Student Version
Background Information
Elements react with each other to form compounds. Zinc and iodine react with each other to form an ionic compound, zinc iodide. The properties of the elements are different from those of the compound. In addition, the Conservation of Mass and the Law of Constant Composition will be observed. Finally, electric current will be used to break down the zinc iodide made back into its original elements.
Safety
Iodine is a corrosive solid and should be carefully massed on the electronic balance. Any spills should be cleaned up with sodium thiosulfate solution. Goggles will be needed as the iodine is corrosive and zinc iodide solution will be evaporated to dryness.
Materials
Zinc, granular (10 – 50 mesh)
Iodine crystals
Hot hands gripping pad
Vinegar (0.9M acetic acid)
Scoops (Spatulas)
Petri Dish
Stir rod
Copper electrodes
Mass of 50mL flask:
50mL Erlenmeyer flask
125mL Erlenmeyer flask
10mL Graduated cylinder
Electronic balance
Heat Plate
Tape
9 V battery
Alligator leads (2)
Procedure (Synthesis)
1.
Determine and record the mass of a 50mL Erlenmeyer flask.
________ g
2.
Approximately 2.0 g of iodine, then 2.0 g of granular zinc are added to the 50mL flask.
Record the mass of iodine and zinc used by massing the filled vial and the empty vial.
Record the appearance of the iodine and zinc. a. Mass of iodine+vial: ________ g
Mass of empty vial: ________ g
Mass of iodine: ________ g b. Mass of zinc+vial: ________ g
Mass of empty vial:
Mass of zinc:
________ g
________ g
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c. Appearance of iodine: d. Appearance of zinc:
3.
Add 5mL of vinegar and swirl. Make observations. Swirl the mixture until it seems the reaction is over. What happens to indicate that the reaction is over?
Call over a graduate student to check that the reaction is over before continuing. a. Observation of chemical reaction: b. What observations indicate that the reaction is over?
4.
Which element was completely used up in the reaction?
What element was not completely used by the reaction? What observations support your answer?
5.
Determine and record the mass of a 125mL Erlenmeyer flask. Decant the clear zinc iodide solution into the 125mL flask. Rinse the remaining zinc three times with 1 mL rinses of vinegar, which are added to the solution in the 125mL flask.
Mass of larger 125mL flask: _________ g
6.
(do step 7 simultaneously) Heat the smaller 50mL flask on a hot plate until the zinc and the flask are completely dry. Remove the flask from the hot plate and allow the flask to cool. Determine and record the mass of the zinc and the dry flask. a. Mass of small, dry 50mL flask + unreacted zinc: _________ g b. Calculate the mass of dry unreacted zinc present: _________ g
Based on the original amount of zinc placed in the reaction flask, how much zinc reacted? c. Mass of zinc that reacted: _________ g d. Mass of iodine that reacted: e. Mass of zinc and iodine that reacted:
_________ g
_________ g
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7.
Heat the zinc iodide solution in the larger 125mL flask using a hot plate set on a low setting. The solid zinc iodide will come out of solution as the water is boiled away. The zinc iodide will be damp and will need to be carefully heated until sounds of crackling stop and the zinc iodide is off-white or light yellow. There should be no water droplets on the upper, inside part of the larger 125mL flask. You do not want the zinc iodide to become dark yellow or orange this means it has started to decompose, because it has been heated too strongly. a. Final color of dry zinc iodide:
Take the 125mL flask out of the sand bath and allow it to cool for 5 minutes and then determine and record the mass of the dry zinc iodide and the large flask. Calculate the mass of the dry zinc iodide. b. Mass of dry zinc iodide and larger 125mL flask: c. Mass of dry zinc iodide:
Procedure (Decomposition)
_________ g
_________ g
1. Add 10mL of distilled water to your solid zinc iodide and dissolve most of the solid. Pour the solution into a small petri dish or watch glass. Secure one electrode (copper wire) to the petri dish with tape. Secure a second electrode to the opposite side of petri dish again with tape. Attach a set of electric leads to a 9 volt battery. Observe and record the appearance of zinc iodide solution, before the alligator clamps on the leads are attached to the secured electrodes.
Observations of zinc iodide solution:
2. Attach the electric leads, one to each of the secured electrodes. Write down your observations. What can you infer is forming at each electrode?
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Prepare a sketch of the equipment and describe what is happening at each electrode.
(Include the petri dish, the zinc iodide solution, the electrodes, the battery, the battery electrode charge and what is observed where the electrodes contact the zinc iodide solution in your sketch.)
Indicate on your sketch what is forming at each electrode.
What forms at electrode attached to the positive battery lead:
What forms at electrode attached to the negative battery lead:
Waste
Nothing goes down the drain!
1.
Pour petri dish into liquid waste. Dissolve any extra zinc iodide with water and pour into liquid waste container, rinse 2 more times into the liquid waste.
2.
Scrape out solid zinc from 50 mL flask into solid waste container. Then when most of the zinc is out, rinse with water 3 times into the waste beaker.
3.
Rinse 125 mL flask with water 3 times into the liquid waste.
4.
Place all glassware into dirty glassware bin provided.
5.
Rinse copper electrodes with water into liquid waste container.
6.
Leave other items on the table.
Analysis
1a. What mass of zinc and iodine were consumed in the reaction?
1b. What mass of zinc iodide was produced by the reaction?
1c. How does the mass of the zinc and iodine (reactants) compare to the mass of the dry zinc iodide (products)?
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1d. Do your results (within experimental error) prove or disprove the Law of Conservation of
Mass?
2. What observations indicated that a chemical change was taking place between the zinc and iodine?
3. How are the original elements different from the compound produced in this reaction?
4a. How many moles of zinc (Zn) were used in the reaction?
4b. How many moles of iodine (I
2
) were used in the reaction? (Remember to change mass into moles, the mass must be divided by the molar mass.)
4c. What is the ratio of moles of zinc to moles of iodine?
5. Based on your answers to question #6 above, what is the chemical formula of zinc iodide?
Write a balanced chemical equation for this reaction.
6. If you were asked to prepare 500 grams of zinc iodide, how many grams of zinc and iodine would be needed?
7a. Did the reaction between zinc and iodine give off (exothermic) or absorb (endothermic) energy?
Zinc + Iodine

Zinc iodide (Exothermic or Endothermic?)
7b. Did the decomposition of zinc iodide (using the 9 volt battery) give off (exothermic) or absorb (endothermic) energy?
Zinc iodide

Zinc + Iodine (Exothermic or Endothermic?)
8a. What type of energy was transferred when the two elements were reacted to form a compound?
8b. What type of energy was transferred when the compound was decomposed into the two elements?
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