Experiment 2 - EXPERIMENT 2 Synthesis of Potassium...

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Unformatted text preview: EXPERIMENT 2 Synthesis of Potassium Tris(oxalato)ferrate(III) Trihydrate Objective: Potassium tris(oxalato)ferrate(III) trihydrate [K3Fe(C2O4)3·3H2O] will be synthesized in a twostep process and the actual, theoretical, and percent yields of this compound will be determined. In addition, the number of waters of hydration will be confirmed by gravimetric analysis. Introduction: A chemical synthesis is the use of one or more chemical reactions to bring about the construction of a desired chemical product or products. The reasons for carrying out a chemical synthesis are varied. It might be the production of a useful substance not found in nature, plastics, for example, or the production of a substance found in nature but difficult or expensive to isolate in large quantities, such as ammonia. Then again, it might be the desire to create a substance with new, useful properties, say, a new medication. Whatever the motivation for the synthesis, maximizing the conversion of reactants to products is always a priority. In this experiment, you will synthesize the compound potassium tris(oxalato)ferrate(III) trihydrate [K3Fe(C2O4)3·3H2O] in a two-step process. The first step involves the reaction of ferrous ammonium sulfate hexahydrate [Fe(NH4)2(SO4)2·6H2O] with oxalic acid (H2C2O4): Fe(NH4)2(SO4)2·6H2O(s) + H2C2O4(aq) → (Pale green) FeC2O4·2H2O(s) + (NH4)2SO4(aq) + H2SO4(aq) + 4H2O(l) (1) (Yellow) Where applicable, the colors of reactants and products are indicated in parentheses below their molecular formula. The iron(II) oxalate dihydrate (FeC2O4·2H2O) produced in the Reaction (1) will be separated from the other products by decantation. In the second step, the iron(II) oxalate dihydrate will be converted to potassium tris(oxalato)ferrate(III) trihydrate [K3Fe(C2O4)3·3H2O] through reaction with oxalic acid, hydrogen peroxide (H2O2), and potassium oxalate (K2C2O4): 2 FeC2O4·2H2O(s) + H2C2O4(aq) + H2O2(aq) + 3 K2C2O4(aq) → (Yellow) 2 K3Fe(C2O4)3·3H2O(s) (2) (Green) The potassium tris(oxalato)ferrate(III) trihydrate crystals then are separated from solution by vacuum filtration. Your objective is to prepare as great a yield of potassium tris(oxalato)ferrate(III) trihydrate as possible. To measure the effectiveness with which this objective is met, you will calculate the percent yield: actual yield (3) Percent yield = ! 100 theoretical yield 2–1 Experiment 2: Synthesis of Potassium Tris(oxalato)Ferrate(III) Trihydrate The theoretical yield is the calculated maximum amount of product that might be obtained under ideal conditions from the reactants. In an experiment, the theoretical yield is seldom, if ever, reached. In this experiment, the theoretical yield is the maximum number of grams of product [K3Fe(C2O4)3·3H2O] that might be obtained from the specified amounts of reactants. To determine the percent yield, it is necessary to calculate the moles of each reactant and then find the limiting reagent. The limiting reagent determines the theoretical yield of potassium tris(oxalato)ferrate(III) trihydrate. Safety Precautions: Protective eyewear must be worn at all times while you are in the laboratory. An acidic solution is produced in the first step of the synthesis. Chemical burns can result if acids come in contact with your skin. If the hot supernatant liquid spills on your skin, immediately wash the affected area with water. Continue washing with water for 15 minutes. Have a classmate notify your instructor. Procedure: Week 1 Measure and record the mass of a 200 mL beaker. Add approximately 5 g of ferrous ammonium sulfate hexahydrate [Fe(NH4)2(SO4)2·6H2O] to the beaker. Measure and record this combined mass. Determine the mass of Fe(NH4)2(SO4)2·6H2O in the beaker by subtraction. Next, dissolve this solid sample in 15 mL