Chemistry 30

Reduction of copper oxide.

An excellent movie from Prentice Hall. General Chemistry: Principles and Modern Applications

Electrolysis of water

An excellent movie from Prentice Hall. General Chemistry: Principles and Modern Applications

A Dramatic Redox Reaction - Lesson 1.1

CAUTION: Do this in a fume hood. Be sure to use a suitable container!!! Very exothermic.

PROCEDURE:

1. Place a small spoonful of potassium permanganate in a heat resistant container.
2. Pour a small amount of glycerin onto the pile (a dropperful or two)
3. The pile will produce smoke and fire in about 15 seconds (I've also had it sit for 15 minutes or more before it ignites)

THE REACTION:

14 KMnO_{4 (g)} + 4 C₃H₅(OH)_{3 (l)} → 7 K₂CO_{3 (s)} + 7 Mn₂O_{2 (s)} + 5 CO_{2 (g)} + 16 H₂O_(l)

 

Reduction of Lead Oxide - Lesson 1.1

PROCEDURE:

1. Place 20 g of lead(II) oxide, PbO, in a large pyrex test tube. Leave unstoppered
2. Heat over a bunsen burner for 30 minutes
3. Beads of lead will form on the inside of the test tube

THE REACTION:

2PbO → 2Pb + O₂

 

Silver - Copper Redox Reaction - Lesson 1.1

This is a very simple demonstration to do if a solution of silver nitrate is kept on hand (store in a brown bottle to keep light out). Within a few minutes the solid copper begins to darken, then turn "fuzzy" as the silver comes out of solution to form a solid. The solution will begin to turn blue as Cu²⁺ ions enter the solution.

This demonstration could be paired with the Zinc - Copper redox reaction to introduce the activity series (Lesson 2.5).

PROCEDURE:

1. Place some clean copper wire or a copper strip in a small beaker or other container.
2. Add silver nitrate solution to cover (prepare by dissolving a few grams of AgNO₃ in about 100 mL of water). The concentration is not critical.
3. Within a few minutes solid silver will form on the metal surface and copper ions will enter the solution , giving the solution a bluish hue.

THE REACTIONS: (the spectator ion, NO₃^-, has been removed)

Oxidation of Copper:    Cu_(s) → Cu²⁺_(aq) + 2e^-

Reduction of Silver:    Ag⁺_(aq) + e^-→ Ag_(s)

Net Reaction:    Cu_(s) + 2Ag⁺_(aq) → Cu²⁺_(aq) + 2Ag_(s)

Zinc - Copper Redox Reaction - Lesson 1.1

Another very simple demonstration. If done with the silver-copper redox reaction demonstration, the concept of an activity series (Lesson 2.5) can be introduced. When a zinc strip is placed in a solution of copper(II) sulfate, the copper ions will be reduced with the zinc is oxidized. In the silver-copper reaction, the copper was the metal undergoing oxidation. In the The solution will begin to turn blue as Cu²⁺ ions enter the solution.

PROCEDURE:

1. Place a clean zinc strip in a small beaker or other container.
2. Add copper(II) sulfate solution to cover (prepare by dissolving a few grams of CuSO₄ in about 100 mL of water).
3. The zinc strip will become corroded as it undergoes oxidation; the blue colour of the solution will fade as the copper ions become reduced and come out of solution.

THE REACTIONS: (the spectator ion, SO₄^2-, has been removed)

Oxidation of Zinc:    Zn_(s) → Zn ²⁺_(aq) + 2e^-

Reduction of Copper:    Cu²⁺_(aq) + 2e^-→ Cu _(s)

Net Reaction:    Zn_(s) + Cu ²⁺_(aq) → Zn ²⁺_(aq) + Cu _(s)

Synthesis of Zinc Iodide (video demonstration shown below )

(Visit this external link to see images of this experiment)

This redox reaction would be useful to demonstrate during the discussion of half-reactions. In this synthesis reaction between zinc and iodine to form zinc iodide, zinc is oxidized and iodine is reduced.

