Synthesis and Decomposition of Zinc Iodide

Objectives:

  1. Synthesize and isolate zinc iodide by reaction of the elements
  2. Observe differences in physical properties that distinguish the substances in this reaction
  3. Use the data to verify the Law of Conservation of Mass and to calculate the empirical formula of the product.
  4. Use electrolysis to decompose the zinc iodide back into its constituent elements.

Introduction:

Synthesis of Zinc Iodide

A balanced chemical equation is a form of chemical bookkeeping stating how many atoms, ions, and molecules (or moles) of reactant substances come together to form given numbers of atoms, ions and molecules (or moles) of product substances. A balanced chemical equation, however, usually provides no information about the process that occurs on the atomic level in a chemical reaction. There are also a number of practical questions in running a reaction and obtaining products that are not answered by the balanced chemical equation:

1. Does the reaction go to completion and how long does it take?

2. Under what set of conditions does the reaction run?

3. Do the reactants or products participate in competing reactions?

4. Can the products be separated in pure form from the reaction mixture?

In this experiment, a combination reaction between the elements zinc and iodine will illustrate the strengths and limitations of a balanced chemical equation and the practical problems encountered in obtaining a pure product. The reaction between zinc and iodine to produce the ionic compound zinc iodide is: Zn(s) + I2(s)→ ZnI2(s), and will go to completion after a period of several minutes. But this reaction is not as simple as its equation looks. If Zn metal and I2 are mixed, no reaction takes place. On the atomic scale the molecules in solid I2 cannot get close enough to the atoms in Zn metal to react. But if a small amount of water is added to this mixture, some of the I2 molecules will dissolve in the water and move close enough to the Zn metal atoms to react. Water is not a substance in the overall reaction, but its presence is one of the conditions needed for the reaction to occur at room temperature.

As this reaction proceeds, another reaction is observed. As the zinc iodide (ZnI2(aq)) forms , the iodide ion reacts with I2 to make I3- and the ionic compound zinc triiodide (Zn(I3)2(aq)) forms. The triiodide ion (I3-) has a distinctive wine-red color that is easy to identify. Fortunately, if excess Zn is present, Zn(I3)2(aq) reacts with Zn(s) to make ZnI2(aq). So this reaction does not affect the overall “chemical bookkeeping” in the chemical equation since ZnI2 is the final product.

Another problem is that the product ZnI2 can react with water in a double replacement reaction: ZnI2(aq) + 2H2O(l) → Zn(OH)2(s) + 2HI(aq). If this reaction occurs it will reduce the amount of product formed and might lead to the conclusion that the Law of Conservation of Mass is not obeyed. This reaction can be prevented if a small amount of a weak acid (such as acetic acid, CH3COOH) is added to the reaction mixture. Acetic acid does not react with any of the substances in the main reaction, and since it is volatile, it can be easily removed from the mixture by heating when it isn’t needed anymore.

Once the reaction is complete, a mixture containing an aqueous zinc iodide solution with solid pieces of Zn or I2 (depending on which one is the excess reagent) is left. How can pure zinc iodide be isolated from this mixture? A good way of doing this is the following two step process.

  1. Since the Zn(s) and I2(s) do not dissolve in water easily, they can be separated from the liquid mixture by carefully pouring off (decanting) the zinc iodide solution.
  2. The liquid contains zinc iodide mixed with water and acetic acid, but since both H2O and CH3COOH are volatile they can be removed by gently heating the mixture. This leaves pure ZnI2(s), the product of the main chemical equation, once evaporation of the water and acetic acid is complete.

