Stoichiometry Lab: Leftover Aluminum Wire

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1 Date; Stoichiometry Lab: Leftover Aluminum Wire introduction The single-displacement reaction between copper(ll) chloride and aluminum metal consistently gives impressive results in just a matter of minutes! The progress of the reaction is easily followed by observing the disappearance of the green-blue copper color in the solution and the formation of solid copperon the aluminum wire. Use the balanced equation and stoichiometry calculations to predict the amount of aluminum that should react withthe copper{ll) chloride and compare this to the actual amount of leftover" aluminum wire. Background Stoichiometry is the branch of chemistry that deals with the numerical relationships and mathematical proportions of reactants and products in chemical reactions. Chemical reactions are represented by balanced chemical equations. Proper interpretation of an equation provides a great deal of infomiation about the reaction It represents and about the substances involved in the reaction. For example, the coefficients in a balanced chemical equation indicate the number of moles of each substance in the reaction. Therefore the ratio of moles of one substance to moles of any other substance in the reaction can be easily detemiined. Consider the following unbalanced chemical equation representing the reaction that will be performed in this lab: CUCI2 (aq) + Al(s) AICI3 (aq) + Cu (s) In the experiment, aluminum wire will be added to an aqueous solution of copper(ll) chloride causing a single-displacement reaction to take place. Aluminum metal will fonn aluminum(lll) ions, which is apparent by the dissolving of the aluminum wire to forni aluminum(lll) chloride (AICI3). The simultaneous formation of copper metal from copper(ll) ions will occur and solid copper metal will precipitate from the solution. As this reaction progresses, the green-blue color will fade until the solution is completely colorless - the indication that the reaction is complete. When performing an experiment such as this, the stoichiometry of the reaction and the actual experimental procedure should be examined to first determine which material is the limiting reactant. The limiting reactant (I.R) is the reagent that is used up in the reaction and on whichthe overall yield of the product depends. The quantities of copper(ll) chloride and aluminum used In this lab are such that the aluminum is in excess and the copper(ll) chloride is the limiting reactant in the reaction (as evident from the title of the lab). The limiting reactant in any reaction, however, can be determined by calculating the starting number of moles of each reactant. The balanced equation is then used to determine which starting material will "run out" first or, in other words, limitthe amount of productformed. Consider, for example, the reaction between hydrogen gas and oxygen gas to produce water. The balanced equation is as follows: 2H2(g) + 02(g) -> 2H20(I)

2 If10.0 grams H2 are mixedwith 10.0 grams of O2, which one will "run out"first and act as the limiting reactant? Wemust first determine the number ofmoles ofeach reactant thatwe are starting with: 10.0gH^x =4.95molH, 10.0g0;x =0.313mo/Q, ' 32.0g0j ' Then we must determine which of these reactants limits the amount of productformed. Consider H2 first. The4.95 moles of H2 could theoretically produce 4.95 moles of H2O. This is determined by looking at the balanced chemical equation to determine the stoichiometricratio, which in this case is two to two. That is, foreverytwomoles of hydrogen that react, two molesof watercan be generated: 4.95mo/ X ^ ^ 2molH2 Now consider O2. The mole of O2 could theoretically produce moles of H2O. This is determined from the stoichiometric ratio, which inthis case is one to two. That Is, for every one mole of oxygen that reacts, two moles of water can be generated: 0.313mo/0, X ImolO^ = 0.626molH,0 ^ Therefore, if all the H2 reacted, 4.95 molesof H2O could theoretically form while only0.626 moles of H2O could form from the available O2. The O2 is therefore the limiting reactant inthis examplesince O2 limits the amount of H2O produced. The O2 will "run out"firstwhile some of the H2 will remain inexcess. then, is the maximum number of grams of product expected from the stoichiometric reaction when the limiting reactant (LR) is completelyconsumed, with side reactions, reversibility, losses, and the like ignored. The theoretical yield is calculated from the expression: Theoretical Yield =moles of ofproduct ^ #g product moles oflr moles ofproduct The theoretical yield is the maximum amountof product that can be produced from the quantities of reactants used. However, the amountof product predicted by the theoretical yield is seldom actually obtaineddue to side reactions, losses, or other complications. The actual yieldof productis oftengiven as a percentage of the theoretical \fleld. This is called the percent yield which describes the efficiency of the reaction and is calculated from the expression: Q..., actual yield % yield = X100% theoretical yield

