CH 112 Special Assignment #4 Chemistry to Dye for: Part A

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1 CH 112 Special Assignment #4 Chemistry to Dye for: Part A PRE-LAB ASSIGNMENT: Make sure that you read this handout and bring the essentials to lab with you. Here are the pre-lab questions for this week. Show your work. 1. The synthesis of alum proceeds in several reaction steps, as described below. The mole ratios of reactants and products can be found by combining the written equations for these separate reactions into an overall equation. Balance the overall equation for the synthesis of alum, KAl(SO 4 ) 2 12 H 2 O, from aluminum, potassium hydroxide, sulfuric acid and water. HINT: Add together the four equations given in the introduction, canceling out any molecules or formula units that appear on both sides. 2. Calculate the amount in moles and the mass in grams of KOH needed to convert 1.50 g of aluminum to KAl(OH) How many moles of H 2 SO 4 are needed to convert the resulting KAl(OH) 4 to potash alum? 4. How many moles and how many grams of potash alum can be prepared from 1.50 g of aluminum? INTRODUCTION: Your customer is a local business called Dyenomite, which hopes to start a new line of custom tie-dyed apparel. They have several projects for the CRaC team: developing a process for using scrap aluminum to make an essential molecule for adhering dyes to fabrics, characterizing the best fabrics for their unique and colorful dyes, and investigating a problem with the rapid fading of one of their dyes. This week s assignment focuses on the preparation of an aluminum compound from discarded cans.

2 Aluminum metal dissolves in hot aqueous potassium hydroxide solution, as represented by the following equation: 2 Al (s) + 2 KOH (aq) + 6 H 2 O (l) 2 KAl(OH) 4 (aq) + 3 H 2 (g) Once dissolved, the aluminum can be converted into a variety of compounds that are useful of themselves or as intermediates in the preparation of other aluminum compounds. The compound of interest in this experiment is potassium aluminum sulfate dodecahydrate, KAl(SO 4 ) 2 12 H 2 O, obtained via the following sequence of reactions: Initial addition of sulfuric acid (precipitation of Al(OH) 3 ): 2 KAl(OH) 4 (aq)+ H 2 SO 4 (aq) 2 Al(OH) 3 (s) + K 2 SO 4 (aq)+ 2 H 2 O (l) Further addition of sulfuric acid (dissolving of Al(OH) 3 ): 2 Al(OH) 3 (s) + 3 H 2 SO 4 (aq) Al 2 (SO 4 ) 3 (aq) + 6 H 2 O(l) Precipitation of alum on cooling: K 2 SO 4 (aq) + Al 2 (SO 4 ) 3 (aq) + 24 H 2 O (l) 2 KAl(SO 4 ) 2 12 H 2 O (s) Potassium aluminum sulfate dodecahydrate, KAl(SO 4 ) 2 12 H 2 O, belongs to the general class of compounds called "alums"; this particular alum is commonly known as "potash alum". Potash alum is used in water clarifying, pickle making, and fabric dyeing. Synthesis of potash alum consists of four parts: (1) dissolution of the aluminum scrap in concentrated potassium hydroxide, (2) neutralization of this solution with sulfuric acid, (3) crystallization of the alum, and (4) collection and washing of the crystals. After synthesizing the compound, you will perform several tests to confirm its identity. LEARNING GOALS: Be able to: Carry out the synthesis of a desired compound. Determine the stoichiometry of the overall reaction given a series of reactions. Calculate expected masses of reactants and products given a known reaction. Characterize a compound through several qualitative tests. 2

