In this experiment you will examine two fundamental laws of chemistry:  the law of constant composition (Proust, 1799) and the law of mass conservation (Lavoisier, 1789).  Although nowadays we take them for granted, these ideas lay the foundation for Dalton’s atomic theory and chemistry as we know it. 

The Synthesis of Zinc Iodide from Zinc and Iodine

The synthesis of zinc iodide from its elements begins by adding 0.3-0.5 g of iodine crystals to about the same amount of granular zinc (10-50 zinc mesh mixture) in a small beaker.  Five drops of 6 M acetic acid is added to 10 mL of distilled water. Add about 3mL of this slightly acidified water to the zinc-iodine mixture and gently swirl the beaker to mix the reactants. In the presence of water, zinc reacts exothermically with iodine producing zinc iodide. During this oxidation-reduction reaction, some of the zinc atoms lose electrons and form cations while the iodine atoms gain electrons and form anions. Solid zinc iodide cannot be seen because water immediately dissolves the soluble product yielding aqueous zinc and iodide ions also in an exothermic reaction.

Once the reaction is over (it takes about 10 min), separate the unreacted zinc metal from the aqueous solution of zinc iodide (don’t forget to save the solution – that’s where the product is!).  Make sure all the zinc is removed from the beaker by washing with a small amount of water which can be added to the zinc iodide solution.  Try to use the minimum volume of wash water to minimize the amount of time spent boiling off the water in the next step.  Once the zinc is isolated, it can be dried in the oven to remove all trace of water and weighed.

The dissolved zinc iodide is isolated by boiling off the water.  This should be done in an evaporating dish.   (Beakers and flasks can crack when heated to dryness.)  Be careful of spattering that may occur near the end of the evaporation, as crystals begin to form, trapping water underneath.  Swirling the dish to spread the water around may reduce the chance of spattering.  Let cool just enough to weigh the product, but don’t wait too long because zinc iodide will absorb water from the air, forming a hydrate. 

 

The Decomposition of Zinc Iodide

Electrolysis

A spatula tip of the zinc iodide product is placed on a watch glass and dissolved with a few milliliters of water. Two metal electrodes connected to a 9-V battery are then placed into the zinc iodide solution for 1-2 min.  Electrolysis produces zinc metal at one electrode while at the other a red-brown color characteristic of iodine-iodide-triiodide is formed. Acquiring electrons from the battery, the reduction of zinc ion occurs at the cathode to produce elemental zinc.

Zn2+(aq) + 2 e- à Zn(s)

The oxidation of iodide ion occurs at the anode, where electrons are lost producing aqueous molecular iodine.

2 I-(aq) à I2 (aq) + 2 e-

Oxidation (optional)

This should be done in the hood because you are producing iodine gas!  One way to recover a sample of the original iodine crystals is by heating solid zinc iodide in a small Erlenmeyer flask which contains a smaller test tube filled with ice. Acting as a cold finger, the small test tube is held in place by a Buchner funnel filter adapter which rests on the mouth of the flask. It may also be necessary to manually hold the test tube using a clamp, so that you can vent the flask slightly to prevent a buildup of pressure.  With heat in the presence of air, the iodide is oxidized to gaseous violet-colored iodine and its crystals are collected on the surface of the cold finger. (This may not be possible because of water condensation on the cold finger). Zinc metal is not formed because zinc reacts with oxygen to produce zinc oxide.


Synthesis of Zinc Iodide

Data workup and report considerations

The purpose of this experiment is ultimately to determine the empirical formula for zinc iodide, or, put another way, x and y in the compound ZnxIy.

This can be done two ways based on the laws of conservation of mass and constant composition.  One way is graphically.  In the Matter and Motion folder on masu, you’ll find a spreadsheet named ZnI_exp.xls.  Before the end of lab, enter the following data in this spreadsheet along with the rest of the class: mass zinc iodide produced, mass zinc consumed, mass iodine consumed, and percent yield of zinc iodide (more on this later).  After everyone has entered their data, create two graphs of the mass of zinc iodide produced: one as a function of the mass of zinc consumed, the other as a function of the mass of iodine consumed.  What does the slope of these lines represent? 

The other way is algebraically.  By knowing the mass of zinc and iodine consumed (there’s an important assumption here – what is it?) and the mass of zinc iodide produced, you can determine the mass percent of each element in the compound.  From this, with consideration for the atomic mass of each element, the empirical formula can be found.

Using your experimentally determined empirical formula, calculate the theoretical yield and percent yield of zinc iodide.  (If you’re unfamiliar with these terms, don’t worry – we’ll discuss the concepts in class on Thursday)  Pool your percent yield with the rest of the class in a histogram.  Comment on trends and discuss a few factors that might account for these (ignoring calculation errors).

In your report you should emphasize: (a) how this experiment relates to the conservation laws, being as quantitative and specific as possible;  (b) a comparison of your graphical and algebraic results, addressing any questions brought up here.  This includes expected results, whether they agree with actual results, and possibilities why.  (Be conscious and clear in how you present your data!)

When writing your report, consider the following statement by Tufte: “In scientific work, both order-of-magnitude reasoning and precise measurements are pervasive.  How are such quantities represented in visual [or written] expression of ideas, experience, evidence?  How are…diagrams, and charts to be scaled and labeled?  And what makes images [and words] quantitatively eloquent?”