of distilled water, measured by graduated cylinder, to which 5 drops of 3 M H2SO4 have been added (to prevent premature oxidation of Fe2+ to Fe3+ by O2 in the air). To this solution add 25 mL of 1 M H2C2O4 (oxalic acid) solution, measured by graduated cylinder. CAUTION: Compounds containing the oxalate ion (C2O42–) are somewhat poisonous. Heat the mixture to boiling on a stirrer-hotplate. Stir the mixture continuously with a magnetic stirbar to prevent spattering. This solution is very susceptible to boiling over; do not leave it unattended. If it should start to boil over, immediately reduce the heat and remove the beaker from the heat source using large beaker tongs. After the mixture has come to a boil, turn the heat off and allow the yellow iron(II) oxalate dihydrate (FeC2O4·2H2O) precipitate to settle. CAUTION: The hot supernatant liquid is somewhat acidic. Chemical burns can result if acids come in contact with your skin. Using large beaker tongs, carefully decant the hot supernatant liquid (the liquid remaining above the solid) into a waste beaker, retaining the solid in the 200 mL beaker. Add 20 mL of distilled water, measured by graduated cylinder, to wash the precipitate, warm, stir, allow the solid to resettle, and again decant the liquid into the waste beaker, retaining the solid. To the solid FeC2O4·2H2O in the beaker add 20 mL of saturated potassium oxalate solution (300 g K2C2O4⋅H2O per liter), measured by graduated cylinder. Support a thermometer using a ringstand, a split one-hole stopper, and a clamp (see Figure 2.1 on the following page). 2–2 Experiment 2: Synthesis of Potassium Tris(oxalato)Ferrate(III) Trihydrate Figure 2.1 Support the thermometer using a ring-stand, clamp, and split one-hole stopper. Place the thermometer in the solution. Carefully heat the solution to 40ºC. Obtain 20 mL of 3% H2O2 (hydrogen peroxide) in a 25 mL graduated cylinder. Add the 20 mL of H2O2 very slowly (a few drops at a time) stirring continuously and keeping the temperature near 40ºC. A small amount of a slimy red precipitate [iron(III) hydroxide, Fe(OH)3] might appear briefly. After adding all the hydrogen peroxide, heat to boiling. To the boiling solution add 8 mL of 1 M H2C2O4—the first 5 mL all at once and the last 3 mL very slowly—keeping the solution boiling. Set up a funnel with filter paper on a ring-stand and gravity filter the boiling solution into a clean plastic specimen cup (see Figure 2.2). Figure 2.2 Gravity filtration apparatus. 2–3 Experiment 2: Synthesis of Potassium Tris(oxalato)Ferrate(III) Trihydrate Allow the solution to cool to room temperature, and then add 10 mL of ethanol to the plastic cup. Cover the plastic cup with its lid, and place it in your laboratory desk until the next laboratory period. The reason that the plastic cup should be placed in your desk is that the product, potassium tris(oxalato)ferrate(III) trihydrate [K3Fe(C2O4)3·3H2O], decomposes in the presence of light. The products of the decomposition are carbon dioxide (from the oxidation of C2O42–) and a complex of Fe2+ (from the reduction of Fe3+). The beaker is covered to prevent complete evaporation of the water. Complete evaporation of the water would make it impossible to separate the pure K3Fe(C2O4)3·3H2O crystals from the other salts, which will otherwise remain dissolved in solution. Also, slower solvent evaporation encourages the development of larger, purer crystals. Week 2 Determine and record the mass of a piece of filter paper. Assemble a Büchner funnel using a second filter paper. Using a stirring-rod equipped with a rubber policeman, transfer the crystals of K3Fe(C2O4)3·3H2O from the plastic cup to the filter paper in the funnel. When the crystals are free of solution, pull air through the sample for 10 to 15 minutes in order to air-dry the crystals. Remove the filter paper and carefully transfer the crystals (using your rubber policeman) to