Synthesis and Decomposition of Zinc Iodide - ACS Publications

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Synthesis and Decomposition of Zinc Iodide. Model Reactions for Investigating Chemical Change in the Introductory Laboratory. Stephen DeMeo. Barnard ...
Synthesis and Decomposition of Zinc Iodide Model Reactions for Investigating Chemical Change in the Introductory Laboratory Stephen DeMeo Barnard College, Columbia University, New York, NY 10027

One of the first experiments that introductory chemistry students perform i n t h e laboratory is the synthesis of a binary compound from two elements. Two classic syntheses of this type are the synthesis of copper sulfide and magnesium oxide, both of which are performed currently by introductory chemistry students and have been for many years. Teachers have their students nerform this t w e of reaction because i t allows students to understand thyfundamental concepts and laws governing chemical change. Furthermore, because this synthesis is a n example of one of the simplest types of reactions, i t establishesa starting place to discuss more complex examples of chemical chanae. The purpose of this pap& is to discuss a synthesis notwidely used by chemical educators in high schools or colleges, a synthesis that when coupled with a decomposition reaction has manv scientific advantages over current syntheses of binary compounds from elements. The discussion of this coupled reaction, referred to a s the synthesis and decomposition of zinc iodide, builds upon earlier work described in this Journal and in other educational sources (1-6). The synthesis of zinc iodide from its elements, zinc and iodine, and the subsequent decomposition of zinc iodide back into its elements are important for chemistry teachers to know about because these reactions can be performed by students to understand different aspects of chemical change such a s the concepts of reaction, compound, bonding, excess and limiting reactants, a n empirical formula, balanced chemical equation, the conservation of matter and energy, the Law of the Conservation of Mass, and the Law of Constant Composition. These concepts, in turn. a r e imnortant because thev are fundamental to chemistry, are widely taught by chemistry teachers, and are deceptively diff~cultfor introductory chemistry students to understand (7-10).Lastly, the synthesis and decornnosition of zinc iodide can be nerformed safelv and quickly, require inexpensive and available materials, are reliable, and produce waste that can be disposed of easily. A Review of Laboratory Manuals Students working in a laboratory often use commercial laboratory manuals to conduct experiments for the synthesis of a binary compound from its elements. A review of introductory college chemistry laboratory manuals published in the United States between 1971 and the present, indicates that onlv a few different svntheses usina elements are available to teachers. seventeen of 29 manuals sampled, contained a t least one synthesis of a compound from two elements. Ten of the 17 described the synthesis of a metallic sulfide. seven described the svnthesis of a metallic oxide, and three described the synihesis of a metallic iodide (zinc iodide and antimony iodide). The popularity for these syntheses stem from the simplicity of these reactions, the low cost of the equipment involved, and the un'A boiling tube, rather than a regular-sized test tube, expedites swirling and enhances visibility.

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Journal of Chemical Education

necessary addition of numerous reagents to prepare the elements for reaction. I n all of these syntheses, the procedures and purposes outlined in each laboratory manual are essentially the same. The procedures require measuring the mass of the elements when nossible. reactinn the elements. measurine the mass of the'compound, andshowing that the physicz and chemical nronerties of the comnound are different from the reactkg elements. The of doing the reactions are to .eenerate emnirical formulas and balanced . 6:qu:ltion;t via quantit:~ti\.ean;ilysi.i. The following is ;I scientific discuss~onof the svnthes~sand decom~ositionof zinc iodide and how this of reactions can provide evidence for the construction of chemical change concepts. The Synthesis of Zinc Iodide from Zinc and Iodine

The svnthesis of zinc iodide from its elements bezins bv adding 2 g of iodine crystals to 2 g of granular zinc'il0-56 zinc mesh mixture) in a boiling tube.' In this reaction, zinc is the excess reactant and iodine, the limiting reactant. These amounts are used for three reasons. First, the amounts allow a n appreciable mass of zinc to react which is detectable on a n analvtical balance (f0.001 a) as well a s the commonly found triple-beam balance ( f 6 0 1 g). Secondlv. iodine must not be used as the excess reactant because it dissolves and cannot be separated readily from aqueous zinc iodide and weighed. Lastly, the masses are small enough to control thekxothermie heat of reaction and deny sublimation of any iodine. Three drops of 6 M acetic acid is added to 5 mL of distilled water. This slightly acidified water is then added to the zinc-iodine mixture and the test tube swirled. I n the presence of water, zinc reacts exothermically with iodine ~roducinezinc iodide. Durine this oxidation-reduction reHction, &me of the zinc atoms lose electrons and form cations (ZU~+), while the iodine atoms gain electrons and form anions (I-). Solid zinc iodide cannot be seen because water immediatelv dissolves t h e soluble nroduct vieldinn : zn2+(aq)and\-(aq) ions also in a n exothermic reaftion. B cause these ions move about in the aqueous solution, further reaction occurs between molecular iodine and elemental zinc (3). Acidification of the 5 mL of water with acetic acid is not necessary to produce a quantitative product of zinc iodide. I t is done mainly for pedagogical reasons. The slightly acidic solution prevents the formation of zinc hydroxide which occurs a t about a pH of 7. This white side product in the solution could distract students from the reaction between zinc and iodine. When water is added to the zinc and iodine mixture, the colorless aqueous solution immediately changes color to an orange-reddish color. As time progresses and with swirling, the color becomes darker red-brown and the tube becomes hot to the touch. These sensory experiences can provide students with evidence t h a t indeed a reaction has taken place. The change of the aqueous solution from colorless to red-brown indicates that a n aqueous molecular

