Experiment 6: Chemistry of the Group 16 Elements

 SAFETY REGULATIONS
Disclaimer: In our choices of test tube tests, we have endeavored to find reactions which, if performed properly and with due care, are reasonably safe. Nevertheless, it is the responsibility of the instructor who choses to utilize these experiments to ensure that procedures meet their own criteria of safety and follow the regulations of their own university. It is also the responsibility of the instructor to follow all Local, State, and National laws, including those that pertain to disposal methods, some of which may differ from those listed here.

1. Safety glasses or goggles must be worn in the laboratory at all times.
2. Laboratory coats are compulsory. Open shoes or sandals must not be worn in the laboratory. Long hair must be tied back.
3. Smoking, drinking, or eating are not permitted in the laboratory.
4. Accidents must be REPORTED IMMEDIATELY.
5. All chemicals should be treated with respect. A notation appears in the experimental instructions where any specific hazard exists. Most chemicals are potentially dangerous if mishandled or misused. For this reason:

(a) Experiments may only be carried out when a supervisor is present. Only authorized experiments may be performed. Read all laboratory instructions carefully.
(b) Never taste chemicals or their solutions.
(c) Do not point the mouth of a test tube towards your neighbour or yourself.
(d) Acids and alkalis are corrosive substances. Any corrosive solution on your skin must be washed immediately and thoroughly with water.

6. Note the locations of the Safety Equipment.
7. Do not wander about the laboratory unnecessarily and do not interfere with other student's work.
8. Handle all organic solvents with care. Many are flammable. Some have long-term cumulative health effects.

Experiment 6a: Solubility of Sulfates

Though many sulfates are appreciably soluble in water, others are quite insoluble.

Reference: Chapter 16, 16.18 Sulfates and Hydrogen Sulfates.

Procedure: Dissolve a pea-size sample of sodium sulfate decahydrate in 2 mL of water. Dissolve a pea-size sample of barium chloride dihydrate in 2 mL of water. Mix the solutions.

Report:

(a) Note your observations.

(b) Write a net ionic equation for the reaction.

(c) Write the name of one other insoluble sulfate.

Experiment 6b: Acidity of the Hydrogen Sulfate Ion

Though many acid anions form acidic solutions, few are as acidic as the hydrogen sulfate ion.

Reference: Chapter 16, 16.18 Sulfates and Hydrogen Sulfates.

Procedure: Dissolve a pea-size sample of sodium hydrogen sulfate monohydrate in 2 mL of water. Add one drop of indicator and compare the color to a pH color chart.

Report:

(a) Note your observations.

(b) Write a net ionic equation for the equilibrium.

Experiment 6c: Sulfate/Hydrogen-Sulfate Equilibrium

Because of acid-base equilibria, a precipitate might not be what initially was predicted.

Reference: Chapter 16, 16.18 Sulfates and Hydrogen Sulfates.

Safety: Lead(II) ions are teratogenic. Wash any spills off your skin immediately and thoroughly.

Procedure: Dissolve a pea-size sample of sodium hydrogen sulfate monohydrate in 2 mL of water. Dissolve a pea-size sample of lead(II) nitrate in 2 mL of water. Mix the two solutions. Dispose of the product in the lead waste container.

Report:

(a) Note your observations.

(b) What is the identity of the precipitate?

(c) Suggest why this product is favored.

(d) Write a net ionic equation for the acid-base equilibrium and one for the precipitation reaction.

Experiment 6d: Effect of Acid on the Sulfite Ion

In solution, the sulfite ion is in equilibrium with the hydrogen sulfite ion and sulfurous acid. Addition of an acid to a sulfite will shift the balance to favor the production of these other ions. As sulfurous acid is formed, it is unstable to decomposition to its acid oxide and water. We can test for the presence of the gas using the yellow dichromate ion, which is reduced to the green chromium(III) ion.

Reference: Chapter 16, 16.15 Sulfur Oxides.

Procedure: To a pea-size sample of sodium sulfite, add 5 drops of 3 mol·L-1 sulfuric acid. Warm gently in a hot water bath in the fume hood. Place a strip of filter paper dipped in acidified 0.2 mol·L-1 potassium dichromate solution into the mouth of the tube. Do not inhale the sulfur dioxide gas. Pour the solution down the fume hood sink.