the previously massed, dry filter paper resting atop a watch-glass; spread the crystals out on the filter paper to assess dryness & purity. Obtain the combined mass of the filter paper and crystals. Determine the mass of K3Fe(C2O4)3·3H2O by subtraction. Show your crystals to your instructor. Finally, take a 0.5 g portion of your sample (massed to the nearest mg in your smallest beaker) and place it in a drying oven for roughly an hour at 115°C. (Note: your beaker need not be clean, but it must be dry prior to use.) Dry the sample to constant mass and determine the number of moles of water per mole of salt. Waste Disposal: All excess solutions can go down the sink. All chemical waste is to be disposed of according to the instructions given to you by your TA in lab. You must write these waste disposal procedures in your lab notebook Used filter papers should be rinsed thoroughly under tap water to remove adhering salts, wrung out to remove excess water, and placed in the wastebasket. 2–4 Experiment 2: Synthesis of Potassium Tris(oxalato)Ferrate(III) Trihydrate Name: _____________________________ Date: _________________________________ Lab Instructor: ______________________ Lab Section: ___________________________ EXPERIMENT 2 Synthesis of Potassium Tris(oxalato)ferrate(III) Trihydrate Pre-laboratory Questions: 1. A student carried out the synthesis described in this experiment. In the first step of the synthesis, the student combined 4.888 g of Fe(NH4)2(SO4)2·6H2O with 25.0 mL of 1.0 M H2C2O4. In the second step of the synthesis, the student added 20.0 mL of saturated K2C2O4, 20.0 mL of 3% H2O2, and 8.0 mL of 1.0 M H2C2O4 to the solid FeC2O4·2H2O produced in the first step. (a) Calculate the number of moles of Fe(NH4)2(SO4)2·6H2O in 4.888 g of Fe(NH4)2(SO4)2 · 6H2O. (b) Calculate the number of moles of H2C2O4 in 25.0 mL of 1.0 M H2C2O4. (c) Calculate the number of moles of K2C2O4 in 20.0 mL of a saturated K2C2O4 (300 g K2C2O4⋅H2O per liter) solution. (d) Calculate the number of moles of H2O2 in 20.0 mL of a solution that is 3% H2O2 by weight. Assume that the density of the solution is 1.01 g/mL. (e) Calculate the number of moles of H2C2O4 in 8.0 mL of 1.0 M H2C2O4. 2–5 Experiment 2: Synthesis of Potassium Tris(oxalato)Ferrate(III) Trihydrate Name: _____________________________ Date: _________________________________ Lab Instructor: ______________________ Lab Section: ___________________________ (f) What is the limiting reactant of this two-step reaction? (g) What is the theoretical yield, in grams, of K3Fe(C2O4)3·3H2O? (h) If the actual yield of K3Fe(C2O4)3·3H2O was 4.763 g, what is the percent yield? 2–6 Experiment 2: Synthesis of Potassium Tris(oxalato)Ferrate(III) Trihydrate Name: _____________________________ Date: _________________________________ Lab Instructor: ______________________ Lab Section: ___________________________ EXPERIMENT 2 Synthesis of Potassium Tris(oxalato)ferrate(III) Trihydrate Results/Observations: 1. Mass of 200 mL beaker: 2. Combined mass of 200 mL beaker and Fe(NH4)2(SO4)2·6H2O: 3. Mass of Fe(NH4)2(SO4)2·6H2O (calculated by difference): 4. Mass of filter paper: 5. Combined mass of filter paper and the K3Fe(C2O4)3·3H2O: 6. Mass of K3Fe(C2O4)3·3H2O (calculated by difference): 7. Appearance of K3Fe(C2O4)3·3H2O: 8. Calculation of theoretical yield of K3Fe(C2O4)3·3H2O. Assume Fe(NH4)2(SO4)2·6H2O is the limiting reactant. 9. Percent yield of K3Fe(C2O4)3⋅3H2O: 10. Mass of K3Fe(C2O4)3⋅3H2O sample prior to drying: 11. Mass of K3Fe(C2O4)3 sample after drying: 12. Mass of H2O removed by evaporation: 13. Experimental ratio: [mol H2O]/[mol K3Fe(C2O4)3] 2–7 ...
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This note was uploaded on 06/10/2011 for the course CHEM 2090 taught by Professor Zax,d during the Spring '07 term at Cornell University (Engineering School).

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