The Synthesis and Electrolysis of Zinc Iodide (ZnI₂)

Conducting this redox reaction helps demonstrate each of the two half-reactions. In this synthesis reaction between zinc and iodine forming zinc iodide, zinc is oxidized and iodine is reduced.

PROCEDURE:

1. Place 0.2 g of powdered zinc in a watch glass.

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 8-10 drops 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 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 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) ® Zn²⁺ _(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

PROCEDURE:

1. Obtain the solid ZnI₂ that was prepared in the Synthesis of Zinc Iodide. 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, color. 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-