Theoretical Test

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8. Chemistry of Iron Oxides (5pts)The nucleus of iron is the most stable among all elements and, therefore,iron accumulates at the core of massive red giant stars where nucleosynthesisof many elements essential for life (such as C, N, O, P, S, etc.) takes place. As aresult, among heavy elements iron is quite abundant in the universe. Iron is alsoabundant on Earth.8-1. Development of a technology for reducing iron oxide to iron was a keystep in human civilization. Key reactions taking place in the blast furnaceare summarized below.C(s) + O 2 (g) → CO 2 (g)∆H ◦ = -393.51 kJ(/mol) ----- 1CO 2 (g) + C(s) → 2CO(g) ∆H ◦ = 172.46 kJ(/mol) ----- 2Fe 2 O 3 (s) + CO(g) → Fe(s) + CO 2 (g) ∆H ◦ = ? ------------------- 38-1-1. Indicate the reducing agent in each reaction.8-1-2. Balance reaction 3 and calculate the equilibrium constant of reaction3 at 1200 o C. (∆H ◦ f (Fe 2 O 3 (s)) = -824.2 kJ/mol, S°(J/mol/K): Fe(s) =27.28, Fe 2 O 3 (s) = 87.40, C(s) = 5.74, CO(g) = 197.674, CO 2 (g) =213.74)8-2. In the manufacture of celadon pottery, Fe 2 O 3is partially reduced in a charcoal kiln to mixedoxides of Fe 3 O 4 and FeO. The amount of thedifferent oxides seems to be related to the“mystic” color of celadon ceramics.Fe 3 O 4 (magnetite) itself is a mixed oxide containing Fe 2+ and Fe 3+ ions andbelongs to a group of compounds with a general formula of AB 2 O 4 . The oxideions form a face-centered cubic array. The figure shows the array of oxygens(gray circles) and representative sites for divalent A and trivalent B cations. The12
dark circle represents a tetrahedral site and the white circle an octahedral site.8-2-1. How many available octahedral sites for iron ions are there in oneAB 2 O 4 unit? Certain sites are shared by neighboring units.AB 2 O 4 can adopt a normal- or an inverse-spinel structure. In normal-spinelstructure, two B ions occupy two of the octahedral sites and one A occupiesone of the tetrahedral sites. In an inverse-spinel structure, one of the two Bions occupies a tetrahedral site. The other B ion and the one A ion occupyoctahedral sites.8-2-2. What percentage of available tetrahedral sites is occupied by eitherFe 2+ or Fe 3+ ion in Fe 3 O 4 ?8-2-3 Fe 3 O 4 has an inverse-spinel structure. Draw the crystal field splittingpattern of Fe 2+ and fill out the electrons. The electron pairing energyis greater than the octahedral field splitting.13
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Theoretical Test

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