METALS: IRON COMPOUNDS Compounds of iron range in complexity from haemoglobin, whose structure is similar to both chlorophyll-a and vitamin-B12, to the examples shown in the reaction scheme.
1. 1. Red blood cells contain the respiratory pigment haemoglobin, Hb. In the absence of oxygen, a water molecule bonds to the iron(II) ion to form deoxyhaemoglobin, HbH2O. Dioxygen reacts reversibly with HbH2O to form oxyhaemoglobin, HbO2; this ligand-exchange reaction, which involves an oxygen molecule replacing the water molecule and bonding to the iron(II) ion, can be summarized in part by this equation:
Carbon monoxide reacts reversibly with HbH2O, but irreversibly with HbO2, to form carboxyhaemoglobin, HbCO; i.e.,
These three forms of haemoglobin colour the blood: dark-blue, HbH2O; bright-red, HbO2; and cherry-red, HbCO. Cells which receive HbCO cannot carry out aerobic respiration, and so cannot release sufficient ATP for vital metabolic processes. Name two organs which contain cells that are most likely to be affected by HbCO. ___________________________________  2. One method of preventing iron from forming rust, Fe2O3.xH2O(s), is 'to paint' the iron with a solution of phosphoric acid, H3PO4(aq); the resulting layer of insoluble iron(III) phosphate provides the iron with a physical barrier to water and dioxygen. Construct the symbol equation for the reaction between iron and aqueous phosphoric acid. ____________ _______________________________________________________________________
3. Except for the precipitation reactions, leading to the formation of iron(II) hydroxide and iron(III) hydroxide, explain carefully why all the other reactions in the scheme are considered to be redox reactions. _______________________________________________________________________ _______________________________________________________________________  4. Using a starting material shown in the scheme, construct the symbol equation for one synthesis of: Iron(II) ethanoate(aq) ________________________________________________ _______________________________________________________________________ Iron(III) fluoride(aq) ________________________________________________ _______________________________________________________________________ Iron(III) hydroxide(s) * ______________________________________________ _______________________________________________________________________  5. State and explain, using ionic equations, the observations at the anode, at the cathode, and for the electrolyte when an aqueous solution of iron(II) chloride is electrolyzed [the ions H1+(aq) and OH1-(aq), though present, are not involved in the electrolysis]. Anode _________________________________________________________________ _______________________________________________________________________ Cathode _______________________________________________________________ _______________________________________________________________________ Electrolyte ___________________________________________________________ _______________________________________________________________________  6. Heterotrophs obtain their chemical energy via the hydrolysis of organic compounds (e.g., carbohydrates, proteins, fats, and oils): by contrast, autotrophs biosynthesize their chemical energy from inorganic compounds (such as CO2, H2O, and H2S). The vast majority of autotrophs are photosynthetic: but a significant minority are chemosynthetic. For example, the bacteria Thiobacillus ferrooxidans obtain their source of energy from the oxidation of iron(II) sulfide; the complex metabolic processes involved can be loosely summarized by the following equation:
One of the metabolic by-products is dilute sulfuric acid. The passage of this acid over ores containing insoluble copper(II) sulfide (CuS) results in the formation of soluble copper(II) sulfate, which collects in blue pools. Copper metal is extracted from these pools either by addition of scrap iron or by electrolysis. (a) Construct the symbol equation for the reaction between: Aqueous sulfuric acid and copper(II) sulfide __________________________ _______________________________________________________________________ Iron and aqueous copper(II) sulfate ___________________________________ _______________________________________________________________________  (b) This extractive method for copper uses micro-organisms, albeit indirectly, and so is an example of biotechnology; i.e., the industrial use of genes from micro-organisms to obtain products considered useful to Man. Genetic engineering, more correctly known as recombinant DNA technology, maybe used to alter the genes of Thiobacillus ferrooxidans. Suggest two possible applications for such altered bacteria. __________ _______________________________________________________________________ _______________________________________________________________________  * The currently accepted view is that the formula 'Fe(OH)3' is little more than a convenient representation for various hydrated iron(III) oxides; these substances are, incidentally, major constituents of soils (and, as such, may be the commonest transition metal compounds observed in everyday-life).
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