Decomposition of Zinc Iodide

The properties of zinc iodide are quite a bit different from the properties of either pure zinc or pure iodine, so how does one know that zinc and iodine are actually present? Once way is to perform a reaction that will decompose the compound back into its constituent elements. By observing the distinctive properties of the elements after decomposition, one can confirm the presence of these elements in the original compound. The decomposition of zinc iodide into zinc and iodine will be done by electrolysis. When the positive and negative electrodes of a battery are placed in a solution of zinc iodide the negatively charged iodine ions are attracted to the positive electrode where they give up electrons to form I2 and the positively changed zinc ions are attracted to the negative electrode where they accept electrons to form Zn metal. By observing the properties (particularly color) of the substances forming at the electrodes, one can confirm the presence of zinc and iodine in the compound.

Procedure:

Part I – Synthesis of Zinc Iodide

  1. Prepare a sample of weakly acidified water by adding 18 drops of 5 M acetic acid solution to 25 mL of distilled water in a 50 mL beaker (use the graduations on the beaker for this measurement) and stir with a glass stirring rod until well mixed. Set this aside for use during the lab.
  2. Measure and record the mass of a clean, dry 20 x 150 mm test tube by laying it sideways on the balance pan.
  3. Obtain a 22 x 175 mm test tube and place one or two boiling stones in it. Measure and record the mass of this test tube + boiling stone(s). Set aside for use later in the experiment.
  4. Weight out 2.0 g of granular Zn metal in a clean weighing boat and add this to the 20 x 150 mm test tube from step 2. Measure and record the mass of test tube + Zn metal by carefully laying the tube sideways on the balance pan.

Mass of Zn = (mass of the tube + Zn) – (mass of empty tube)

  1. Weight out 2.0 g of iodine crystals (I2) in a clean weighing boat and add this to the 20 x 150 mm test tube from step 4 (contains Zn metal). Measure and record the mass of test tube + Zn metal + I2.

Mass of I2 = (mass of tube + Zn + I2) – (mass of tube + Zn)

  1. Add 5 mL of the acidified water (prepared in step 1) to the test tube from step 5 and gently swirl the contents. Note the physical properties (color, luster, physical state) of zinc and iodine at the very start of the reaction.
  2. After one minute, compare the color of the liquid in the test tube to the colors of the standard solutions of I2 and Zn(I3)2in the lab. Which color does the liquid in the reaction test tube look like most? Is Zn(I3)2 present at this point in the reaction? Feel the bottom of the test tube. Is it hot, cold, or at room temperature? Is the reaction endothermic or exothermic?
  3. Continue to gently swirl the mixture in the test tube until the color completely disappears from solution. This could take as long as ten minutes. Once the liquid is colorless, carefully pour the liquid into the 22 x 175 mm test tube that you weighed in step 2. Make sure that none of the Zn metal is poured over with the liquid.
  4. Add 1 mL of the acidified water (from step 1) to the Zn metal remaining in the 20 x 150 mm test tube and swirl the contents for a few seconds. After the Zn metal has settled to the bottom of the tube, carefully add this liquid to the 22 x 175 mm test tube. Once again, make sure you don’t pour any Zn metal pieces over with the liquid.
  5. Repeat step 9 two more times, adding the liquid to the 22 x 175 mm test tube. Then repeat step 9 three more times, but now discarding the liquid down the drain each time.
  6. Dry the washed Zn metal in the 20 x 150 mm test tube by heating it gently in the flame of a Bunsen burner. Don’t heat the test tube directly in the flame. Instead, pass the test tube slowly through the very top of the flame until the water in the tube starts to boil and evaporate. (If the test tube is heated too strongly, the Zn metal in the tube will react with oxygen in the air.) The Zn metal is dry when the metal pieces no longer clump together or adhere to the side of the test tube. Be sure to heat the sides of the test tube to remove any droplets of water. Be patient, this step could take several minutes.
  7. Let the tube containing the Zn metal (20 x 150 mm test tube) cool down, then weight and record the mass using the balance.