iodine solution has formed, that is, triiodide ion in equilibrium with iodine and iodide ion (11).As the reaction progresses, the red-brown solution fades to orange, to yellow, and finally turns colorless. As these color changes occur, the temperature of the solution decreases until it finally reaches room temperature. The resulting colorless solution indicates that the reaction has ceased and all of the solid and dissolved iodine have reacted with the zinc to oroduce iodide and zinc ions. The absence of dark-colored iodme crystals or a red-brown colored 8olution can ullow students to understand that iodine is the limiting reactant, while the concept of excess reactant can be understood by the observation of some unreacted zinc observed at the bottom of the boiling tube appearing as it did when it was initially weighed out. Eventually, the quantification of the excessand Emiting reactants dlows the masses of the reacting elements to be compared to the mass of the product. The Law of the Conservation of Mass is demonstrated if the sum of the masses of the reactants which have reacted equal, to within the uncertainty of the balance, the mass of the product. Once the reaction is over (it takes about 10 m i d , the aqueous solution of zinc iodide in the boiling tube is then decanted into a oreviouslv " weiehed larw test tube containing a boiling chip. The remaining zinc in the boiling tube is rinsed three times with 1mL washes of the acidified water. the washes being added to the solution in the large test tube.2 To find the mass of the excess zinc, the boiling tube containing the zinc is further rinsed with a few milliliters of acidified water, the washes this time being discarded. The boiling tube and excess zinc are then dried over a flame, allowed to reach room temperature and together, are weighed on a balance. Drvness is evident when the manular zinc no longer adheres to the sides of the boiling tube. The mass of the excess zinc is found to be about 1.5 g. This indicates that of the 2.0 g of zinc initially weighed, 6.5 g of zinc reacted with 2.0 g of iodine. To isolate the zinc iodide, the large test tube containing about 8 mL of the aqueous zinc iodide solution is heated gently over a Bunsen burner flame until the aqueous solvent is evaporated. This step takes another 10 min, after which a white to off-white solid of zinc iodide is f ~ r m e d . ~ Next, the test tube containing the solid is allowed to reach

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21t acet c aclo s not Lsed to lower !he pH, !hen wh te z1nc hydrox de w I form when !he excess z nc metal, wnose s~rface contam some remnant aqbeoLs z nc loo de 1s wasned wctn water 3The formation of the solid is not the end of the heating process because zinc iodide can form at least two hydrates (Znlp .ZH,O and ZnL. 4H"OI. the more common one beina a dihvrate (201.To acauire theanhv&& iodide. heatino is coninued inti1 sou"ds of crack~~, ,~~ zinc ~ mg cease and wnen some of the prod~cttuns on-whte or I ghl yelow Excesswe heating cases lhe lioeral on of lod ne lhal Lrns tne prod~cta oark yellow color Th s can oe avo ded eas ly by movong tne test tube rapidly over the flame. 4An average percent yield of 98.83%with an average deviation of f 1.07% was found for five trials when acetic acid was not used with the distilled water. As can be seen, the yields with and without acetic acid differ by about 1%. From this, it is argued that acetic acid does not meaningfully effect the data, the elucidation of the Law of Constant Composition nor the Law of the Conservation of Mass. S~lectrolysis units using copper wires connected to small stainless steel nails function adequately. Pencils or sticks of carbon work also. but do not allow the zinc metal to be observed very clearly. The most effective and inexoensive electrodes are the exDosed ends of wire clips from Rao o shack Tnese wares are made of copper that enhances u s b l y ofzlnc meta tormlng at the calhooe Altno~ghcopper 1s not an nen meta becaJse 11 slow y reams w th oo~ne,nevertheless, it functions perfectly well in this context because only zinc and iodine are observed. Copper electrodes are not recommended to be used when performing formal electrochemistry experiments. ~

