Experiment 5 Empirical Formula of Zinc Iodide

Transcription

1 Experiment 5 Empirical Formula of Zinc Iodide Purpose: The purpose of this experiment is to react elemental zinc and iodine to form the binary compound zinc iodide (Zn x I y ) and to use the amounts of reactants consumed in the reaction to determine the Empirical formula of the compound. Backround: The simplest type of chemical reaction is the combination of two pure elements to from a binary compound. For instance, copper metal reacts with sulfur to form the binary compound copper sulfide, accordin the chemical equation: 2Cu + S Cu S 2 The term binary refers to the number of different elements in the compound. A binary compound is composed of just two elements althouh there may be more than one atom of either element in the compound. The quantities (masses) of the substances consumed in such a reaction obey three important laws: Law of Conservation of Mass (Antoine Lavoisier, 1789) The mass of the system must remain constant over time, as system mass cannot chane quantity if it is not added or removed. Hence, the quantity of mass is "conserved" over time. In other words, the mass of any one element at the beinnin of a reaction will equal the mass of that element at the end of the reaction. If we account for all reactants and products in a chemical reaction, the total mass will be the same at any point in time in any closed system. Law of Constant (Definite) Proportions (Proust's Law) A pure compound will always have the same elements combined in the same (constant) proportion by mass.

2 Law of Constant Composition The Law of Constant Proportions applies reardless of how the substance was made, where the substance is found, or how many different paths (reactions) are followed to form the compound. Mass vs. Moles Chemical reactions relate the amounts of reactants (elements or compounds) to the amount of product compound(s) in simple whole number ratios of the number of moles of each substance participatin in the reaction. Each mole of a substance represents the same number of atoms and since each atom of a different element has a unique mass (protons + neutrons), the molar mass (molecular weiht) is different for each element. Thus, accordin to the Law of Definite Proportions, the ratio of the masses of elements participatin in a chemical reaction will be a simple multiple of the ratio of the atomic weihts of those elements. mass of A reacted mass of B reacted = X * atomic weiht A atomic weiht B moles A X = = moles B mass of A reacted atomic weiht A mass of B reacted atomic weiht B In the above reaction between copper and sulfur, two moles of copper (A) react with 1 mole sulfur (B) to form copper sulfide (X = 2/1 = 2) Empirical Formula vs. Molecular Formula A chemical formula is a way of expressin information about the proportions of atoms that constitute a particular chemical compound. The simplest types of chemical formulas are called empirical formulas and represent the smallest whole number ratio of the relative masses of elements (moles) in the compound. An actual molecule of a substance, as represented in a chemical reaction and representin the actual number of atoms participatin in the reaction, could be a multiple of the empirical formula, i.e., a molecular formula. Examples include: Hydroen peroxide has an empirical formula of HO and a molecular formula of H 2 O 2 Glucose has an empirical formula of CH 2 O and a molecular formula of C 6 H 12 O 6 Example Empirical Formula of Tin Iodide (SnI 2 or SnI 4 ) Tin (Sn) metal can react with elemental iodine (I 2 ) to produce tin iodide with the eneral formula Sn x I y. Dependin on conditions, catalysts, etc., it is possible to form either Sn(II)I 2 or Sn(IV)I 4., i.e., tin could be oxidized (loose electrons) to either Sn 2+ or Sn 4+. The only way to be certain which compound is actually produced and to determine the

3 empirical formula is to compare the moles of each element actually consumed in the reaction. To do this, it is necessary to: 1. Determine the masses of the elements used in the experiment. 2. Determine which element was consumed in its entirety, i.e., the limitin reaent, and which element had an unreacted excess at the end of the reaction. 3. Convert the masses of reactants actually consumed in the reaction to moles. 4. Determine the molar ratio of tin to iodine consumed 5. Express the empirical formula from the molar ratio Iodine (I vs. I 2 ) A complicatin factor in this reaction and the subsequent determination of the empirical formula is the iodine molecule, which exists in nature as a diatomic molecule (I 2 ). Chemical reactions usually involve individual atoms of iodine, as iodide ions (I - ). I + 2e 2I 2(s) - - Two moles of iodide ion (I - ) are produced for each mole of iodine (I 2 ) reacted. Experimental Results Startin amount of Sn metal: Amount of Sn metal remainin (after all I 2 is completely consumed): Mass of Sn Consumed: = Startin amount of I 2 solid: Mass of I 2 Consumed: Conversion of mass quantities to molar quantities: Moles Sn Consumed: Sn x (1 mol Sn/ Sn) moles Moles I 2 Consumed: I 2 x (1 mol I 2 /253.8 I 2 ) moles I 2 Moles I - Consumed: moles I 2 x (2 mol I - /1 mol I 2 ) moles I - Remember that since a metal is bein reacted with a non-metal, the reaction is really the combination of a tin (Sn) positive ion with an iodide neative ion (I - ). A calculation of the molar ratio of Sn to I - is required to determine the valence of the Sn ion. Takin the larer number and dividin by the smaller number yields the followin molar ratio for I - and Sn: Moles I - to Moles Sn: mol I - / mol Sn = 3.98 mol I - / 1 mol Sn Remember that this is the molar ratio of the final product accordin to the followin reaction:

