CHEM 443L Inorganic Chemistry Laboratory Revision 2.0 The Goldschmidt Reaction In this laboratory we will carry-out several aluminothermic reactions, reactions involving the oxidation of Aluminum (Al) powder by a metal oxide (M x O y ): 2 Al(s) + M x O y (s) Al 2 O 3 (s) + 3x M(s) This class of reactions is sometimes known collectively as the Goldschmidt Reaction, after their developer Hans Goldschmidt, or more generally, the Thermite Reaction. Common oxidizers include Fe 2 O 3, Cr 2 O 3 and Mn 3 O 4. These reactions are notable because of their extreme exothermicity. The amount of heat released is usually sufficient to produce the metal (M) in a molten state. An Example of a Thermite Reaction http://upload.wikimedia.org/wikipedia/commons/6/6f/thermitefe2o3.jpg The Goldschmidt Reaction is driven in large measure by the Heat of Formation of Aluminum Oxide: 2 Al(s) + O 2 (g) Al 2 O 3 (s) H f o (298) = -1675.7 kj/mol So, for example, the reduction of Fe 2 O 3 by Aluminum:
P a g e 2 2 Al(s) + Fe 2 O 3 (s) Al 2 O 3 (s) + 2 Fe(s) occurs with an enthalpy change of: H o (298) = H f o (298),Al2O3 - H f o (298),Fe2O3 = -1675.5 - (-824) = -851.5 kj/mol This is large enough to overcome the reaction's unfavorable entropy change (S o ) of -36.48 J/molK, resulting in a Free Energy (G o ) of -838 kj/mol at 298K for this reaction. The large heat of formation for Al 2 O 3 is generated by strong ionic bonding within the compound as evidenced by its large Lattice Energy (U o ), the energy released when gaseous ions come together to form the crystalline substance: 2 Al 3+ (g) + 3 O 2- (g) Al 2 O 3 (s) In general, lattice energies are difficult to measure but can be estimated using Hess' Law and a Born-Haber cycle. The Born-Haber cycle for a generalized metal halide MX is represented below: M(g) H IonEnergy M + (g) H Atom + X(g) H ElecAffin X - (g) H Atom U o H f M(s) + ½ X 2 (g) MX(s) If experimental values are obtainable for the atomization enthalpies (H Atom ), the heat of formation (H f ), the ionization energy for M (H IonEnergy ) and the electron affinity for X 2 (H ElecAffin ), then the lattice energy can be estimated to a high degree of accuracy using this cycle. (You will have an opportunity to perform this calculation for Al 2 O 3.) To start the Goldschmidt reaction, it is usually sufficient to provide an initial ignition. Then, the reaction will proceed without further input of heat. The necessary reaction temperature is maintained by the reaction itself. The temperature is so high (~2000-2500 o C), that the reaction mixture will melt the resulting metal, which will subsequently separate from the other reaction residues. Addition of CaF 2 will support this separation as it decreases the solubility of the metal in the produced melt. Due to the high reaction temperatures involved, metals with a high vapor pressure cannot be obtained by this method. A superheated gaseous metal produced in the high temperature environment of the reaction could violently react with the Oxygen of the Air. This is undesirable.
P a g e 3 For this laboratory, we will carry-out the following three Goldschmidt Reactions: 2 Al(s) + Fe 2 O 3 (s) Al 2 O 3 (s) + 2 Fe(s) 8 Al(s) + 3 Mn 3 O 4 (s) 4 Al 2 O 3 (s) + 9 Mn(s) 2 Al(s) + Cr 2 O 3 (s) Al 2 O 3 (s) + 2 Cr(s) In each case, we will be interested in recovering the metal plug that results from the reaction. This yield can then be compared with the reaction's theoretical yield to determine the efficiency of the reaction in producing the free metal.
P a g e 4 Procedure For each reaction, we require an ignition source. In one case, we will use powdered Potassium Permanganate (KMnO 4 ) mixed with Glycerine. KMnO 4 is a strong oxidizing agent and will produce sufficient heat, when mixed with an organic fuel source, to ignite the Thermite Reaction. Alternatively, we will use a mixture of Barium Peroxide (BaO 2 ) and Magnesium (Mg) powder. This mixture will need to be ignited with a strip of Magnesium metal that itself has been ignited with a torch. In either case, the charge (reaction mixture) is placed in the reaction vessel and a hole is pressed into its center. This hole is then filled with the ignition mixture. In the permanganate case, the permanganate is added to the hole as a mound with a dimple in the top. The Glycerine is poured into the dimple and will, in a few moments, be spontaneously ignited by the permanganate. In the case of the BaO 2 /Mg mix, a thin strip of Mg is inserted into the mound and this is ignited using a propane torch. The burning Magnesium will ignite the peroxide mixture which will in turn ignite the Thermite Reaction. In each case the reaction will be carried out in a Hessian crucible, a crucible made of a clay which can absorb the heat produced during the reaction. If the crucible is small and is filled to at least three quarters its volume, it should be embedded in a sand bath. Hessian Crucible A few cautions: If the Thermite Reaction fails to ignite, consult with your laboratory instructor. Do not attempt to extinguish the reaction with Water. This will result in an explosion. Have a FIRE EXTINGUISHER ready at all times. Carry-out these reactions outdoors, as sparks and debris will be thrown from the reaction vessel for a considerable distance. Move a few yards away from the reaction after the ignition has been initiated. Always handle metal powders in a well ventilated fume hood.