3 PROCEDURE: Part 1. Synthesis of potash alum 1. Weigh out approximately 0.5 g of scrap aluminum on weighing paper. Record the weight to the nearest g. Put the aluminum pieces into a 250 ml beaker. 2. In the hood, add 25 ml of 1.5 M potassium hydroxide solution to the aluminum. CAUTION: DO NOT SPLATTER THE SOLUTION. 3. Heat the beaker gently. Record your observations during this reaction. There should be a fizzing around the aluminum scraps due to the evolution of hydrogen gas. Heat until hydrogen evolution slows appreciably, ceases altogether, or half an hour has elapsed. If the liquid level drops while dissolving the aluminum, water may be added, but the liquid should not exceed its original volume. 4. Filter the hot solution into a small beaker through gravity filtration, using either a very small plug of glass wool or folded filter paper in a glass funnel. (A large plug of glass wool, or repeated filtration, will result in considerable loss of material.) The solution should now be clear. 5. After cooling, make the solution acidic by slow addition (with continuous stirring) of 10 ml of 9 M H 2 SO 4 in the hood (USE CAUTION!). During the acidification, lumps of aluminum hydroxide may form, but these should dissolve as the rest of the H 2 SO 4 is added. If solid remains, heat gently. 6. Cool the beaker in an ice bath for approximately 5 minutes. Crystals of alum should start to form. If crystals do not start to form, scratch the inside walls of the flask with a glass stirring rod. This provides sites at which crystallization can begin, followed by crystal formation throughout the liquid. Swirl the flask when you notice the onset of crystal formation and allow it to cool in the ice bath for another 10 minutes. If crystals still do not form, you have added too much water. In this case evaporate the solution to one-half to one-third of the original volume and cool again. 7. Collect the crystals on filter paper in a Buchner funnel by vacuum filtration. Wash the crystals by pouring 10 ml of cold 1:1 ethanol-water mixture over the crystals on the filter paper. Allow the crystals to dry for five minutes on the filter paper with full suction. 8. After the crystals are dry, weigh them on the lab balance by the following procedure. Weigh a clean beaker on the balance, then place the crystals in the beaker and weigh the beaker plus crystals. Obtain the mass of the alum by subtraction. Part 2. Characterization of potash alum 1. Melting point: Pulverize a few mg of the sample with a clean mortar and pestle. Push the open end of a capillary tube into the powder; invert the tube and tap the powdered sample down until the capillary tube contains sample to 3

4 a depth of about 0.5 cm. Insert the tube into a cooled melting point apparatus and determine the melting point range. A narrow range is indicative of high purity. 2. Qualitative test for sulfate: When an aqueous solution of a barium salt such as BaCl 2 is mixed with an aqueous solution containing sulfate, a white precipitate of insoluble BaSO 4 forms. A positive test for SO 4 2-, therefore, is the observation of a white precipitate when an aqueous BaCl 2 solution is mixed with the aqueous test solution. In a clean test tube, dissolve a few crystals of your alum sample in a small volume of deionized water. Add a few drops of 0.1 M BaCl 2. Record your observations. Does this sample contain sulfate? 3. Qualitative test for potassium: When placed in a flame, many elements produce a distinctive color. Potassium produces a distinctive lavender flame that we can use as a qualitative test for its presence. Potassium's flame is often difficult to see because sodium, which is often present as an impurity, has an intense yellow flame that masks other colors. The potassium flame can be seen in the presence of sodium by viewing the flame through a dark blue filter, which absorbs the yellow light from Na, but allows the light from K to pass. In the hood, heat the provided wire loop in the flame to remove impurities. Once the loop is clean, carefully scoop up a small amount of alum on the end of the hot loop. Place the alum in the flame and heat it until the crystals begin to melt and the solid glows. Note the color of the flame. If your flame is bright yellow (indicating the presence of sodium), try cleaning your loop again, or use the blue filter. Record your observations. Does this sample contain potassium? 4. Save your crystals for further use in next week s experiment. Reference: Smith, W. H.; Sager, E. E. and Siewers, I. (1949) J. Anal. Chem. 21, Adapted from an experiment originally developed by Wayne L. Smith of the Colby College Chemistry Department. 4

5 CraC Special Assignment #4A REPORT SHEET NAME PARTNER DATE DATA SUMMARY Initial mass of aluminum Moles of aluminum Theoretical moles of alum * Mass of alum obtained Moles of alum obtained Percent yield of alum ** * Theoretical moles of alum is the maximum number of moles that could be made, assuming complete conversion of all the initial aluminum. ** Percent yield of alum is the actual moles of alum divided by the theoretical moles of alum times 100. Experimental melting point of alum Literature value Reference: Test for sulfate (yes/no) Test for potassium (yes/no) POST-LAB QUESTIONS 1. What causes the Al clippings to rise and fall as they are being dissolved in the KOH solution? 2. Briefly explain how you might get a yield greater than 100% and how you could get one less than 100%. 5

6 3. What is the evidence that you actually synthesized alum? 4. Assuming that the aluminum cans are free, about how much does it cost to synthesize 1 mole of alum? Note that potassium hydroxide pellets cost about 1 cent per gram and sulfuric acid costs about $2 per mole. (Use your actual percent yield for the synthesis.) CONCLUSIONS Use the space below to report on your experiment to Dyenomite, making sure that you address the original goals of this work. 6