Diffusion of Two Gases in Two Dimensions

Introduction

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. This is the principle you will study in this experiment.

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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 Beral pipets with pulled 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.
  6. Describe any changes to the drop of reagents (bleach and acid mixture).
    A chemical reaction takes place between the bleach and hydrochloric acid to form chlorine gas bubbles Cl2(g). A simple equation describes this reaction:
    bleach + acid --> chlorine gas
    The presence of chlorine gas will be detected by its reaction with potassium iodide as it moves through the system.
    The concentrated ammonia solution loses gaseous ammonia which is extremely soluble in water. Ammonia is detected by phenolphthalein.

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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 25°C is 2.6 x 10° cm/sec Did Cl2(g) travel this fast in your experiments? Was your rate faster or slower? Why?
  4. Compare the rate of diffusion of the chlorine to that of the ammonia.

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Safety

Bleach is caustic and damages skin and eye tissue. Wear eye protection. 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 you hands before leaving the laboratory.

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TG Answers

  1. Bleach contains chloride ions, and any strong acid will cause chlorine gas to be released from the solution.
    Cl- + ClO- + 2 H+ --> Cl2 + H2O
  2. Dichlorine oxidizes iodide ions to form colored iodine:
    Cl2 + 2 I- --> I2 + 2 Cl-
  3. 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.
  4. The light molecules of ammonia (17 g/mol) diffuse more rapidly than do the dichlorine molecules (71 g/mol).

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TG Lab Hints

Dispose of the chemicals at the sink with lots of water. 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. After 20 minutes, rinse the system. The iodine stain is partially reversed by immersing the plate in a 5% sodium thiosulfate solution.

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TG Materials

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TG Reference

A microscale version of this lesson based upon the diffusion of chlorine was developed for Chemistry 111 at the USAF Academy, Colorado Springs. Colorado.

The lesson was extended by:

Dianne N. Epp
Lincoln East High School
1000 S. 70th St.
Lincoln, NE 68510

Micheal T. Morton
Crete Public Schools
1500 E. 15th
Crete, NE 68333

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