Report:

(a) Note your observations.

(b) Write the net ionic equations for the two-step formation of sulfurous acid from sulfite ion, the decomposition of sulfurous acid, and the overall equation.

(c) Write the half-reaction for the reduction of the dichromate ion and that for the oxidation of the gas to the sulfate ion. Then write the overall equation.

Experiment 6e: Sulfite Ion as a Reducing Agent

A typical redox reaction of the sulfite ion is that with the permanganate ion. The sulfite ion is oxidized to sulfate ion while the permanganate ion is reduced to the manganese(II) ion.

Reference: Chapter 16, 16.16 Sulfites.

Procedure: Dissolve a pea-size sample of sodium sulfite in 2 mL of water and acidify with three drops of 3 mol·L-1 sulfuric acid. Add two drops of 0.02 mol L- 1 potassium permanganate solution. Warm if necessary.

Report:

(a) Note your observations.

(b) Write the two half-reactions and then the overall reaction (the sulfuric acid is only present to supply hydrogen ions).

Experiment 6f: Effect of Acid on the Thiosulfate Ion

Addition of a strong acid to the thiosulfate ion results in the formation of unstable thiosulfuric acid.

Reference: Chapter 16, 16.19 Other Oxo-Sulfur Anions.

Procedure: Dissolve a pea-size sample of sodium thiosulfate pentahydrate in 2 mL of water and add two drops of 3 mol·L-1 hydrochloric acid.

Report:

(a) Note your observations.

(b) Write a net ionic equation for the formation of thiosulfuric acid, then an equation for the decomposition of thiosulfuric acid, and finally a combined equation.

Experiment 6g: Thiosulfate Ion as a Reducing Agent

The thiosulfate ion can act as a reducing agent; it is itself oxidized to the tetrathionate ion.

Reference: Chapter 16, 16.19 Other Oxo-Sulfur Anions.

Procedure: Dissolve a pea-size sample of sodium thiosulfate pentahydrate in water and add two drops of iodine solution.

Report:

(a) Note your observations.

(b) Write a half-reaction for the reduction of iodine and one for the oxidation of thiosulfate ion; then write an overall equation.

Experiment 6h: Unique Reaction of the Thiosulfate Ion

A unique reaction of the thiosulfate ion is that with the iron(III) ion. At low temperatures, a complex is formed; then the thiosulfate ion reduces the iron(III) ion while the thiosulfate ion is oxidized to the tetrathionate ion.

Reference: Chapter 16, 16.19 Other Oxo-Sulfur Anions.

Procedure: Dissolve two pea-size samples of sodium thiosulfate pentahydrate in 2 mL of water, cool the solution in an ice bath, and add two drops of 0.1 mol·L-1 iron(III) chloride solution. Then place in a hot water bath.

Report:

(a) Note your observations.

(b) Write an equation for the formation of the complex.

(c) Write a half-reaction for the reduction of iron(III) ion and one for the oxidation of thiosulfate ion, then write an overall equation.

Test 6i: Peroxodisulfate Ion as an Oxidizing Agent I

Whereas the sulfite ion is reducing, the peroxodisulfate ion is strongly oxidizing. Here you will use the peroxodisulfate ion to oxidize iodide ion.

Reference: Chapter 16, 16.19 Other Oxo-Sulfur Anions.

Procedure: Dissolve a pea-size sample of your product in 2 mL of water and add one small crystal of copper(II) sulfate (catalyst). Add five drops of 3 mol·L-1 potassium iodide solution. Stir. Warm in a hot water bath.

Report:

(a) Note your observations.

(b) Write half-reactions and a combined net ionic equation for the reaction.

Test 6j: Peroxodisulfate Ion as an Oxidizing Agent II

Many oxidizing agents will oxidize the iodide ion, but few will oxidize the chloride ion, as you do in this experiment with the peroxodisulfate ion.

Reference: Chapter 16, 16.19 Other Oxo-Sulfur Anions.

Procedure: To a pea-size sample of your product, add five drops of 3 mol·L-1 hydrochloric acid and warm in the fume hood. Test the gas produced with damp litmus paper and also identify it cautiously by odor.

Report:

(a) Note your observations.

(b) Write half-reactions and a combined net ionic equation for the reaction.