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room temoerature and weiehed. Time spent cooline the product tdroom temperature should be kept to a minikum because zinc iodide slowly absorbs moisture from the air at a rate of about 0.001 g ofwater per minute (5).The rate of water absorption does not significantly effect the calculations if the product is weighed fairly quickly. If available, a desiccator or a zip-lock bag containing desiccant would minimize the absorption of water when zinc iodide is cooled andlor stored. After the reaction, about 2.5 g of zinc iodide is usually produced. Because a 2.5-g yield was expected (0.5 g of zinc plus 2.0 e of iodine). the Law of the Conservation of ~ a sisi supp&ted. using a n analytical balance, an averaee oeroent vield of 99.70% was achieved based on five t r i a i ~ The . ~ k e r a g e deviation of these five trials was f0.47%. From the masses of the reactine elements and their molar masses, a n empirical formula can be written. Exoerimentallv. the reaction between zinc and molecular iodine is stbichiometric with reactants existing in a 1 : l : l mole ratio with the product, zinc iodide. In terms of atoms, zinc iodide consists of one atom of zinc bonded to two atoms of iodide. From the empirical formula and knowledge of diatomic molecules, a n overall balanced equation can be written: Z d s ) + 12(s)+Zn12(s)

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In support of the Law of Constant Composition, zinc iodide is produced in a fixed mass ratio. The experimental mass ratio of zinc to molecular iodine is about 0.25:l. which corresponds to the molar mass ratio of 0.2576:l drtermined from the Penod~cTable I 121. The Decompositionof Zinc iodide into Zinc and iodine A spatula tip of the zinc iodide product is placed on a watch elass and dissolved with a few milliliters of distilled water.-TWOmetal electrodes connected to a 9-V battery are then placed into the zinc iodide solution for 1-2 min.5 Electrolysis produces blue-grey colored zinc metal at one electrode while a t the other, a red-brown color characteristic of iodine-iodide-triiodideis formed. Acquiring electrons from the battery, the reduction of zinc ion occurs a t the cathode to elemental zinc. Zn"bq) + 2 e + Znk)

The oxidation of iodide ion occurs a t the anode, where electrons are lost producing aqueous molecular iodine. 2 Uaq) + 12(aq)+ 2 eDuring electrolysis, the f a d that zinc iodide is a white compound which dissolves to give a clear and colorless solution is important. The red-brown color of the iodine-iodide-triiodide and the grey zinc metal appear against the background of a colorless solution. Because these colors are different and highly visible, and because no other products are visibly formed, the visual identification of these substances can be rendered without a high degree of conjecture. If desired, qualitative tests can be conducted to identify positively zinc and iodine. One way to recover a sample of the original iodine crystals is by heating solid zinc iodide in a test tube 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 large test tube. With heat, gaseous violet-colored iodine is liberated and its crvstals are collected on the surface of the cold finger. Zinc metal is not formed because zinc reacts with oxygen to produce zinc oxide. The solid iodine can be removed from the exterior of the cold finger and qualitatively tested. The notion that elements are conserved in a compound can be understood by first the synthesis between zinc and Volume 72 Number 9 Se~tember1995

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iodine to produce zinc iodide, then the decomposition of a sample of zinc iodide back into, and only into, its constituent elements, zinc and iodine. If the concept of atoms is understood and it is known that elements consist of atoms, then conservation of atoms through chemical change can be likewise demonstrated. The re-appearance of the original elements with electrolysis does not involve the addition of new substances (e.e.. . - . reagents). . Electrolvsis is mef e r r e d because t h e source of t h e u n d e r s t a n d i n g of conservation is to know that both the elements were in the compound and that the compound was no more than a transformed combination of the two elements. If this were not the case, some inexperienced students might find it difficult to determine, for example, if the zinc is actuallv contained in the zinc iodide or brought in by a reagent (e.g., magnesium added to aqueous zinc iodide precipitates zinc). The simplicity of electrolysis mitigates this p;oblem. The synthesis and decomposition of the compound also begs the question: "If the eiementdatoms are-present in the compound, why aren't they observable in that compound?" The answer to this question entails a t least two other critical topics of chemical change: atomic structure and electron interaction during bonding. Lastly, a comparison between the heat energy given off during the synthesis with the electrical energy needed to decompose zinc iodide into the elements, can lead to a discussion of the conservation of energy on a qualitative level.