3 Name; Procedure 1. Set upa hotwaterbath in a 150 ml beaker using approximately 100 ml oftap water. Bring the water to a boil using a hot plate while performing steps Obtain approximately 0.4 grams of copper(ll) chloride dihydrate crystals. Use a balance to determinethe exact mass to the nearest hundredth of a gram. Record the mass in the data table on page 4. (Safety precaution: Copper(ll) chloride solution, also known as cupric chloride solution, is toxic by ingestion and is a body tissue imtant; avoid contact withbody tissues.) 3. Place the copper(ll) chloride crystals in a small test tube. Fill the test tube about 4/5 full with distilled water and stir to dissolve the crystals. 4. Cut a piece of aluminum wire about 12 cm long with wire cutters. 5. Measure and record the exact mass of the aluminum wire to the nearest hundredth of a gram in the data table on page Lower the aluminum wire into the test tube containing the copper(ll) chloride solution. The wire will extend beyond the height of the tube. Place the test tube into the boiling water bath for approximately 5-10 minutes. The reaction is complete when the solution becomes completely colorless. 7. Measure the mass of a clean, dry weigh-boat and recordthe mass in the data table on page Remove the wire from the test tube. USE CAUTION SO AS NOT TO DISTURB THE COPPER ON THE WIRE. 9. Use a spatula to scrape as much solid copper as possible from the aluminum wire into the weigh-boat. 10. Rinse the aluminum wirewith tap water to remove any impurities and dry the wire with a paper towel. Once dry, measure and recordthe "leftover" mass of the aluminum wire in the data table on page Allow the solid copper in the weigh-boat to dry overnight. (Note: the copper will slightly oxidize and change color to green or dark brown) Once dry, measure the mass of the copper and the weigh-boat. Recortf the mass in the data table on page 4.

4 _ Prelab Questions Complete prior to starting the lab procedure. 1) Write the balanced chemical equation for the reaction that occurred between the aluminum wire and the copper(ll) chloride solution. 2) What is the molar mass of CuCl2-2H20? 3) What is the molar mass of Al? 4).Define oxidation and reduction a. Using this lab as a reference, provide an example of each. 5) How will you know when the reaction is done? 6. Whatare the products inthis reaction?

5 PER Partners: Data Table Mass of CuCl2-2H20 used (g) Initial mass of Al wire used (g) Mass of "leftover" Al wire (g) Mass of clean, dry weigh-boat (g) Mass of weigh-boat and Cu (g) Calculations SHOW YOUR WORK &put your answers on the line 1) Calculate the mass of Al that reacted (initial mass - leftover mass). 2) Using the mass of the limiting reactant, calculate the theoretical yield of Cu in grams. 3) Calculate the actual yield of Cu in grams. 4) Calculate the percent yield of Cu.

6 Analysis Questions The answers these questions should not include phrases such as "human error" or "calculation error^ or "measured incorrectly." You need to specifically talk about parts of the procedures or assumptions that the lab makes... DO NOT talk about mistakes that you may have made. 1. Discuss reasonable and potential sources of error in this experiment. In other words, explain what could have caused mistakes to occur. Be Specific!! 2. Discuss potential reasons why the % yield of recovered copper may be less than 100%. 3. Discuss potential reasons why the % yield of recovered copper may be greater than 100%. 4. Why do you think that scientists add excess of one or more chemicals when performing a reaction rather than combine the exact stoichiometric ratio?