Oxidation States of Manganese

Description

Solutions of manganous ion (+2) and permanganate ion (+7) are used to prepared samples of other manganese oxidation states.

Go to Top

1. Prepare a concentrated solution of manganous ion by dissolving about 15 cm³ of solid manganous sulfate in an equal volume of distilled water. Add 1 mL of 3 M H₂SO₄ to the solution. (A 50-52% solution of manganous nitrate is commercially-available.) Note the color of the solution.
2. Note the characteristic purple-violet color of the permanganate ion is observed in 0.02 M solutions of KMnO₄.
3. Use a thin-stemmed polyethylene transfer pipet to add 10 drops of the concentrated manganous sulfate solution to a 50-mL beaker. Add 20 drops of concentrated sulfuric acid to the manganous sulfate solution using a thin-stemmed polyethylene transfer pipet. Use a thin-stemmed polyethylene transfer pipet to add 40 drops of 0.02 M KMnO₄ to the resulting solution with continuous stirring.
4. The resulting colored solution is suggestive of manganese (III) compounds. Note this color.
5. Use a thin-stemmed polyethylene transfer pipet to add 10 drops of 0.02 M KMnO₄ to a clean 50-mL beaker. Use a thin-stemmed polyethylene transfer pipet to add 10 drops of 0.02 M NaHSO₃ to the solution with stirring.
6. The resulting brown solid is manganese dioxide, MnO₂, with manganese in the +4 oxidation state.
7. Use a thin-stemmed polyethylene transfer pipet to add 20 drops of 50% NaOH to a clean 50-mL beaker. Place 5-10 pellets of sodium hydroxide on a plastic spoon. Transfer these to the beaker. Use a thin-stemmed polyethylene transfer pipet to add 10 drops of 0.02 M KMnO₄ to mixture with stirring.
8. Use a thin-stemmed polyethylene transfer pipet to add 10 drops of 0.02 M NaHSO₃ to the mixture with stirring. The blue ion MnO₄^3- is formed.
9. Note the color of the solution.
10. Use a thin-stemmed polyethylene transfer pipet to add 20 drops of 0.02 M KMnO₄ to a clean 50-mL beaker. Use a thin-stemmed polyethylene transfer pipet to add 5 drops of 50% NaOH to the solution. Use a thin-stemmed polyethylene transfer pipet to add 10 drops of 0.02 M NaHSO₃ to the mixture with stirring. The green ion MnO₄^2- is formed.
11. Note the color of the solution.
12. Set up an overhead projector and screen.
13. Display all 6 oxidation states at once.

Go to Top

Name ___________________________ Class ________

Teacher__________________________

DoChem 055 Oxidation States of Manganese

Makeup students should watch the movie and record the colors.

Oxidation state Color Stable acidity range

Go to Top

Purpose

To illustrate several oxidation states of manganese.

Go to Top

• overhead projector and screen
• suitably labeled transparency
• distilled water
• 25 g manganese sulfate (or 15 mL 50-52% aqueous manganese nitrate)
• 1 mL concentrated sulfuric acid
• 2 mL 0.02 M KMnO₄
• 2 mL 0.02 M NaHSO₃
• 1 g NaOH pellets
• 1 mL 50% aqueous NaOH
• plastic spoon
• 6 thin-stemmed polyethylene transfer pipets
• 6 50-mL beakers
• 3 glass stirring rods
• 1-L plastic container (for disposal)
• gloves, apron

Go to Top

• Practice the demonstration before performing in class. Check all solutions carefully. Do not permit students to perform this demonstration experiment.

Go to Top

Teacher preparation: 30 minutes

Presentation: 20 minutes

Go to Top

Concentrated sulfuric acid is extremely corrosive. Sodium hydroxide pellets and 50% aqueous sodium hydroxide are extremely corrosive. Dilute sulfuric acid is corrosive. Solid potassium permanganate is a strong oxidant. Potassium permanganate and sodium bisulfite are toxic.

Go to Top

• Handle the concentrated sulfuric acid with extreme care. Wear goggles, apron, and thin gloves. Use only the amount suggested in a transfer pipet.
• Handle the sodium hydroxide pellets and 50% aqueous solution with extreme caution. Wear goggles, apron, and thin gloves. Use only the amount suggested. Handle the pellets on a plastic spoon. Handle the solution in a transfer pipet.
• Use caution when handling the dilute sulfuric acid solution.
• Do not permit the potassium permanganate to come in contact with any reducing agents. Save any spilled material for disposal according to the procedure given.
• Do not ingest any of the chemicals used in the experiment.

Go to Top

• The objective is to convert all of the manganese compounds to manganese (II), and to discard the solution at the sink.
• Fill a 1-L plastic container with 200 mL of tap water. Add the contents of each beaker to this container, one at a time. Acidify the resulting solution with 1 M HCl if necessary. Add 2% NaHSO₃ solution in small portions, with stirring, until all of the brown manganese dioxide is reduced. Neutralize the resulting solution. Add just enough acid to dissolve any solid hydroxides. Discard the solution at the sink with 10 volumes of tap water for each volume of solution.

Go to Top

Manganese compounds exhibit several oxidation states. The (relatively) stable states are +2 as in Mn²⁺, +4 as in MnO₂, and +7 as in MnO₄^-. Some insoluble compounds of the +3 state are stable. Each oxidation state has an acidity range in which it is stable. The +5 and +6 states are stable in strongly alkaline solutions.

Go to Top

• redox
• oxidation
• reduction
• acid
• base
• oxidation state
• oxidation number
• manganese
• manganese compounds

Go to Top