Synthesis and Decomposition of Zinc Iodide

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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) 5. 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) 6. 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. 7. 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)2 in 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? 8. 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. 9. 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. 10. 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. 11. 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. 12. Let the tube containing the Zn metal (20 x 150 mm test tube) cool down, then weigh 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) 13. 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. 14. 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 2. Mass of empty 20 x 150 mm test tube………………………….____________ 3. Mass of 22 x 175 mm test tube + boiling stones……………….____________ 4. a. Mass of tube + Zn……………………………………………____________ b. Mass of Zn = (mass of tube + Zn) – (mass of empty tube)….____________ 5. a. Mass of tube + Zn + I2.............................................................____________ b. Mass of I2 = (mass of tube + Zn + I2) – (mass of tube + Zn)...____________ 6. a. Two physical properties of Zn metal i. ii. b. Two physical properties of I2 i. ii. 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% 3. Empirical formula of zinc iodide for your measured masses 4. 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)?


Writing a Formal Report

General Instructions The formal lab report should be written in your own words and should describe concisely the purpose of the lab, what you did in the laboratory, and what your results and conclusions were. Extensive copying of the laboratory handout signifies a lack of originality and will be graded as such. Extensive description of the minute details of the laboratory procedure is also unoriginal as well as boring. Please avoid both of these tempting writing techniques in your laboratory report. Two to four double-spaced typewritten pages (Times New Roman font in 12 pt) should be sufficient length for the laboratory report. The report should be written in past tense and passive voice (except the introduction). Your instructor will give you a list of three “post-lab’ questions at the start of the laboratory period. The answers to these questions must appear in the appropriate section in your report. Do not confine your report to answering these questions alone. You will turn in the completed data sheet for the lab along with your formal report. Your grade for the formal lab will consist of points from your data sheet results, points for you answers to the “post-lab” questions, and points for overall style and content.

Report Format To help both the writer and the reader organize their understanding of the topics of the report, scientists have agreed to use a six-part format when writing their lab reports. The six parts are, in their order of appearance: 1. Title: This is simply the name of the experiment. For our purposes this will be a separate page and also include your name, your partners name (if applicable), class and section number, and date the experiment was performed. 2. Abstract: This is a very brief (2-3 sentences) statement that tells what was done, the main techniques used, and the major results obtained. 3. Introduction: In this section the theory on which the experiment was developed is described. This is an important section, because it shows the reader that the experiment was thought out on scientific principles, and was not simply a collection of random observations or guesswork. 4. Experimental Procedure: This section outlines what you actually did in the experiment (note: this may differ from the procedure given in the lab handout!!) Describe what you did in sufficient detail that another person with at least your level of expertise in the lab could read the report and duplicate the experiment. 5. Results and Discussion: This section showcases your raw data, observations, and calculated results. Limit your observations to complete sentences about what you actually saw, heard, smelled, or felt while you performed the experiment. Numerical data should be in table form whenever possible, and should always include units and the correct number of significant figures. The discussion should include sample calculations written out for each different calculated quantity and it should include information on any special or unusual circumstances associated with the data collection, such as high or low room temperature, and unexpected odor given off during a reaction, or the presence of a visible contaminant in a beaker of solution. 6. Conclusion: In this section you will discuss what you learned from the experiment. You will essentially be answering the following question. “Did the experiment verify the theory presented in the introduction? If not, why not?” If possible discuss sources of error that lower the quality of the results, such as weighing errors or failure to account for temperature effects. Suggestions for improving the quality of the results, such as modifications to the experimental procedure, should be described in this section, along with reasons why you think the suggestions could lead to better results. This section should not introduce any new material; instead it should be a summary of the key points in the report.

Grading for Style and Content The style and content of your laboratory report will be graded in three categories (1) spelling, grammar, and usage, (2) originality, and (3) organization. The following is expected of a satisfactory performance in each category: 1. Spelling, Grammar, and Usage: Spelling and grammatical rules are generally followed; any mistakes that occur do not interfere with the comprehension of your report. Sentences are constructed to convey meaning clearly and unambiguously. There are no incomplete or run-on sentences. The report should be written in past tense and passive voice (except the introduction). 2. Originality: The experiment has been described in the student’s own words, not copied from the lab handout. Understanding of the experimental procedure is shown by including details that are not in the laboratory handout and by identifying possible sources of error in the experiment. An understanding of the scientific principles involved in the experiment is shown by stating how those principles apply to the experiment. 3. Organization: The report shows a clear progression from each section to the next. The report is organized into paragraphs with obvious headings at the beginning of each section. Conclusions follow logically from experimental results.

Examples of past tense and passive voice 1. The density of carbon tetrachloride at 25% was determined by weighing a known volume of carbon tetrachloride on an analytical balance. From the data collected, the density was calculated to be 1.60 g/mL. 2. The purpose of this experiment was to separate and identify mixtures of dyes found in commercial drink mixes into their constituent components. 3. A piece of paper, approximately 10 x 15 cm in dimension was obtained from the chemistry stockroom. A horizontal pencil line was made on the long edge of the paper, approximately 1 cm from the bottom.


Synthesis and Decomposition of Zinc Iodide Formal Report Information

Be sure to look over your “Molar Mass of Magnesium” formal report and correct any style and content errors in this week’s formal report. Also, remember to summarize important data in a table and show two example calculations (one in words and one with numbers) within your formal report. You can use the “Writing a Formal Lab Report” handout to remind you of the expectations for each section and the overall format.


Be sure the following questions and calculations are shown in your report. If these are missing you will lose points on your report!

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% 3. Empirical formula of zinc iodide for your measured masses 4. 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)?


Post-lab questions for Synthesis and Decomposition of Zinc Iodide

Be sure the answers to these questions appear in the appropriate section your formal report!

1. Introduction: Describe how this experiment relates to any two of the following concepts: law of conservation of mass, ions, percent yield, limiting reagents, stoichiometry (reactant mass versus reactant moles), chemical reaction, oxidation states. 2. Experimental: Why is the reaction of Zn + I2 ran in weakly acidified water? 3. Results: Do your measurements support the Law of Conservation of mass (use percent difference to compare)? Explain why or why not. 4. Conclusion: Discuss 2 possible sources of error.

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