Expt 023 -- Oxidation States of Manganese

Potassium permanganate is reduced under different conditions to form three different substances.

Introduction

Atoms in compounds may be found in various oxidation states. Atoms of some elements, like manganese, have many possible oxidation states. Oxidation-reduction reactions lead to changes in the oxidation state.

Safety

• Dilute sulfuric acid and dilute sodium hydroxide solutions are corrosive. Solid potassium permanganate is a strong oxidant. Potassium permanganate and sodium bisulfite are toxic. Wear goggles and apron. Use caution when handling the dilute sulfuric acid solution. Do not permit the potassium permanganate to come in contact with any reducing agents. Wash spills with water. Save any spilled material for disposal according to the procedure given. Do not ingest any of the chemicals used in the laboratory activity.
• Thoroughly wash hands before leaving the laboratory.

Procedure

1. Use plastic transfer pipets with constricted tips.
2. Place 10 drops of KMnO₄ in 4 wells of a 24-well plate, A-1 through A-4.
3. Add 4 drops of NaOH to well A-2.

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4. Add 3 drops of H₂SO₄ to well A-4.

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5. Place the plate on a sheet of white paper or other white background.
6. Add 3 drops of NaHSO₃ to well A-2 while stirring with a clean toothpick until there is evidence of a reaction. Note and record the evidence for reaction.
7. Add 3 drops of NaHSO₃ to well A-3 while stirring with a toothpick. Note and record the evidence for reaction.
8. Slowly add NaHSO₃ to well A-4, while stirring with a toothpick, until there is evidence of a reaction.

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9. Note and record the evidence for reaction.

   !!!Click here to See Picture.

10. Predict the effect of replacing NaHSO₃ with NaHSO₄. Test the prediction.

Questions

1. NaHSO₃ is frequently used to remove brown rust stains. Suggest a reason for this use. What must be known about the rust stains to make this suggestion consistent with principles?
2. Predict the outcome if a student mistakenly uses NaHSO₄ instead of NaHSO₃.
3. Suggest a reason why aqueous solutions of KMnO₄ are unstable.

Handout

Name ___________________________ Class ________

Teacher__________________________

SmallScale 023 Oxidation States of Manganese

MnO₂ is an insoluble brown solid. Mn²⁺ is faintly pink in solution. MnO₄^2- is dark green in solution. Based upon these colors, identify the manganese reaction products formed in wells A-2, A-3, and A-4.

For each well, determine whether the Mn in MnO₄^- undergoes oxidation or reduction. For each well, decide whether MnO₄^- acts as an oxidizing agent or a reducing agent. Identify the other agent in the reaction, and indicate whether it is an oxidizing agent or a reducing agent.

Complete the following table involving reactions between KMnO₄ and NaHSO₃:

	Well	Change Oxidation State of Mn	Change Oxidation State of S	Electrons gained by Mn	Electrons Lost by S
Reaction 1	A-2	7 --> 6	4 --> 6	1	2
Reaction 2	A-3
Reaction 3	A-4

Write a balanced chemical equation to describe each reaction.

Curriculum-

Use when discussing oxidation, reduction, redox, oxidation states, transition metals, or solution stoichiometry. Also, this is an excellent way to see how the acidity of a medium affords different possible reactions.

Safety-

• Dilute sulfuric acid and dilute sodium hydroxide solutions are corrosive. Solid potassium permanganate is a strong oxidant. Potassium permanganate and sodium bisulfite are toxic. Wear goggles and apron. Use caution when handling the dilute sulfuric acid solution. Do not permit the potassium permanganate to come in contact with any reducing agents. Wash spills with water. Save any spilled material for disposal according to the procedure given. Do not ingest any of the chemicals used in the laboratory activity.
• Thoroughly wash hands before leaving the laboratory.
• Goggles and aprons worn. Wash spills of H₂SO₄ and NaOH immediately. Do not ingest chemicals.
• Although not a part of the experiment, teachers should know that mixtures of potassium permanganate and concentrated sulfuric acid lead to the formation of explosive manganese oxide, Mn₂O₇.

Time-

Teacher Preparation: 20 minutes

Class Time: 30 minutes

Materials-

• 1 mL of 3 M H₂SO₄ sulfuric acid (use stock solution).
• 1 mL of 2.0 M NaOH sodium hydroxide (place 67 mL of 3 M NaOH in a 150-mL beaker and add 33 mL of distilled water.)
• 3 mL of 0.01 M KMnO₄ potassium permanganate (add 0.158 g KMnO₄ to 60 mL of water and add enough water to bring the total volume to 100 mL.)
• 1 mL of 0.02 M NaHSO₃ sodium bisulfite (add 0.208 g NaHSO₃ to 60 mL of water and add enough water to bring the total volume to 100 mL.)
• 1 mL of 0.02 M NaHSO₄ sodium bisulfate (add 0.272 g NaHSO₄ to 60 mL of water and add enough water to bring the total volume to 100 mL.)
• The KMnO₄ and NaHSO₃ solutions are reactive and change upon storage.
• 24-well plate (or small test tubes)
• plastic transfer pipets
• White (bond) paper
• Round toothpicks

Disposal-

Collect all of the reaction mixtures in a disposal jar or beaker. Neutralize the solution (with 3 M H₂SO₄ or Na₂CO₃). Add 0.01 M KMnO₄ until the solution is pink. Add 0.02 M NaHSO₃ until the pink color disappears. Filter. Discard the solid MnO₂ with ordinary solid wastes. Discard the filtrate at the sink.

Data Table-

The alkaline solution turns green and then slowly turns murky. A brown precipitate forms in the well with no added acid or base. The acid solution turns colorless.

Data Analysis-

	Well	Change Oxidation State of Mn	Change Oxidation State of S	Electrons gained by Mn	Electrons Lost by S
Reaction 1	A-2	7 --> 6	4 --> 6	1	2
Reaction 2	A-3	7 --> 4	4 --> 6	3	2
Reaction 3	A-4	7 --> 2	4 --> 6	5	2

Balanced equations:

2 MnO₄^- + 2 OH^- + SO₃^2- --> 2 MnO₄^2- + SO₄^2- + H₂O

2 MnO₄^- + 3 HSO₃^- --> 3 SO₄^2- + H₂O + 2 MnO₂ + H⁺

2 MnO₄^- + 5 HSO₃^- + H⁺ --> 5 SO₄^2- + 3 H₂O + 2 Mn²⁺

Answers-

Q1. NaHSO₃ is frequently used to remove brown rust stains. Suggest a reason for this use. What must be known about the rust stains to make this suggestion consistent with principles?

A1. Iron(III) oxide and hydroxide are much less soluble in acid solution than iron(II) hydroxide and oxide. The NaHSO₃ reduces iron(III) to iron(II).

Q2. Predict the outcome if a student mistakenly uses NaHSO₄ instead of NaHSO₃.

A2. NaHSO₄ is not a reducing agent. No reactions are expected to occur.

Q3. Suggest a reason why aqueous solutions of KMnO₄ and NaHSO₃ are unstable.

A3. The KMnO₄ is extremely reactive. (It reacts slowly with most plastic containers, for example.) Solutions of NaHSO₃ are readily air oxidized.

Reference-

See the Redox module of ChemSource.

Key Words 1-

oxidation, reduction, redox, oxidation states, transition metal, solution stoichiometry

Elements-

Mn, S