Expt 029 -- Diffusion of Two Gases in Two Dimensions

The diffusion of two gases across a 96-well plate filled with substances which are sensitive to the presence of these gases is observed.

Background

• Diffusion is the spontaneous mixing of two or more gases. Under controlled conditions (constant pressure and temperature), Thomas Graham found the rate of diffusion to be inversely proportional to the square root of the density of a gas. We now substitute molar mass for density. A more generalized statement under less restrictive conditions relates the rate of diffusion to the concentration of a gas.
• Diffusion is observed in all states of matter -- gas, liquid, and even solid. Particles move at relatively high speeds but, because of collisions with other particles, their paths are deflected.
• In this experiment, chlorine gas is produced by a chemical reaction which expels a large burst on initial mixing. In contrast, ammonia diffuses from a concentrated solution. Consequently, no quantitative comparisons of diffusion rates are reasonable.
• This experiment does illustrate the dependence of chemical reactions on the diffusion of the reactants. Most chemical reactions require collisions of molecules which in turn depend on mixing and diffusion of these molecules.

Safety

• Bleach is caustic and damages skin and eye tissue. Wear goggles and apron. Several of the chemicals are toxic. Do not ingest the chemicals. Chlorine and ammonia are toxic by inhalation. Work in a well ventilated area. Wash hands before leaving the laboratory.
• Should the ammonia and bleach mix, chloramine, a very toxic material is produced. Do not mix the bleach and ammonia.

Procedure

1. Identify the wells under the "corner holes" in the cover of a 96-well plate. Leave these wells empty.
2. To each of the remaining 94 wells, add 2 drops of 0.1 M KI/phenolphthalein solution.
3. Select one of the empty corner wells. Place 5 drops of bleach in this well.
4. Prepare plastic transfer pipets with restricted tips, one with 3 M HCl and the other with concentrated aqueous ammonia.
5. Work quickly. Replace the cover. Add 2 drops of 3 M HCl to the bleach. Add 3 drops of concentrated ammonia to the remaining empty well. Cover both holes with small pieces of tape in order to restrict the gases to the container.

   !!!Click here to See Movie.

   !!!Click here to See Movie.

6. Describe any changes to the corner wells (bleach and acid mixture; ammonia). Observe the plate for several minutes, Note and record the observations.

   !!!Click here to See Movie. This movie is accelerated to 5 times faster than the actual experiment.

7. Go to a sink. Turn on the water tap, and perform this disposal while water is running. Shake the tray's contents into the sink. Rinse the tray with water and shake again.

Questions

1. Write a balanced chemical equation for the production of chlorine gas from bleach (NaClO.)
2. Write a balanced equation for the reaction of chlorine gas with aqueous potassium iodide.
3. The average molecular speed of Cl₂(g) at 25ºC is 2.6 x 10⁶ cm/sec Did Cl₂(g) travel this fast in the experiments?

   Was the observed rate faster or slower? Why?

4. Compare the rate of diffusion of the chlorine to that of the ammonia.

Handout Makeup

Name ___________________________ Class ________

Teacher__________________________

SmallScale 029 Diffusion of Two Gases in Two Dimensions

Watch the movies.

Describe the reaction carefully.

Answer the questions.

Curriculum-

Use this experiment when studying gases, kinetic molecular theory, or diffusion. This experiment works well in applied chemistry.

Safety-

• Bleach is caustic and damages skin and eye tissue. Wear goggles and apron. Several of the chemicals are toxic. Do not ingest the chemicals. Chlorine and ammonia are toxic by inhalation. Work in a well ventilated area. Wash hands before leaving the laboratory.
• Should the ammonia and bleach mix, chloramine, a very toxic material is produced. Do not mix the bleach and ammonia.
• Control the amounts carefully. This experiment can be performed handily using no more than 2 mL each of the reactants per class distributed among 5 or 6 pipets, each with 30-40 drops, but all together serving a class of 30 students.

Time-

Teacher Preparation: 15 minutes

Class Time: 30 minutes

Materials-

• Dispense the following solutions in plastic transfer pipets.
  • 0.5 mL of bleach
  • 0.5 mL of concentrated ammonia
  • 0.5 mL of 3 M HCl
  • 10 mL of 0.1 M KI/phenolphthalein mixture. (Add 1.66 g KI to a 0.5 % phenolthalein solution prepared in the usual way, or dissolve 0.5 g phenolphthalein in 60 mL of 95% ethanol and dilute to 100 mL with distilled water; add 1.66 g KI; add enough distilled water to make 100 mL of solution)
• 96-well plate
• cover for 96-well plate with holes drilled over one pair of opposite corner wells
• Clear tape for holes

Disposal-

Disposal presents a potential safety problem. The amounts of material are so small that sink disposal is appropriate. However, if bleach and ammonia mix, chloramine gas may be produced. Dispose of small amounts of the chemicals used in this experiment into running water at a sink. Wash the plates as soon as observations are completed.

Lab Hints-

Bleach degrades the plastic material of the 96-well plate. It is appropriate to always use the same well to contain bleach. Iodine stains the plastic. Do not let the plate sit for a long period before washing. After 20 minutes, rinse the system. The iodine stain is partially reversed by immersing the plate in a 5% sodium thiosulfate solution.

Answers-

Q1. Write a balanced chemical equation for the production of chlorine gas from bleach (NaClO.)

A1. Bleach contains chloride ions, and any strong acid will cause chlorine gas to be released from the solution.

Cl^- + ClO^- + 2 H⁺ --> Cl₂ + H₂O

Q2. Write a balanced equation for the reaction of chlorine gas with aqueous potassium iodide.

A2. Dichlorine oxidizes iodide ions to form colored iodine:

Cl₂ + 2 I^- --> I₂ + 2 Cl^-

3. The average molecular speed of Cl₂(g) at 25ºC is 2.6 x 10⁶ cm/sec Did Cl₂(g) travel this fast in the experiments? Was the observed rate faster or slower? Why?

A3. The chlorine is not diffusing into a vacuum, but into air. Collisions with other molecules cause the chlorine to be deflected back on its path. Although the speed of a given molecule may be extremely high, the path is deflected many, many times so that net progress in a given direction is slow.

Q4. Compare the rate of diffusion of the chlorine to that of the ammonia.

A4. The light molecules of ammonia (17 g/mol) diffuse more rapidly than do the dichlorine molecules (71 g/mol).

Reference-

Early versions of this experiment were developed for Chemistry 111 at the USAF Academy, Colorado Springs, CO.

It was extended at an EESA workshop at Lincoln East HS, Lincoln, NE, in 1988 by Dianne N. Epp, Lincoln, NE, and Micheal T. Morton, Crete, NE. Epp, D. N. et al J. Chem. Educ. 1989, 66, 436.

.Key Words 1-

gases, diffusion, Graham's Law, molar mass, oxidation, reduction, redox, acid, base, pH indicator

Elements-

Cl, N, I