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Safety and Disposal Concerns The synthesis and decomposition of zinc iodide can be performed safely. With the exception of iodine gas, the forms of these chemicals are non-toxic or only pose slight hazards to the user (13). The possibility of iodine sublimation during the synthesis can be rendered a non-issue by using.. manular zinc (10-50 zinc mesh mixture), certain . glasiw;~r(,, and a prtwrihrd nmonnt of water. Ihring the decnmpositinn rc;ictinn whm. zinc iodide is heated, iodine gas solidifies a s a solid and is contained in the test tube by the cold finger apparatus and later by corking. Furthermore, iodine's characteristic color and odor are easily detectable. This enables the user to recognize quickly if contact with this substance has been made and then respond for treatment. For example, iodine stains the skin a yellowish color. Once detected. the stain can he removed with rubbing alcohol or a weak 8olotlon of sodium thioiulfat(:. Zinc, iodine and zinc iodide can be d ~ s ~ o s of e dreadill bv first, changing the solids to ions, dilution to standard% levels. and finallv. in a sewer svstem (13.14). The .. disposal . disposal of these chemicals do not pose"a threat to the environment, infringe on federal laws, nor preclude a costlv removal program(a1ways check with local authorities before disposing of any chemicals). Recycling of the excess zinc after the synthesis also can be done if desired. Practical and Economic Benefits There are practical and economic benefits for using zinc, iodine, and zinc iodide. These chemicals and the equipment used in the synthesis and decomposition reactions are readily attainable from chemical supply companies and are inexpensive to purchase (15). Moreover, they are easv to dispense., weigh. - , and test because thev exist as solids a t room temperature. Lastly, the synthesis and decomposition reactions take minutes to do res~ectivelv.while a quantitative synthesis can be performedAbystudints during a 45-min class period. The Analysis of Zinc and lodide in Zinc Iodide Because a quantitative analysis of a product is a n important aspect of chemistnr. two additional activities can be performed easily that extend the utility of the synthesis 838

Journal of Chemical Education

and decomposition of zinc iodide. The first involves quantifying the mass of zinc in the synthesized zinc iodide and then comparing that value to the mass of zinc that was initially reacted to form that compound. This is done in order to reestablish that the amount of zinc reacted is conserved in the zinc iodide. The quantification of zinc in zinc iodide is achieved by a titration of zinc ion with a standard solution of EDTA ions using Calmagite or Eriochrome Black T indicators (16). Likewise, the quantification of the iodide ion in zinc iodide can demonstrate that the anion - - -is conserved. The amount of iodide in zinc iodide is determined bv a volumetric titration of iodide ions with a standa r d solution of silver n i t r a t e using 2'J-dichlorofluorescein. a n absorption indicator (17). If a oualitative analysis is deswable, a varirty of simple chemical tests cun bc, wrformed and standards used in order to further identify reactants and products. For example, excess zinc can be identified in part by reacting it with hydrochloric acid and testing for hydrogen gas. ~

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The Disadvantages of Currently Used Syntheses From a review of laboratory manuals, introductory chemistry students currently perform other syntheses of binarv com~oundsfrom the elements in order to understand chemical change concepts. These syntheses have certain limitations and simificant drawbacks in comoarison to the synthesis and decomposit~onof zlnc iodide. The metalhc .sulfide svnth(!.sei (.ommonlv involve the roaction between iron, cogper, nickel, or lead-with sulfur. The major drawback of these syntheses is the inability to quantify sulfur, the excess reactant, after it has reacted with the metal. The excess sulfur is virtually impossible to quantify because some of it bums away as sulfur dioxide. Therefore, the amount of sulfur that has reacted cannot be measured directly; it can be derived only from the mass of the metal sulfide. I n this way, the mass of reacting sulfur is a n artifact of the product and does not demonstrate the Law of the conservation of Mass a s previously discussed. Moreover, sulfur dioxide is a toxic gas that can adversely effect the health of students in the laboratory if inadequate ventilation is not provided. Because these metallic sulfides are not soluble in water, simple electrolysis equipment cannot produce the elements in aqueous media. Reheating the compound also will not decompose the compound into its elements. This prevents the conservation of matter to he discussed readily from empirical evidence. Even more problematic, the synthesis of copper and sulfur is not stoichiometric due to the formation of CU& in CuzS (18). I n other words, when the compound is produced, a variable composition is observed and consequently, the Law of Constant Composition cannot be demonstrated adequately. The other synthesis of a compound from its elements is the reaction of magnesium, tin, or copper with oxygen to produce a metallic oxide. When considering these reactions, the measure of the reacting masses an; the product is problematic on three accounts. First, if air is used a s a n oxygen source, the oxygen cannot be separated easily and auantified bv students. If the source of oxwen is from a " reaction in the laboratory or from a cylinder, a measuring device such a s a gas syringe would have to be used. This would necessitate higher equipment costs and laboratory time. Second, in order to produce the oxides. heat is applied usually to crucibles containing the metal's. These c&cibles sometimes crack during the hour-long heating process and therefore, invalidate the quantitative analysis. Third, magnesium oxide ash is easily airhome when crucibles are left open. The subsequent lbss of any product certainly would invalidate the analysis and could easily produce nonstoichiometric results. Because the mass of the excess reactant cannot be determined, the capacity of

