Synthesis Of Alum Preliminary Lab Assignment 6-1

Abstract The Synthesis of Alum lab compromised mixing a piece of aluminum with 3M potassium hydroxide solution (KOH). The purpose of this lab was to receive a sample of alum crystals. We obtained the crystal by mixing a piece of alum with the solution to have a reaction. However, we have done many filtrations processes in order to separate the solution from the alum. With these processes, we identified the actual yield, the theoretical yield and the percent yield of alum. Introduction The goal of this experiment is to properly synthesize a type of alum called potassium aluminum sulfate dodecahydrate, KAl(SO4)2 ●H2O, in order to calculate the theoretical, actual, and percent yields. A hydrate (e.g. dodecahydrate) is a substance that contains water or its elements. According to chemistry, an alum is a general family name for a crystalline substance composed of cations with 1+ and 3+ charges (chemistry). Aluminum hydroxide is considered to be an amphoteric hydroxide because it reacts with both acids and bases. To

Unformatted text preview: Pre-laboratory Assignment: SYNTHESIS OF A CHEMICAL COMPOUND Consider carefully the chemical equation below that describes the first reaction in the synthesis of alum. 2 Al (s) + 2 KOH (aq) + 6 H20 (1) e 2 K+ (aq) + 2 Al(OH)4' (aq) + 3 H2 (g) What would you expect to observe as this reaction occurs, or what would be visible evidence that this reaction is occurring? Once the synthesis is complete, the alum will be recovered from the reaction mixture by filtration. Why is the reaction mixture cooled immediately before filtration? What is the molar mass of alum, AlK(SO4)2'12H20 ? [Show your calculation in the space below] What is the mass of alum you would expect to obtain from this synthesis if you use 0.496 g of solid aluminum in the first reaction in the synthesis? [Shuwycur calculation in the space balcw] In addition to wearing safety goggles, name three other safety precautions described in the laboratory notes for this experiment. UW-Waukesha General Chemistryl (CHE 145) LAB Synthesis of a chemical compound Page 1 SYNTHESIS OF A CHEMICAL COMPOUND Introduction In this experiment you will perform a chemical synthesis. Ajchemical synthesis is the process of making a new substance from other, usually simpler substances. The synthesis you will perform is the transformation of aluminum fi'om a beverage can into the substance alum, aluminum potassium sulfate dodecahydrate, which has the chemical formula of AlK(SO4)2'12H20. Alum is a Very versatile and useful substance, being used in dying fabrics, tanning leather, clarifying water in municipal water treatment plants and as an astringent or styptic. Derivatives of alum are also widely used in antiperspirants. While we are starting this synthesis with elemental aluminum, it would be economically and energetically better to start with aluminum ore in which the aluminum is already at the' ionic state which we hope to achieve. Before beginning the synthesis it is necessary to remove the decorative paint from the outside of the can and the protective plastic coating from the inside. The first step in the synthesis 1s the oxidation of the aluminum with potassium hydroxide solution. 2Al(s) + 2KOH(aq) + 6HZO —-> 2K (aq) + 2A1(0H)4 (aq) + 3H2(g) In the oxidation process the aluminum atoms lose three electrons. These electrons are gained by the hydrogen atoms-of water in the reduction process. . The net reaction involves the conversion of elemental aluminum into aluminum ions and the conversion of hydrogen ions into elemental hydrogen. In this basic KOH solution the aluminum ion forms the complex tetrahydroxyaluminate ion. In the second step a slight excess of sulfuric acid is added to the products of step one. This sulfuric acid neutralizes the excess KOH and the four OH' groups of the tetrahydroxyaluminate complex ion: 2KOH(aq) + H2304(aq) —-> KZSO4(aq) + 2H20 Al(OH)4 (aq) + 21123045111) —> A1 (aq) + 41120 + 2304'2(aq) These are acid -base base reactions in which the H+ 1011 of the acid combines with the OH ion to form water. The resulting solution contains K+, Al”, and 804-2 ions in the same proportions as they are found in alum, AlK(SO4)2'12H20. Alum is quite soluble in water, but its ‘ solubility decreases as the temperature is lowered. Therefore, upon sufficient cooling, crystals of alum will form. - ' After the crystals have formed they are filtered to separate them from the solution. The crystals are then washed with methanol to remove any excess sulfluic acid. Methanol is used for washing since alum is quite insoluble in methanol and methanol evaporates readily. X—ray crystallographic studies show that m the crystal lattice of the lonic