Iron energy from reaction

Because free energy changes are additive, it is ofien possible to bring about a nonsponta-neous reaction by coupling it with a reaction for which AG° is a large negative number. As an example, consider the preparation of iron metal from hematite ore. The reaction... 

Electron-transfer reactions occur all around us. Objects made of iron become coated with mst when they are exposed to moist air. Animals obtain energy from the reaction of carbohydrates with oxygen to form carbon dioxide and water. Turning on a flashlight generates a current of electricity from a chemical reaction in the batteries. In an aluminum refinery, huge quantities of electricity drive the conversion of aluminum oxide into aluminum metal. These different chemical processes share one common feature Each is an oxidation-reduction reaction, commonly called a redox reaction, in which electrons are transferred from one chemical species to another. 

The "iron bacterium" Thiobacillus ferrooxidans obtains energy from the oxidation of Fe2+ to Fe3+ with subsequent precipitation of ferric hydroxide (Eq. 18-23). However, it has been recognized recently that a previously unknown species of Archaea is much more important than T. ferrooxydans in catalysis of this reaction.3243... 

Oxidation-reduction reactions are among the most important in chemistry, biochemistry, and industry. Combustion of coal, natural gas, and gasoline for heat and power are redox reactions, as are the recovery of metals such as iron and aluminum from their oxide ores and the production of chemicals such as sulfuric acid from sulfur, air, and water. The human body metabolizes sugars through redox reactions to obtain energy the reaction products are liquid water and gaseous carbon dioxide. 

In seawater (pH = 8.1) the reported iron concentration ranges from to 10 - M with a mean value of perhaps 10 " M (cf. Holland 1978 Hem 1985). It is interesting to question whether this amount represents dissolved or suspended ferric iron. Examination of Fig. 12.4 shows that the dominant aqueous species in seawater at pH = 8.1 is Fe(OH) . (Fe(IIl) chloride complexing can be ignored.) Assuming that Fe(III) is controlled by the solubility of amorphous HFO (p p = 37.1) as an upper limit, and with free energies from Table A 12.1, we find for the reaction Fe(OH)3(am) = Fe(OH)3, that = [Fe(OH)y] = 10" M. This value is close to the mean iron concentration reported... 

Thermodynamic data on the iron oxides are not directly available but calculations based on free energy values determined from equilibrium data between steam and iron and from specific heat data at low temperatures by the entropy principle91 show that the formation of methanol by the reactions shown is very unfavorable. 

Normally, tbe oxidation of ferrous iron ro rente is slow. The bacterium ThiobaciUus Jerrooxidans, an aerobic chemnauiotroph deriving its energy from the oxidation of ferrous iron, greatly accelerates the oxidation of ferrous iron according to the reaction... 

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