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these two reactions to provide results i n support of the Law of the Conservation of Mass is inadequate. Finally, the conservation of matter cannot be demonstrated since all of the above compounds cannot be decomposed readily to produce the elements. he l a s t reaction found i n t h e survey of laboratory manuals is the synthesis of antimony (111) iodide. The first limitation of this reaction involves evaporating the solvent, toluene, to isolate the product. Because toluene is toxic and flammable, safety precautions (e.g., fume hoods and avoidance of flames) must be taken when students work with these chemicals. Secondly, the synthesis of antimony (111) iodide cannot be followed readily by a decomposition reaction because the compound forms SbOI when This does not allow the conservation placed i n water (19). of matter to he demonstrated a s well a s zinc iodide.

Summary If introductory students have only enough time in the laboratow to perform one exoeriment for each new chemistry topic that is taught, a s h a l l y i s the case, then i t is imperative for teachers to choose the best available experiment. The best experiment is one that can convincingly allow students t o understand a concept. is aestheticallv pleasing, can be performed safely and ; n a timely fashion, is inexpensive, reliable, and produces waste that i s manageable. I t i s i n this context that the synthesis and decomposition of zinc iodide has some important advantages over the current syntheses of binary c&npounds that are commonly found in the literature. Summarized below are the reasons for using the synthesis and decomposition of zinc iodide with introductory chemistry students:

The synthesized zinc iodide can be analyzed readily by two different titrations. Quantification of zinc and iodide ions can be used to demonstrate the Law of the Conservation of Mass from the product side. While the synthesis and decom~ositionof zinc iodide have been described i n the literature, significant improvements have been made by the author. These include the incorporation of a 10-50 mesh mixture of granular zinc, specific volume of water and its acidification with acetic acid, the ability to isolate solid iodine from zinc iodide, and safety glassware. Currently research is being conducted to determine how and to what extent students, who perform these reactions, are able to construct a n understanding of specific concepts and laws governing chemical change.

A Collection of Teacher Activities This author has developed a small collection of teacher activities describing the synthesis, drrornposltion, and a n d v s ~ of s zinc iodide. All ol'the activitirs contain deta~lcd procedures, diagrams, safety and disposal requirements, and extensive discussions only touched upon i n this article. Appropriate for high school and first-year college chemistry teachers, the activities are intended to be a chemistry resource not a collection of prescriptive worksheets that can be handed out to students i n the lab. They are available on a 3.5-in. computer disk for the Macintosh a t no charge from the author. Requests should he accompanied with a self-addressed stamped envelope to Stephen Demeo, 2 Oakledge Drive, East Northport, NY 11731. Acknowledgment The author would like to acknowledge P a m Fallon, the reviewers of this article, and the nine chemists-Arlene Ferko, Olympia Jebejian, Albert Kodjo, J e a n Lythcott, Marvlou Powderlv. Meenakshi Rao. Earle Scott. Keith shepard, and Nicholas Turro-who initially commented on the activities involvine the svnthesis and decom~ositionof zinc iodide a s they were devhoped.

Zinc and iodine are commerciall~available and are inexpensive. Zinc and iodine are elements and readilv form a stable anhv, drous compound in one proponton. The renrtion is colorfdl nno produces small amount of hmt. Idcnrifiention can be made visually btcauie all three of there chcmtcnls have dif: ferent colors. A n anhydrous sample of zinc iodide can be synthesized in Literature Cited less than half an hour. The svnthesis is stoichiometric to 1% 1. Ka8imer.P J. C k m . Ed-. 1958,35,A387 of theoretical mass and this, can demonstrate quantita2. Walker. N.J. Chem. Educ. 1980.57.738. tivelv the Law of Constant Camuosition. An emuirical for3. Shakhashiri. B. chemimiDemo~amtim8; University of Wisconsin: WI, 1983:Vol 1. mula and a balanced equatmn r a n he written. pp. 49-50. Thr mdss of the rractlnx elements can be mensurcd d:rectb: 4. Korenic. E. In The Wwimw WilsonFoundoIion Chemistry Institute Curri