compound alum there are six water molecules surrounding each ion of potassmm and aluminum. This is the reason for the twelve waters indicated in the chemical formula of alum. Experimental Procedure Chemicals: 1.4 M KOH, 9 M H2804, methanol, NaHC03 Special Supplies: aluminum can, sturdy scissors, steel wool, board on- which to clean aluminum surfaces, ice, filter funnel, Buchner funnel, filter flask, filter paper, trap assembly, aspirator, vacuum tubing, hot plate, container for the alum produced. Safety Precautions: WEAR EYE PROTECTION AT ALL TIMES. Potassium hydroxide (KOH) and sulfuric acid (H2804) are very corrosive. Both can cause bums if left in contact with the skin. If either chemical gets on your skin, rinse it immediately { with lots of cool water for several minutes. If either should get in your eyes rinse them " out at the eye wash station. Spills should be covered with copious amounts of sodium bicarbonate (N aHC03) and then wiped up and disposed. Methanol vapors and liquid are toxic. All work with methanol should be performed in the fume hood and special care should be taken to avoid contact with skin. Since methanol is flammable no open flames I will be allowed in the laboratory during this experiment. ‘ Synthesis ofan 7 Cut a piece of aluminum about 5 x 5 cm fiom an aluminum beverage can, being careful of the sharp edges. Place it on the board provided and clean both surfaces with steel wool. When the aluminum is clean and bright, rinse. it with water and wipe it with paper towel. Using scissors, cut the clean aluminum into small squares about 0.5 x 0.5 cm. . , Place a clean, dry 150 mL beaker on a balance and tare, touch the bar or button to obtain a reading of 0.000 g. Add between 0.5 and 0.6 g of aluminum pieces and read the mass to the nearest 0.001 g. Record the mass. Moist filter paper Fill the funnel with water and let it Fold and crease lightly run untll the air ls washed out ol the stem. When the water level drops to the top of the stern. add the mixture to be filtered Fold again Tear off the corner unequally (The torn corner prevents air irum leaking down the fold.) Then open the litter out'llke this The weight olthis ' column of water hastens filtration “I! ‘— anr-u alum G— plinth! Lulu“; Place the filter paper-In the tunnel and seal the edge against the tunnel wall by “um um/ \ moistening the paper 7‘— "" u “5:221; unuum tu'lrlru- ' Infill-h“. rtnnltlnd The filtrate should Hm, .1"... run down the walls to nun...“ ' -( of the beaker B I run- I‘ll-k - ll‘lp hut.“- Figure 2 Theprocess of vacuum filtr tion using a Buchner funnel A , ef- Via .t‘l’ “hwy. Figure 1 The process of filtration is; w‘: flv “l rtr Obtain 25 mL of 1.4 M KOH solution. Avoid getting this KOH solution on your skin. Working in a hood, carefully add the 25 ml. of 1.4 M KOH solution to the beaker- containing the aluminum pieces. You can use a glass stirring rod to direct the solution as you pour it. Place the beaker on a hot plate set at low heat. Warm the solution, but do not allow it to boil. With occasional stirring with a glass rod, all the aluminum should react in about 20 minutes. . While the aluminum is dissolving, set up a glass funnel using a ring stand and iron ring. See Figure 1. Place a clean 150 mL beaker below the end of the funnel. Fold a piece of coarse filter paper into quarters as in Figure 1 and place it in the funnel. Pour a little distilled into the paper to wet it and seat it in place. Discard any water that runs through the fiinnel into the beaker. , When all of the aluminum has reacted, as detected by no visible pieces of 7 aluminum or bubbles of hydrogen gas, remove the beaker fi'om the hot plate. Be careful handling the beaker because it is hot and contains a corrosive solution. Obtain 10 mL of 9 M H2304 solution and carefully but rapidly pour all of the 9 M H2804 into the beaker, stirring as you do so. If you add the H2804 solution too slowly, you may observe the temporary appearance of a white aluminum hydoxide (Al(OH)3 precipitate. The reaction between H2304 and the excess KOH is exothermic, releases heat, so the solution will get hotter. Be carefirl. Stir the solution thoroughly. If white crystals are visible, return the beaker to the hot plate and stir until all white crystals dissolve. Any tiny black specks may be disregarded. Remove the beaker from the hot plate and allow to cool. When the beaker is cool enough to be safely handled, place your glass stirring rod against the lip of the beaker to guide the solution, and pour the solution into the previously prepared funnel. Do not touch the filter paper with the stirring rod to avoid tearing the paper. Do not overfill the funnel. The liquid level should not be closer than about 1 cm fiom the