(Introduction) | (Reaction) | (Decomposition) | (Data Sheet Walkthrough) | Glossary
Objectives: In this experiment you will (1) synthesis and isolate zinc iodide by reaction of the elements, (2) observe differences in physical properties that distinguish the substances in this reaction, (3) use your data to verify the Law of Conservation of Mass and to calculate the empirical formula of the product, and (4) use electrical methods to decompose the zinc iodide product back into its constituent elements.

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 run to completion and how long does it take?
2)Under what set of conditions does the reaction run?
3)Do any of the reactants or products participate in competing reactions?
4)Can the products be separated in pure form from the reaction mixture?

In this experiment you will conduct a combination reaction between the elements zinc and iodine that will illustrate the strengths and limitations of a balanced chemical equation and the practical problems in obtaining a pure product. The reaction between zinc and iodine to produce the ionic compound zinc iodide is written: Zn(s) + I2(s) -> Znl2(s), and will run to completion after a period of several minutes.

As this reaction proceeds, one can easily observe the presence of another reaction. As the zinc iodide (ZnI2(aq)) forms it reacts with I2 to make the ionic compound zinc triiodide (Zn(I3)2(aq)). The triiodide ion (I3-) has a distinctive wine-red color that is easy to identify.
One final problem is that the product ZnI2 can react with water in a double replacement reaction: ZnI2(aq) + 2H20(l) -> Zn(OH)2(s) + 2HI(aq). If this reaction occurs it will reduce the amount of product formed and might lead us to believe that the Law of Conservation of Mass is not obeyed. This extra 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 does one isolate pure zinc iodide from this mixture? A good way of doing this is in a 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 the zinc iodide solution. (2) The liquid contains zinc iodide mixed with water and acetic acid, but since both H20 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? One 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. You will use electrical means to decompose an aqueous solution of zinc iodide into zinc and iodine. When the positive and negative electrodes of a battery are placed in a solution of zinc iodide, the negatively charge iodide ions are attracted to the positive electrode where they give up electrons to form I2 and the positively charged zinc ions are attracted to the negative electrodes where they accept electrons to form Zn metal. By observing the properties (particularly the color) of the substances forming at the electrodes, one can confirm the presence of zinc and iodine in the compound.