Mass of Zn consumed = (mass of the tube + Zn before the reaction) – (mass of tube + Zn after the reaction)

  1. Gently heat the 22 x 175 mm test tube containing the zinc iodide solution. Once again, do not heat it directly in the flame instead pass the test tube slowly through the very top of the flame until the water in the tube starts to boil and evaporate. After a period of several minutes the water will evaporate to leave a white or off-white residue. The wet residue will make crackling noises as it finishes drying. Once the crackling has stopped, the residue should be totally dry. Do not heat the reside too strongly since excess heat will decompose the zinc iodide (indicated by a dark yellow to purple color in the residue). Make sure you heat the test tube along its length to remove any drops of water. Set the test tube aside to cool. Measure the mass of the tube + zinc iodide when the tube is still warm, but not too hot to touch.
  2. Add together the masses of the zinc and iodine consumed in the reaction. Compare this value to the mass of zinc iodide obtains in step 13. Are the results consistent with the Law of Conservation of mass? Use the masses of zinc iodine consumed in the reaction to calculate the percent composition of zinc and iodine in zinc iodide. Use the percent composition of zinc and iodine to determine the empirical formula of zinc iodide. How well does the calculated empirical formula agree with the accepted value?

Part II – Decomposition of Zinc Iodide

  1. To the dry zinc iodide from step 13, add the remainder of the acidified water and stir the mixture with a glass stirring rod until the zinc iodide is dissolved. Pour half of this solution into a clean porcelain evaporating dish. (Save the other half in case you need to repeat this part of the procedure.)
  2. Obtain a 9 volt battery and two lengths of copper wire from the stockroom. Wrap one end of each length of copper wire around each of the two electrodes on the top of the battery.
  3. Bend the copper wire attached to the positive electrode in the evaporating dish until the end is completely immersed in the zinc iodide solution. Now carefully bend down the end of the copper wire attached to the negative electrode until it too is immersed in the zinc iodide solution. Observe what happens at the electrodes noting particularly any color changes on or about the ends of the wires. Are the colors you observe characteristic of zinc and iodine? What does this say about the composition of the solid product from the original reaction?

Synthesis and Decomposition of Zinc Iodide Data Sheet

Name______Date______Section ______

Part I – Synthesis of zinc iodide

  1. Mass of empty 20 x 150 mm test tube………………………….______
  2. Mass of 22 x 175 mm test tube + boiling stones……………….______
  3. a. Mass of tube + Zn……………………………………………______

b. Mass of Zn = (mass of tube + Zn) – (mass of empty tube)….______

  1. a. Mass of tube + Zn + I2...... ______

b. Mass of I2 = (mass of tube + Zn + I2) – (mass of tube + Zn)...______

  1. a. Two physical properties of Zn metal
  2. Two physical properties of I2

c. Color of the liquid in the test tube one minute after reaction started______

d. Does the color match that of I2(aq) or Zn(I3)2(aq)……………...______

e. Is the reaction exothermic or endothermic……………………______

11. a. Mass of 20 x 150 mm test tube + remaining Zn………………______

b. Mass of Zn consumed in the reaction………………………...______

13. a. Mass of 22 x 175 mm test tube + zinc iodide………………...______

b. Mass of Zinc iodide………………………………………….______

c. Two physical properties of zinc iodide:

1.

2.

14. Mass of Zinc iodide expected if Law of conservation of mass is obeyed

= (mass of Zn consumed + mass of I2)………………………______

Part II – Decomposition of Zinc Iodide

3. a. Color appearing at the positive electrode………………….______

b. Substance produced at the positive electrode.…………….______

c. Color appearing at the negative electrode…………………______

d. Substance produced at the negative electrode…………….______

Formal Report

Answer the following questions in your formal report. If the answer requires a calculation, show your work.

  1. How well do your results agree with the Law of Conservation of Mass (use percent difference to compare)? What errors might have led to any violation of the Law?
  2. Percent composition of Zn and I

% composition = (mass of element / mass compound) x 100%

  1. Empirical formula of zinc iodide for your measured masses
  2. What do the results of the decomposition of zinc iodide say about the composition of the solid product from part I of the experiment (i.e. what elements are present)?