Pair this demonstration/lab with the Electrolysis of Zinc Iodide demonstration/lab. The ZnI₂ synthesized in this demonstration will be decomposed.

These demonstrations could be done as a student lab rather than as a demonstration.

This demonstration could also be used to demonstrate the effect of a catalyst on reaction rate (Unit 2, Lesson 4.4). This reaction will be very slow until catalyzed by water.

PROCEDURE:

1. Place about 0.2 g of powdered zinc in a watch glass. Fine powder works best, but course ground works well, also . The quantities are not critical and may be approximated.
2. Weigh out 0.2 g of iodine crystals and add to the powdered zinc. If the iodine crystals are large, grind them into a finer powder before mixing.
3. Mix the two solids together with a glass stirring rod. Although the reaction will begin upon mixing, it will occur too slowly at room temperature to produce an immediate observable change.
4. Place the watch glass on some white paper in a fume hood.
5. Add a ½ dropperful of water. Stir to mix well. The water will catalyze the reaction, allowing it to occur at room temperature. Heat will be generated, causing some of the iodine to vaporize, producing a violet "smoke".
6. The watch glass will now contain a gray slurry of unreacted zinc. The product of the reaction, ZnI₂, is water soluble so will not be visible yet.
7. To obtain the product, filter the contents of the watch glass. Place a clean beaker or watchglass beneath a funnel fitted with filter paper, then pour the entire contents of the watch glass onto the filter paper. Extra water may be added to completely rinse the watch glass. The ZnI₂ will be pass through the filter and collect in the beaker as part of the filtrate.
8. Discard the filter paper from the funnel and its contents. Pour the filtrate (the solution collected in the beaker below the funnel) onto a clean watch glass.
9. Allow the water to evaporate from the watch glass. This may take a day or two. A white solid, ZnI₂, will remain.

THE REACTIONS:

Oxidation of Zinc:    Zn_(s) → Zn ²⁺_(aq) + 2e^-

Reduction of Iodine:    I_{2 (s)} + 2e^- → 2 I^-_(aq)

Net Reaction:    Zn_(s) + I_{2 (s)} → ZnI_{2 (s)}

Electrolysis of Zinc Iodide - Lesson 3.2

(Visit this external link to see images of this experiment)

In this demonstration, which could be done as a student lab, the product of the Synthesis of Zinc Iodide demonstration is decomposed into it's elements through electrolysis.

PROCEDURE:

1. Obtain the solid ZnI₂ that was prepared in the Synthesis of Zinc Iodide demonstration. Place it on a watch glass if it is not already there.
2. Add a few drops of water to dissolve all of the white zinc iodide. Stir to make sure it all dissolves.
3. You will need a battery (9-volt works well), two wire leads with alligator clips at both ends, and to small pieces of copper wire (the electrodes), each a few cm long.
4. Assemble the apparatus - attach a wire lead to each post of the battery, and clamp the small copper wire in the other ends of each wire lead. The copper wire electrodes will be placed in the zinc iodide solution, keeping the two electrodes at opposite sides of the watch glass. You may want to use clay or masking tape to help hold the electrodes in place.
5. A gray coating of zinc will begin to form on the copper electrode that is attached to the negative post of the battery as the zinc ions become reduced:
   
         Zn ²⁺_(aq) + 2e^- → Zn_(s)
   
   This electrode is the cathode, the site of reduction. The electrons required for this half-reaction are supplied by the battery.
   
6. At the electrode attached to the positive post of the battery iodine ions become oxidized to form solid iodine crystals. These crystals will dissolve in the water producing an amber, then red, colour. This electrode is the anode, the site of oxidation. The electrons produced by this half-reaction return to the positive post of the battery.
   
            2 I^-_(aq) → I_{2 (s)} + 2e^-

THE REACTIONS:

Reduction of zinc ions:      Zn ²⁺_(aq) + 2e^- → Zn_(s)

Oxidation of iodide ions:     2 I^-_(aq) → I_{2 (s)} + 2e^-

Synthesis of Zinc Iodide