top of the filter paper. The filtrate coming fiom the bottom of the funnel should be clear and colorless. Allow the 150 mL beaker and filtrate to cool. Fill a 600 mL beaker about half fiill of ice and add water to the same level as the ice. Place the 150 beaker with the filtrate into the ice bath, taking care that no water from the ice bath spills into the 150 mL beaker. Keep the beaker in the ice bath about 20 minutes, stirring frequently, to allow the formation of alum crystals While the filtrate is cooling and crystals are forming, set up the filtration apparatus pictured' in Figure 2. After the 20 minute cooling period 1s over carefirlly stir the filtrate with a thermometer and read the temperature. If the temperature reads 6°C or lower the crystallization may be considered to be complete. If the temperature is not low enough, continue cooling with stirring. When the filtrate is sufficiently cool remove the 150 mL ‘ beaker from the ice bath. Place a piece of appropriate size filter paper into the bottom of the Buchner fimnel (it should just fit in the bottom). Add a little water to seat the filter paper in place. Turn on the water to make the aspirator fimction. Stir the contents of the 150 mL beaker with stirring rod to loosen all of the alum crystals. Transfer all of the filtrate and crystals to the Buchner funnel. A rubber policeman on the end of a glass rod may be used to scrape out as many crystals as possible. Do not overfill the Buchner funnel. Allow the'aspirator to pull air through the crystals for a minute after the last of the liquid has been pulled through the funnel. Turn ofi‘ the aspirator water. Add 10 mL of methanol the crystals and allow to stand for a minute to wash the crystals. Turn on the aspirator water and draw the methanol through the funnel. Turn ofl‘ the aspirator water and wash the crystals with a second 10 mL portion of methanol. Afler the methanol has been drawn through the fiamel, continue to draw air through the crystals for about 15 minutes to dry the alum crystals. During this drying process obtain a clean, dry 150 beaker and weigh it to the nearest 0.001 g and record the mass. After the 15 minute drying period, tum oh0 the aspirator water. Remove the Buchner funnel fi‘om the filter flask. Using small spatula, remove the ' filter paper and crystals from the funnel. Using a small spatula, carefiflly scrape the alum crystals from the paper and transfer them into the previously weighed 150 beaker. Reweigh the beaker with the crystals and record the mass. Dispose your sample in the labeled container under the hood. Weigh out about 10 g of sodium bicarbonate (NaH003). Add about 80% of this Nal-ICOs to the filtrate left in the filter flask. Swirl the flask until the bubbling stops and the NaHC03 has dissolved. Then add the rest of the NaHC03 and note whether any additional bubbles of C02 are produced. When there is no bubbling upon further additionsof NaHCO3, rinse the s_0_lution_down_the_drain_with_lots_oflap water. Name Synthesis of Alum: Report Data mass of aluminum g temperature of cooled crystals °C mass of 150 mL beaker & alum - g mass of empty 150 mL beaker g mass of alum crystals - g Calculations Calculate the number of moles of aluminum that you used. Show your calculations. moles Al Determine the formula mass (molar mass) of alum, KA1(SO4)2'12H20. Be sure to include the waters of crystallization. me0 1e Since there is one aluminum atom in each formula unit of alum one can conclude that one mole of alum can be produced fiom each mole of aluminum used in the synthesis. Based , on the number of moles of aluminum that you began with calculate the maximum number of grams of alum that you could produce. Assume that aluminum is the limiting reagent and all other reagents are present in excess. Show your calculations. —____g.a13.1_rn Using the grams of alum that you actually obtained, calculate the percentage yield of alum for your synthesis: %yield = g alum from experiment x 100 = % yield maximum g of alum from calc. Show your calculations. % yield Questions 1. Experiments show that the solubility of alum in 35 mL of water (about the volume in this experiment) is 1.0 g at 1.0°C and 1.7 g at 6.0°C. Using your measured temperature, estimate the grams of alum left in your cooled solution. Show your calculations. ' g alum left in soln. 2. Does the amount of alum left in solution account for most of the difference between the maximum grams of alum calculated and your experimental grams of alum obtained? Explain. 3. consider the chemical reactions that were employed in this experiment. Why would one not want to store basic solutions of KOH or NaOH in aluminum containers? ' _/A‘ ...
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