Published on January 11, 2008
Slide1: Background K3Fe(C2O4)3 3H2O Potassium Trioxalatoferrate (III) Trihydrate was prepared in a previous experient. This compound is photosensitive. In acid solution (acetic acid in this case) in the presence of sunlight the reaction results in the reduction of Iron (Fe3+ to Fe2+) and the oxidation of an oxalate ligand to carbon dioxide as follows: Reduction of Iron (Gain of Electrons) Half Cell Reaction Fe3+ + e- Fe2+ Oxidation of Oxalate (Loss of Electrons) Half Cell Reaction (C2O4)2- 2CO2 + 2 e- Slide2: Background (Con’t) In acidic solution, the Ferric (Fe3+) ion is coordinated with one (1) oxalate ion. [Fe3+) (C2O4)2-]+ In sunlight the coordinated oxalate is oxidized (loses an electron) reducing the ferric ion to the ferrous (Fe2+) ion. Ferrous Iron (Fe2+) is sparingly soluble and forms a yellow precipitate of ferrous oxalate dihydrate. Some of the oxalate (C2O4)2- is oxidized to CO2 2 [Fe3+(C2O4)2-]+ + (C2O4)2- + 4H2O → 2 Fe2+(C2O4)2- 2H2O + 2CO2 The overall ferrioxalate photoredox reaction is: 2K3Fe3+(C2O4)3 3H2O +H2O + H+ 2Fe2+(C2O4) 2H2O + 2CO2 + 3K2(C2O4) Slide3: Procedure Week One – End of Redox Lab Session Precisely weigh about 1 gram of the potassium trioxalatoferrate complex to the nearest 0.001g and mix with 15 mL of 10% (v/v) acetic acid. Place in sunlight for one week. A yellow precipitate should have formed. Slide4: Procedure (Con’t) Week Two You will be doing a vacuum filtration with a glass filter crucible instead of the normal Buchner funnel. Note: The glass filter is delicate, treat with care. Pre-dry the glass filter crucible under lamp or in oven. Weigh the dried glass filter crucible. Note: Use same balance for all weight determinations. Vigorously stir contents of test tube containing sample with a glass stirring rod. Pour quickly into glass filter crucible. Use additional acetic acid to transfer any remaining sample from test tube to crucible. Slide5: Procedure (Con’t) Apply vacuum to apparatus until all liquid passes through filter. Release vacuum momentarily, add about 2 mL additional acetic acid, and reapply vacuum. Repeat washing process twice with acetone. Dry sample under vacuum for a couple of minutes. Place crucible under heat lamp in hood until dry. Weigh crucible and contents Dry 10 minutes more and weigh again. Remove sample from crucible and weigh again on a pre-weighed watch glass. Slide6: Calculations Protocol Applicable to All Experiments Set up equations showing variable names and relationships Show data substitutions Show appropriate units Report results to appropriate accuracy, i.e. correct number of significant figures and decimal places Set up equations and calculations in a neat, easy to read format Slide7: Calculations Assumptions: The photoreaction is complete All the ferric iron (Fe3+) has been reduced to (Fe2+) The mass of iron in the product must be the same as in the reactant Recall that you have weighed your Ferrous Oxalate Dihydrate product three times, with the final weight on the pre-weighed watch glass. mol wgt = grams / moles moles = grams / mol wgt mol wgt Fe(C2O4) 2H2O = 179.90 mol wgt K3Fe(C2O4)3 3H2O = 491.25 mol wgt Fe = 55.85 Slide8: Record the 3 weight determinations on the Data Report form. Convert the product on the watch glass value to moles. Compute the theoretical weight % of Fe2+ in product Fe(C2O4) 2 H2O Compute the theoretical weight % of Fe3+ in reactant K3Fe(C2O4)3 3H2O From the weight % of iron in the Fe(C2O4) 2 H2O (the product) and the mass of product produced experimentally, calculate the experimental mass of iron in the product. Slide9: Determine the experimental mass of iron (Fe3+) in the reactant - Potassium Trioxalatoferrate Trihydrate. Compute the experimental weight % of iron (Fe3+) in the reactant. Compute the % error for the theoretical and experimental values of the weight percent of (Fe3+) in the Potassium Trioxalatoferrate Trihydrate complex. Slide10: Compute the theoretical weight % of Potassium in the reactant. Recall the experimentally determined weight percent values of the following in K3Fe(C2O4)3 3H2O: Oxalate Water Iron Compute, by difference, the weight % of Potassium in Potassium Trioxalatoferrate Trihydrate.