E2Iron_III_Oxalate - EXPERIMENT 2 Synthesis of Potassium...

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Unformatted text preview: EXPERIMENT 2 Synthesis of Potassium Ferric Oxalate Trihydrate Objective: Potassium ferric oxalate trihydrate [K3Fe(C2O4)3 3H2O] will be synthesized in a two-step process and the actual, theoretical, and percent yields of this compound will be determined. 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 ferric oxalate 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) (Yellow) (1) Where applicable, the colors of reactants and products are indicated in parentheses below their molecular formula. The ferrous oxalate dihydrate (FeC2O4 2H2O) produced in the Reaction (1) will be separated from the other products by decantation. In the second step, the ferrous oxalate dihydrate will be converted to potassium ferric oxalate 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) (Green) (2) The potassium ferric oxalate trihydrate crystals then are separated from solution by vacuum filtration. Your objective is to prepare as great a yield of potassium ferric oxalate 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 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, 21 Experiment 2: Synthesis of Potassium Ferric Oxalate Trihydrate 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 ferric oxalate trihydrate. Safety Precautions: Protective eyewear must be worn at all times while you are in the laboratory. Sulfuric acid (H2SO4) is a product of the first step of the synthesis. Chemical burns can result if H2SO4 (a component of the hot supernatant liquid) comes 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. 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 ferrous oxalate dihydrate (FeC2O4 2H2O) precipitate to settle. CAUTION: The hot supernatant liquid contains H2SO4. Chemical burns can result if H2SO4 comes 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 K2C2O4H2O 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). 22 Experiment 2: Synthesis of Potassium Ferric Oxalate Trihydrate Figure 2.1: A thermometer is used to monitor the solution temperature while heating. Support the thermometer using a ringstand, clamp, and split one-hole stopper. Place the thermometer in the solution. Carefully heat the solution to 40C. 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 40C. 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 ringstand and gravity filter the boiling solution into a clean plastic specimen cup (see Figure 2.2 on the following page). 23 Experiment 2: Synthesis of Potassium Ferric Oxalate Trihydrate Figure 2.2: Gravity filtration. 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 ferric oxalate 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 would have remained 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 gravity filtration setup (see Figure 2.2 above) 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, remove the filter paper and transfer the crystals (using your rubber policeman) to the previously massed, dry piece of filter paper resting atop a watchglass; spread the crystals out on the filter paper so they can air-dry. 24 Experiment 2: Synthesis of Potassium Ferric Oxalate Trihydrate 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. Waste Disposal: Place K3Fe(C2O4)3 3H2O crystals in the labeled collection bottle. All other chemical waste is to be flushed down the sink with plenty of water. 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. 25 Experiment 2: Synthesis of Potassium Ferric Oxalate Trihydrate Name: _____________________________ Date: _________________________________ Lab Instructor: ______________________ Lab Section: ___________________________ EXPERIMENT 2 Synthesis of Potassium Ferric Oxalate 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. 26 Experiment 2: Synthesis of Potassium Ferric Oxalate 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? 27 Experiment 2: Synthesis of Potassium Ferric Oxalate Trihydrate Name: _____________________________ Date: _________________________________ Lab Instructor: ______________________ Lab Section: ___________________________ EXPERIMENT 2 Synthesis of Potassium Ferric Oxalate 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 3 H2O (calculated by difference): 7. Appearance of K3Fe(C2O4)3 3 H2O: 8. Calculation of theoretical yield of K3Fe(C2O4)3 3 H2O. Assume Fe(NH4)2(SO4)2 6H2O is the limiting reactant. 9. Percent yield of K3Fe(C2O4)3 3 H2O: 28 ...
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