7# $G`# <^y^y^y^y^y^<^^^^^_ _ z_x^_ ``A*`k^y`A`5 `A`A`k`A`A`A`A`A`A Experiment V Preparation of an Aluminum Compound from Aluminum Cans pre-lab Assignment Reading: Before coming to your discussion section, read the following: 1. Pp. 870-873 in Olmstead and Williams. 2. The remainder of this experiment in this manual. Pre-lab assignment to hand in: Write separate equations showing Al(OH)3 acting rst as an acid and then as a base. Submit your equations to your instructor at the beginning of your discussion period. Introduction The main objectives of this experiment are to convert scrap aluminum into a useful compound, to demonstrate some of the chemistry of aluminum and to illustrate some of the techniques of synthetic inorganic chemistry. Aluminum metal dissolves in the basic solution hot aqueous potassium hydroxide as represented by the following net ionic equation: 2 Al(s) + 2 OH-(aq) + 6 H2O(l) F( , )> 2 Al(OH)A(-,4)(aq) + 3 H2(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. In this experiment you will prepare potassium aluminum sulfate, K2SO4.Al2(SO4)3.24H2O, which belongs to the general class of compounds called "alums"; this alum is also known as "potash alum." It is sometimes called a "double salt," but this is a misnomer because the crystalline lattice contains K+, Al3+, and SO42- ions in a regular array. Six water molecules are arranged in an octahedral configuration about each Al3+ and each K+. "Potash alum" is used in water clarification, paper sizing, fabric fireproofing, and as a mordant in dyeing fabrics. You will synthesize the alum this week in the laboratory. Next week, when it is dry, you will calculate your yield and then determine some of its physical properties, including its melting point and acid-base properties. The experimental procedure this week 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. PLEASE NOTE that some of these procedures involve use of hazardous chemicals. Glassware is etched by strong base; glassware that is used for strongly alkaline solutions should be washed immediately. Nomenclature: The ability to associate names and formulas of chemical compounds is important for anyone working in a scientific field. Understanding the accepted systematic process for naming compounds is essential for communication among scientists. We must all be able to speak the same language. Practice is the best method to develop a knowledge of the language of chemistry. This practice can be more memorable if one actually observes the compound along with the name and formula and notes that it is, for example, a colorless liquid or a light blue solid. This experiment provides you with references to the accepted system for naming inorganic compounds and some experience with associating names and formulas with real samples of inorganic compounds. The nomenclature portion of this lab will be performed in WEEK 2. PROCEDURE WEEK 1 1. Weigh out approximately 0.5 g of aluminum scrap in a weighing bottle using the analytical balance. Record the weight to the nearest 0.0002 g. Put the aluminium pieces into a 250 ml beaker. Add 25 ml of 1.5 M potassium hydroxide solution to the aluminum. CAUTION: Pour the KOH solution slowly so that it does NOT SPLATTER. Heat the beaker gently on a hot plate. There should be fizzing around the aluminum scraps due to evolution of hydrogen gas. Sometimes there are unpleasant fumes associated with this reaction; if you wish, do this step under a hood. Heat the solution until hydrogen evolution ceases. If the liquid level drops while the aluminum is dissolving, water may be added. It is important that the liquid not evaporate completely, but the liquid level should not exceed its original volume. If solids remain after hydrogen evolution ceases, filter the hot solution by gravity through a very small plug of glass wool in a glass funnel. 2. Cool the solution and then make it acidic by slowly adding (with continuous stirring) 10ml of 9 M H2SO4. During the acidification, lumps of aluminum hydroxide may form, but these should dissolve as the rest of the H2SO4 is added. If solid remains, heat gently. Al(OH)A(-,4)(aq) + H+(aq) F( , )> Al(OH)3(s) + H20(l) Al(OH)3(s) + 3 H+(aq) F( , )> Al3+(aq) + 3H20(l) 3. Cool the beaker in an ice-water bath for approximately 20 minutes. Crystals of alum should form during the cooling period. If crystals do not begin to form, you have added too much water. In this case, evaporate the solution to about one-half of its original volume and cool it again. 4. Collect the crystals on filter paper in a Bchner funnel by vacuum filtration. Wash the crystals with 10 ml of cold 1:1 alcohol-water mixture in which alum is not very soluble. The wash is accomplished by (1) pouring the alcohol-water mixture over the crystals in the Bchner funnel with the suction turned off, (2) quickly and gently stirring the mixture to assure the all of the surfaces of the crystals are wet by the solvent, and then (3) immediately turning the suction on to draw the wash solvent away from the crystals. The crystals are then dried by the stream of air which is drawn through them by the aspirator. Allow the crystals to completely dry by placing them on a watchglass in your desk until next week. Discard the filter paper on which you collected your crystals. WEEK 2 Part I Alum 1. Yield: Weigh the crystals in a weighing bottle or small beaker and determine the yield of alum. 2. Melting Point: Pulverize a few mg of the sample (with a clean mortar and pestle if necessary). Push the open end of a melting point capillary tube into the powder, invert the tube and tap the powdered sample down until the capillary tube contains sample to a depth of about 1/2 cm. Meltemps and instructions will be provided for you to take your melting points. 3. Acid-Base Properties: Dissolve a few crystals of your alum sample in a small volume of distilled water in a small, clean test tube. Heat the mixture if necessary to attain dissolution, then cool the solution before testing it. Test the acidity of the solution with both litmus paper and pH paper. An acid solution turns blue litmus red and shows a pH < 7. A basic solution turns red litmus blue and shows a pH > 7. When aluminum compounds dissolve in water, Al3+ ions are formed. These ions are coordinated by six water molecules to form the complex ion, [Al(H2O)6]3+. The following equilibrium is then established: [Al(H2O)6]3+(aq)  [Al(H2O)5(OH)]2+(aq) + H+(aq) In aqueous solution, an acidic substance is one that increases the concentration of H+(aq) ions; a basic substance decreases the concentration of H+(aq) ions. 4. Qualitative Test: Use your dissolved alum sample from part 3, above, or in a clean test tube, dissolve a few crystals of your alum sample in a small volume of distilled water. Add a few drops of 3M HCl to acidify the solution further. Then add a few drops of 0.1M Ba(NO3)2. Record your observations and name any solid product that may form. 5. Mass Percent of Water: Calculate the mass percent of water in potash alum. In principle it is easy to remove this water by heating and determining the moles of water per mole of anhydrous salt from the weight loss. Unfortunately, in practice it is difficult to dehydrate an aluminum sulfate completely without some decomposition to a "basic sulfate" such as Al(OH)(SO4). In your notebook, calculate the theoretical loss in weight on dehydration of a 1.00 g sample of potash alum. Part II Nomenclature There will be 24 vials of inorganic compounds in the laboratory. Each will be designated either by its chemical formula or chemical name. In your laboratory notebook, set up a table for formula, name, and physical description. Supply the missing information. For naming cations, the Stock system is now preferred and its use is encouraged. Questions The color of compounds is one of their more appealing and distinctive characteristics. From your observations, what group of compounds is most frequently colored? What is unique about these materials that might be responsible for the color? report Your report should include the following (with all calculations): 1. The weight of alum theoretically obtainable from the weight of scrap aluminum used; 2. Your percent yield; 3. A brief explanation of how it might be possible to obtain a yield apparently greater than 100% and an explanation of yields less than 100%; 4. Your observed capillary melting point ( reported as a range). 5. Your completed nomenclature table. CH 141 Lab: Expt. V -- -- CH 141 Lab: Expt. 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