P a g e 5 Iron Oxide Mix 55g of Fe 2 O 3 with 20g of Al powder (325 mesh or finer). Add this Charge to a Hessian crucible. Use 25g of KMnO 4 crystals that have been powdered using a mortar and pestle (in a fume hood wearing gloves and an apron) as an ignition source. When ready, ignite the KMnO 4 by adding ~5-10 ml of Glycerine. When the reaction is complete and the crucible has cooled, break open the crucible with a hammer to recover the metal button that forms on the bottom of the crucible. Calculate the percentage yield for the metal. Chromium Oxide Mix 40g of green Cr 2 O 3 with 13g of CrO 3 (if not available, use 10g K 2 Cr 2 O 7 ). Add 21.4g of 30 mesh Al powder and mix well. Pack 5g of CaF 2 onto the bottom of a Hessian crucible and pack the Charge on top of it. Use a mix 15g BaO 2 and 2g Mg powder for ignition. (Never grind a mix of BaO 2 and Mg with a mortar and pestle.) When the reaction is complete and the crucible has cooled, break open the crucible with a hammer to recover the metal button that forms on the bottom of the crucible. Calculate the percentage yield for the metal. Manganese Oxide In a fume hood, heat 80g of MnO 2 to a bright redness for an hour. Let this cool down completely. Re-weigh the mixture once it has cooled. Heating MnO 2 in Air forms Manganese (III) Oxide: 4 MnO 2 (s) 2 Mn 2 O 3 (s) + O 2 (g) The resulting Manganese Oxide mixture is much less likely to result in a Thermite mixture that will explode than if Mn 3 O 4 is used directly. Mix 30g of the heated MnO 2 with 19g of 30 mesh Al powder and mix well. Pack 10g of CaF 2 onto the bottom of a Hessian crucible and pack the Charge on top of it. Use a mix of 15g BaO 2 and 2g Mg powder for ignition. (Never grind a mix of BaO 2 and Mg with a mortar and pestle.) When the reaction is complete and the crucible has cooled, break open the crucible with a hammer to recover the metal button that forms on the bottom of the crucible. Calculate the percentage yield for the metal.
P a g e 6 Post Lab Questions 1. Why might the Goldschmidt Reaction be preferable to a standard smelting process for the reduction of metal oxides? 2. What are the two forms of Al 2 O 3 and which is likely produced in the Goldschmidt Reaction? 3. What are some metals with a high vapor pressure that should not be produced using the Goldschmidt reaction? 4. Use a Born-Haber Cycle to determine the Lattice Energy for Al 2 O 3. Needed data include the Heat of Formation and the data listed below: Sublimation Enthalpy for Al (kj/mol) H = 330.0 Bond Dissociation Energy for O 2 (kj/mol) BDE = 493.6 kj/mol Ionization Potentials for Al (kj/mol) IP1 = 577.57 IP2 = 1816.6 IP3 = 2744.8 Electron Affinities for O (kj/mol) EA1 = 141.0 EA2 = - 779.6 Speculate as to why the Lattice Energy is so large for this compound. Which step in the Born-Haber cycle is the most costly energetically?
P a g e 7 References Cotton, F.A. and Wilkinson, G. Advanced Inorganic Chemistry: A Comprehensive Text Interscience Publishers, New York, 1972. Huheey, James E.; Keiter, Ellen A. and Keiter, Richard L. Inorganic Chemistry: Principles of Structure and Reactivity Harper Collins, 1993. Purcell, Keith F. and Kotz, John C. An Introduction to Inorganic Chemistry Saunders College Publishing, Philadelphia, 1980. Shakhashiri, Bassam Z. Chemical Demonstrations: A Handbook for Teachers Vol. 1 The University of Wisconsin Press, Madison, WI, 1983. Snyder, Paul E. and Seltz, Harry "The Heat of Formation of Aluminum Oxide" JACS 67 1945. Walton, H.F. Inorganic Preparations: A Laboratory Manual Prentice-Hall, New York, 1948.