4 The Experiment: Sn (s) + I 2(s) SnxI y Proust s law of Definite Proportions requires the molar subscripts in the tin iodide molecule be the smallest whole number molar ratio of the elements Thus, convertin 3.98 to 4 ives SnI, where the v alence of tin is + 4 (Sn I ) 4 4 Measured masses of zinc metal (Zn) will be reacted with iodine (I 2 ) to form zinc iodide. Zn + I Zn I 2 x y The x and y subscripts are to be experimentally determined. Actually, the x and y themselves cannot be determined directly, only their mass ratio x:y. The x and y values will be adjusted to the smallest whole number ratio. For example, if the final ratio is determined to be 1.51, then the smallest inteers to ive this ratio would be x = 3 and y = 2. The masses of zinc and iodine used in the experiment will not represent the balanced molar amounts required. One of the two reactants will be in excess. This means that some of the reactant will not be consumed in reaction. The other reactant, referred to as the limitin reaent, will be totally consumed in the reaction, its total mass used in the formation of the product. The moles of the two reactants actually consumed in the reaction will be computed. The balanced stoichiometric equation for the reaction will reflect equivalent moles of the limitin element in both the reactant and product. Pre-Lab Report & Notebook: AZn + BI CZn I 2 x y Download from the department data base to your hard drive or flash drive a copy of the lab report template and the data summary tables for the Zinc Iodide experiment. Print the Zinc Iodide summary table and use it to record the laboratory results durin class. Empirical Formula of Zinc Iodide Data Summary table Prepare the Pre-lab report accordin to instructor s instructions.

5 Materials & Equipment: Materials Equipment Zinc Metal Electronic Balance Iodine Crystals Evaporatin Dish 6 M Acetic Acid Watch Glass Distilled Water Thin Spatula Fume Hood Dryin Oven Calculator Procedure: Safety Precautions To avoid potential alleric reactions to iodine vapor, the experiment should be carried out in the fume hood up to the point at which the iodine (limitin reaent) is totally. The end of the reaction is indicated by the total disappearance of the reddish iodine crystals and the absence of any as evolution or bubblin action in the reaction mixture. Be careful to avoid inhalation of the iodine vapor and avoid contact between the iodine and your skin. 1. Attach an identification label with your name and section number on an empty, clean, dry porcelain evaporatin dish. 2. Weih the evaporatin dish to the nearest on the electronic balance. 3. Add approximately 2 zinc metal to the dish and weih the dish with contents aain. Note: it is not necessary to obtain exactly 2 of zinc, only that whatever the amount actually used is weihed to the nearest Place the dish containin the zinc metal in the fume hood. 5. Take a small test tube from the equipment drawer in your bench; take it the prep room or the instructor s desk and exchane it for another rubber stoppered test tube containin the iodine crystals. 6. Place a 150 ml beaker on the diital balance and tare it Place the test tube containin the iodine and stopper into the beaker and determine the combined mass to the nearest Transfer the iodine crystals in the test tube to the evaporatin dish containin the zinc metal. 9. Add 5 ml distilled water to the dish.

6 10. Add a few drops of 6 M acetic acid. Note: The acetic acid in not really involved in the reaction; its purpose is to prevent simultaneous side reactions which miht confuse observations. 11. Usin a lass stirrin rod or thin metal spatula, carefully stir and rind the reaction mixture for minutes. Initially, a very dark red color is seen in the reaction mixture, eventually fadin to a pale yellow color, and finally colorless. When the reaction is complete, the colorless mixture will show no further bubblin action or evolution of as. 12. Reweih the now empty test tube plus stopper in the same tared150 ml beaker to determine the mass of the iodine crystals. The mass of the iodine should be in the rane of Decant the liquid in the evaporatin dish into the sink (no hazard to environment). 14. Wash the remainin reaction mass with two portions of 10 ml distilled water, discardin the aqueous solution into the sink. 15. Place the evaporatin dish into the dryin oven for at least 20 minutes. 16. Remove the dish from the oven and allow it to cool to room temperature (about minutes). 17. Usin the same electronic balance as before, reweih the dish and determine the mass of the unreacted zinc metal. Note: the actual product, zinc iodide is soluble in water and was discarded, as it was not needed for the final calculations. 18. Return the unreacted zinc to the instructor s desk; it can be recycled. 19. Determine the mass of zinc reacted from the initial and final masses of zinc. Calculations: The followin alorithms are used in the determination of the empirical formula for Zinc Iodide. Instructions for enterin the alorithms into the Excel data base are described in the Data Processin section. 1. Compute the moles of reacted zinc moles reacted zinc = total mass zinc - unreacted mass of zinc molecular weiht zinc ( / mol)

