Balancing oxidation states

The classical geochemical material balance (12) assumes that the balance and the electron balance (oxidation state) ki our environment have been estabhshed globally by the kiteraction of primary (igneous) rocks with volatile substances (Table 7). 

Rutile pigments, prepared by dissolving chromophoric oxides in an oxidation state different from +4 in the mtile crystal lattice, have been described (25,26). To maintain the proper charge balance of the lattice, additional charge-compensating cations of different metal oxides also have to be dissolved in the mtile stmcture. Examples of such combinations are Ni " + Sb " in 1 2 ratio as NiO + Sb202, + Sb " in 1 1 ratio as Cr202 + Sb O, and Cr " +... 

In all of these oxide phases it is possible that departures from the simple stoichiometric composition occur dirough variation of the charges of some of the cationic species. Furthermore, if a cation is raised to a higher oxidation state, by the addition of oxygen to tire lattice, a conesponding number of vacant cation sites must be formed to compensate tire structure. Thus in nickel oxide NiO, which at stoichiomen ic composition has only Ni + cations, oxidation leads to Ni + ion formation to counterbalance the addition of extra oxide ions. At the same time vacant sites must be added to the cation lattice to retain dre NaCl sUmcture. This balanced process can be described by a normal chemical equation thus... 

It also follows that, when three (or more) oxidation states lie approximately on a straight line in the volt-equivalent diagram, they tend to form an equilibrium mixture rather than a reaction going to completion (provided that the attainment of thermodynamic equilibrium is not hindered kinetically). This is because the slopes joining the several points are almost the same, so that E° for the various couples (and hence AG°) are the same there is consequently approximately zero change in free energy and a balanced... 

The four oxygen anions in the tetrahedron are balanced by the -i-4 oxidation state of the silicon cation, while the four oxygen anions connecting the aluminum cation are not balanced. This results in -1 net charge, which should be balanced. Metal cations such as Na", Mg ", or protons (H" ) balance the charge of the alumina tetrahedra. A two-dimensional representation of an H-zeolite tetrahedra is shown ... 

As stated above, a typical zeolite consists of silicon and aluminum atoms that are tetrahedrally joined by four oxygen atoms. Silicon is in a +4 oxidation state therefore, a tetrahedron containing silicon is neutral in charge. In contrast, aluminum is in a +3 oxidation state. This indicates that each tetrahedron containing aluminum has a net charge of -1, which must be balanced by a positive ion. 

Zeolites are crystalline alumina-silicates having a regular pore structure. Their basic building blocks are silica and alumina tetrahedra. Each tetrahedron consists of silicon or aluminum atoms at the center of the tetrahedron with oxygen atoms at the comers. Because silicon and aluminum are in a +4 and +3 oxidation state, respectively, a net charge of -1 must be balanced by a cation to maintain electrical neutrality. 

K.25 The following redox reactions are important in the refining of certain elements. Balance the equations and in each case, write the name of the source compound of the element (in bold face) and the oxidation state in that compound of the element that is being extracted ... 

Balance the equation, (b) Identify the elements that change oxidation state, (c) Explain the metathesis portion of the reaction using hard-soft acid-base arguments. 

Anderson RF (1987) Redox behavior of uranium in an anoxic marine basin. Uranium 3 145-164 Anderson RF, Fleisher MQ, LeHuray AP (1989) Concentration, oxidation state, and particulate flux of uranium in the Black Sea. Geochim Cosmochim Acta 53 2215-2224 Back W, Hanshaw BB, Pyler TE, Plummer LN, Weiede AE (1979) Geochemical significance of groundwater discharge in Caleta Xel Ha, Quintana Roo, Mexico. Water Res 15 1521-1535 Barnes CE, Cochran JK (1990) Uranium removal in oceanic sediments and the oceanic U balance. Earth. Planet. Sci. Lett 97 94-101... 

We learned to write formulas of ionic compounds in Chaps. 5 and 6. We balanced the charges to determine the number of each ion to use in the formula. We could not do the same thing for atoms of elements in covalent compounds, because in these compounds the atoms do not have charges. In order to overcome this difficulty, we define oxidation numbers, also called oxidation states. 

The more electronegative element will take the negative oxidation state, (a) The maximum oxidation state of sulfur is +6 the most common negative oxidation number of oxygen is -2. Therefore, it takes three oxygen atoms to balance one sulfur atom, and the formula is SO v (b) The maximum oxidation state of carbon is +4 the only oxidation number of fluorine in its compounds is - 1. Therefore, it takes four fluorine atoms to balance one carbon atom, and the formula is CF4,... 

First balance the C atoms. Then balance the elements changing oxidation state, Then balance the rest of the atoms by inspection. 

Still another method is to balance the equation by the oxidation state change method ... 

Fig. 1.14. A scheme of the effect of the rise in atmospheric oxygen on the balance of oxidation states of several elements in the environment. The lines are drawn as oxidation strengths of elements at pH 7.0 against an H+/H2 oxidising power of —0.42 V at this pH. The initial balance of each element some 4 x 109 years ago is close to the H+/H2 line, hence many are reduced, while the state today is close to the 02/H20 line, whence many are oxidised. Thus in time, elements have moved from reduced states to oxidised states seen in their balances, which can be calculated, see Table 6.1. We are only concerned here with inorganic and not with bio-organic chemistry.

A flexible method for modeling redox disequilibrium is to divide the reaction database into two parts. The first part contains reactions between the basis species (e.g., Table 6.1) and a number of redox species, which represent the basis species in alternative oxidation states. For example, redox species Fe+++ forms a redox pair with basis species Fe++, and HS- forms a redox pair with SO4. These coupling reactions are balanced in terms of an electron donor or acceptor, such as 02(aq). Table 7.1 shows coupling reactions from the llnl database. 

The oxidation state of an element in a compound is an indication of how many electrons each atom of that element has lost (positive oxidation state) or gained (negative). Since oxidation state is determined by a set of rules, rather than by experiment, its connection to the number of electrons actually transferred is rather tenuous. It is used in naming compounds and balancing some chemical equations. 

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