2 *(1/2 m_bv_b²⁾

m_av_a^{2 =} m_bv_b²

In the equation on the previous page m_a is the molecular mass of Gas "a" while m_b is the molecular mass of Gas "b". v_b is the velocity of Gas "b" and v_a is the velocity of Gas "a". This can be stated in words as Graham's law of diffusion. Under the same conditions of temperature and pressure, the rates of diffusion of two gases vary inversely as the square roots of their molecular masses.

In this experiment, you will determine experimentally the relative rates of diffusion of two gases, hydrogen chloride and ammonia. You will then express your experimental results in the form of a ratio and will establish a relationship between the rate of diffusion of a gas the molecular mass of the particles.

Student Objectives

* Students will observe the reaction of HCl(g) with NH₃(g) in a diffusion apparatus

_{* Students will measure the distance traveled by each gas to the point where the reaction occurs}

* Students will calculate the diffusion rates for the gases based on distance traveled

* Students will compare their experimental ratio of diffusion rates with the theoretical ratio predicted by Graham's law

* Students will relate what they learned about diffusion of gasses to diffusion within a cell

Class Time Needed

One 50-60 minute class period.

Materials (Per pair of students)

1.  GOGGLES AND APRON 2.  plastic       5. wax marking pencil or overhead       
tubing (10 mm X 60 cm long) 3.  2       marking 6.  pen 7.  ruler (metric) 8.   
cotton swabs 4.  2 dropper bottles      string 9.  tissue                       
filled with concentrated HCl and NH3                                            
into which cotton swabs may be                                                  
dipped.                                                                         

Procedure

1. Obtain or prepare two cotton swabs

2. CAUTION: FUMES OF BOTH HCl(aq) and NH₃(aq) are very irritating to eyes and lungs. Solutions of both can cause skin burns. Avoid contact. Notify teacher of spills. Rinse spills with plenty of water. Dip one cotton swab in the concentrated HCl and the other cotton swab in the aqueous ammonia. At exactly the same time, push the swabs back into the opposite ends of the plastic tube, and plug the ends of the tubes with damp paper towels as shown in Figure 1.. Be sure to note which end of the tube contains the HCl plug and which end contains the NH₃ plug. Leave the tube on the table; do not move it until you see the reaction.

Figure 1. Gas diffusion apparatus.

CAUTION STUDENTS ABOUT THE CONCENTRATED HCl AND NH₃. SPILLS SHOULD BE CLEANED UP WITH WATER AND PAPER TOWELS USING GLOVES. TUBES NEED TO BE DRY IN ORDER TO GET ACCEPTABLE RESULTS.

3. Mark the location of the tip of both cotton swabs on the tube using a wax pencil or overhead pen.

YOU MAY WISH TO HAVE STUDENTS MAKE A NOTE OF THE HCl END AND NH₃ END DIRECTLY ON THE TUBE.

4. A reaction will occur at the point where the two gases meet. It will take from 2 to 6 minutes to start. A white vapor ring will form as in Figure 2. Mark the point where the reaction begins with a wax pencil or overhead pen.

Figure 2. Reaction point for diffusing gases.

INSTRUCT THE STUDENTS TO BE PATIENT ON THIS ONE. IT CAN TAKE A FEW MINUTES.

5. After you have observed the beginning of the reaction and marked the tube, remove the swabs and paper towels. Be sure to remember which end of the tube contained the HCl swab was which contained the NH₃ plug. Place the cotton swabs into beakers of tap water provided by your teacher.

PROVIDE ONE BEAKER OF TAP WATER FOR EVERY FOUR STUDENTS.

6. With a metric ruler, measure carefully the distance from the cotton swab mark in each end to the reaction make you have made towards the middle of the tube.

7. Tie a string around a damp tissue and pull it through the tube to clean it. See Figure 3.

Figure 3. Cleaning tube with damp paper towel and string.

8. Repeat the experiment two more times.

Data

Fill in the chart below.

Below is typical student data.

       Trial Number         Distance from HCl to       Distance from NH3 to      
                           reaction point  (mm)       reaction mark (mm)         
            1                         31              29                         
            2                         36              24                         
            3                         45              21                         
Average                     36 +/- 5                   24+/-5                    

Analysis

1. Since the time interval was the same for both gases in this experiment, you can use the distance traveled as a measure of velocity or diffusion rate. What is the ratio of the average diffusion rate of the faster molecules to the average rate of diffusion of the slower molecules. (Divide the larger number by the smaller number in the average row from the chart on the previous page.)

Student answers will vary.

2. Calculate the molecular mass of HCl and NH_3.

HCl = 36.46 g/mole NH₃ = 17.04 g/mole

3. Substitute these masses into the equation given above.

Ratio = 1.46/1

4. Find the percent error when you compare your experimental results in question 1 with the theoretical results in question 3. Remember that % error =

Expect 10-25% error

5. True or false, a molecule with a mass of 2.0 travels twice as fast as a molecule with a mass of 4.0.

False, the relationship would be the square root of 2 or 1.4/1

Conclusion

1. Complex molecules found in food have molecular masses of >100,000 grams/mole. What would be the diffusion rate of these large molecules though a cell membrane?

Very slow or non-existent

2. What must your body do in order to get these nutrients into the cell where they can be used?

Break them down into smaller molecules.

3. You want quick energy. Would you choose to eat glucose (C₆H₁₂O₆) or steak (C₂₆H₅₀O). Why?

Glucose. Smaller, easier to break down and get into the system.

4. DNA has a molecular mass of >100,000 grams/mole. Why doesn't DNA leak out of the cell nucleus?

It is much larger than cell pore size.