7 2. Compute the moles of iodine reacted assumin the measure mass of iodine is monatomic iodine (I) as opposed to diatomic iodine (I 2 ) as used in the example above. measured mass (I) moles (I) = = mol wt (I) mass () / mol 3. Compute ratio of moles I : moles Zn 4. Determine the empirical formula. 5. Compute the % Error of the molar ratio (I/Zn) % error molar ratio = 100 measured ratio - expected ratio expected ratio Analysis and Conclusions: Develop aruments explainin how the experimental results produced the empirical formula? Explain how you determined the expected ratio of iodine to zinc? How would you determine if the empirical formula is also the molecular formula? Data Processin: Use the printed Pre-lab report as a notebook to record the experimental results in the results section of the applicable procedure. Enter the experimental results into the printed Zinc Iodide summary result table and attach to report. If required by the instructor, transfer the laboratory results to the electronic files and attach tables to the final laboratory report. Spreadsheet Processin: Input Results into Laboratory Excel Database Use one of the laboratory computers and the web-based data entry forms shown in Fiure 5.1 below to enter the Zinc Iodide lab results into an Excel spreadsheet.

8 Fiure 5.1: Input form for enterin Zinc Iodide results into Excel data base Class Data Retrieval: Retrieve class data for Empirical Formula experiment from website (select Empirical Formula ) Copy the data presented on the screen and insert into a spreadsheet (Excel, Goole, other) and save the spreadsheet on your hard drive or flash drive with an appropriate file name. Your default sheet should be renamed (RawData) and have the followin columnar format:

9 Col Row A B C 1 Chemistry 211 Section 2A1 Empirical Formula ZnI 2 2 m_initial_i2 m_initial_zn m_excess_zn 3 4 Column Definitions: Sheet #1 (RawData) Lab results entered durin lab (all students) Col A Initial Mass of Iodine (I 2 ) Col B Initial Mass of Zinc Col C Mass of zinc remainin after reaction (Excess zinc) Create computation Sheet Open a new sheet and rename it Results. Follow the instructions below to define the columns and setup the appropriate computational alorithms. The final form should look somethin like the followin. Col Row A B C D E 1 Chem 211: Sec 204 Empirical Formula of Zinc Iodide 2 Mass Zn Moles Moles Mol I/ Zinc I Mol Zn % error 3 4 Column Definitions: Col A Final Mass of Zinc Col B Mole of Zn Col C Moles of Iodine (I) Col D Ratio Moles I/Moles Zn Col E % Error

10 Enter info and alorithms into Results sheet Compute mass of zinc remainin after the reaction Enter your alorithm into Cell B Compute the moles of zinc consumed in the reaction Enter your alorithm into Cell C3 Compute moles of Iodine consumed in the reaction Enter your alorithm into Cell D3 Compute mole ratio of Iodine to Zinc Enter your alorithm into Cell E3 Compute % error of the molar ratio Enter your alorithm into Cell F3 Copy alorithms for class results into rows A4:Ax; B4:Bx; C4:Cx; D4:Dx; E4:Ex Select cells B3:Fx From Editin box under Home on Menu bar select Fill Down For Goole Sheets press Ctrl D Note: This computes the results for all students

11 Results Summary Table Empirical Formula of Zinc Iodide Mass of Evaporatin Dish (empty) Mass of Evaporatin Dish plus Zinc Metal Initial Mass of Zinc Mass of Evaporatin Dish (final) Final Mass of unreacted Zinc Mass of Zinc Reacted Mass of 150 ml Beaker Plus Test Tube, Stopper, Iodine Mass of 150 ml Beaker Plus Test, Stopper Mass of Iodine Moles of Zinc Metal Reacted Moles Iodine (I 2 ) reacted Moles I atoms reacted Ratio Moles Iodide Ion to Moles Zinc Ion mol mol mol mol Empirical Formula of Zinc Iodide