Zinc, chemical state changes

In some instances, an ore is treated to change its chemical state before being reduced. The most common ores of zinc, for example, are the sulfides. These compounds are first roasted in an excess of air, converting zinc sulfide to zinc oxide. The zinc oxide is then reduced either by reacting it with coke (as in the case of iron) or by electrolyzing it (as in the case of aluminum). 

We determine whether a given chemical reaction is an oxidation-reduction reaction by keeping track of the oxidation numbers oxidation states) of the elements involved in the reaction. — (Section 4.4) This procedure identifies whether the oxidation state changes for any elements involved in the reaction. For example, consider the reaction that occurs spontaneously when zinc metal is added to a strong acid (T FIGURE 20.1) ... 

It is not certain whether Sir Humphrey Davy (Fig. 1-7) knew of these considerations. He accepted a commission from the Admiralty for the protection of copper-clad wooden ships, which had been introduced in 1761. During his numerous laboratory experiments, he discovered the cathodic protection of copper by zinc or iron [3]. Davy had already put forward the hypothesis in 1812 that chemical and electrical changes are identical or at least arise from the same material property. He believed that chemical reaction forces could be reduced or increased by altering the electric state of the material. Materials can combine only if they have different electric charges. If an originally positive material can be artificially negatively... 

Chemistry is concerned with the properties of matter, its distinguishing characteristics. A physical property of a substance is a characteristic that we can observe or measure without changing the identity of the substance. For example, a physical property of a sample of water is its mass another is its temperature. Physical properties include characteristics such as melting point (the temperature at which a solid turns into a liquid), hardness, color, state of matter (solid, liquid, or gas), and density. A chemical property refers to the ability of a substance to change into another substance. For example, a chemical property of the gas hydrogen is that it reacts with (burns in) oxygen to produce water a chemical property of the metal zinc is that it reacts with acids to produce hydrogen gas. The rest of the book is concerned primarily with chemical properties here we shall review some important physical properties. 

The substitution of another metal for that present in the native state or the removal of any metal is the simplest chemical modification for a metalloenzyme. Marked changes in activity are usually observed in either case. Substitution of cadmium for zinc first demonstrated a difference in the esterase and peptidase activities of carboxypeptidase A (47). The activity of [(CPD)Cd] toward Bz-Gly-L-OPhe is increased, but that enzyme is virtually inactive toward Cbz-Gly-L-Phe. 

Other zinc compounds for which the Zn NMR spectra have been reported include ZnS04, in which the zinc is in octahedral coordination with oxygen. Comparison of this spectrum with that of tetrahedral Zn in ZnO indicates that the two coordination states are separated by about 200 ppm (Wu 1998). The Zn NMR spectra of zinc acetate and its hydrate show that on hydration the zinc coordination changes from four to six, with a large change in the isotropic chemical shift. (Figure 8.34). Distortions in the octahedral units of the hydrate are reflected by a large increase in the Xq value... 

Environmental Fate. Zinc partitions to the air, water, and soil (Callahan et al. 1979 Guy and Chakrabarti 1976 Houba et al. 1983 Pita and Hyne 1975). Zinc occurs in the environment mainly in the +2 oxidation state (Lindsey 1979). Adsorption is the dominant fate of zinc, resulting in enrichment of zinc in suspended and bed sediments (Callahan et al. 1979). The mobility of zinc in soil has been characterized (Base and Sharp 1983 Bergkvist et al. 1989 EPA 1980d Hermann and Neumann-Mahikau 1985 Kalbasi et al. 1978 Saeed and Fox 1977 Tyler and McBride 1982). No estimate for the atmospheric lifetime of zinc is available. Development of pertinent data on the atmospheric processes important for zinc speciation in the atmosphere would be helpful. Development of this information would permit construction of a comprehensive model for the transport and interaction of zinc not only in air but in other media as well. Transformation in air and water can occur as a result of changes in chemical speciation (Anderson et al. 1988 Callahan et al. 1979 EPA 1980d Stokinger 1981). Data that describe the transformation processes for zinc in soil or the fate of zinc in soil are needed. A model of zinc flux from all environmental compartments would be useful for providing information on the overall environmental fate of zinc. 

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