One oxidation

Anhydrous halides, however, are obtained when the metal is heated with the dry hydrogen halide or the halogen. In the case of elements with more than one oxidation state, the hydrogen halide produces a lower halide and the halogen a higher halide, for example... 

Halides of non-metals are usually prepared by the direct combination of the elements. If the element exhibits more than one oxidation state, excess of the halogen favours the formation of the higher halide whilst excess of the element favours the formation of the lower halide (e.g. PCI5 and PCI3). 

When an element has more than one oxidation state the lower halides tend to be ionic whilst the higher ones are covalent—the anhydrous chlorides of lead are a good example, for whilst leadfll) chloride, PbCl2, is a white non-volatile solid, soluble in water without hydrolysis, leadflV) chloride, PbC, is a liquid at room temperature (p. 200) and is immediately hydrolysed. This change of bonding with oxidation state follows from the rules given on p.49... 

Although the reduction process is not always a reversible one, oxidation and reduction potential values can be sometimes related to the Hiickel energies of the highest and lowest filled molecular orbital of the dye (108). 

In general, all iron pigments are characteri2ed by low chroma and excellent Hghtfastness. They are nontoxic, nonbleeding, and inexpensive. They do not react with weak acids and alkaHes, and if they are not contaminated with manganese, do not react with organic solvents. However, properties vary from one oxide to another. 

As shown in equation 12, the chemistry of this developer s oxidation and decomposition has been found to be less simple than first envisioned. One oxidation product, tetramethyl succinic acid (18), is not found under normal circumstances. Instead, the products are the a-hydroxyacid (20) and the a-ketoacid (22). When silver bromide is the oxidant, only the two-electron oxidation and hydrolysis occur to give (20). When silver chloride is the oxidant, a four-electron oxidation can occur to give (22). In model experiments the hydroxyacid was not converted to the keto acid. Therefore, it seemed that the two-electron intermediate triketone hydrate (19) in the presence of a stronger oxidant would reduce more silver, possibly involving a species such as (21) as a likely reactive intermediate. This mechanism was verified experimentally, using a controlled, constant electrochemical potential. At potentials like that of silver chloride, four electrons were used at lower potentials only two were used (104). 

Special containers have been developed for anesthetic ether to prevent deterioration before use. Their effectiveness as stabHizers usuaHy depends on the presence of a lower oxide of a metal having more than one oxidation state. Thus the sides and the bottoms of tin-plate containers are electroplated with copper, which contains a smaH amount of cuprous oxide. Staimous oxide is also used in the linings for tin containers. Instead of using special containers, iron wire or certain other metals and aHoys or organic compounds have been added to ether to stabHize it. 

Oxidation of metals forming more than one oxide... 

The ability of N to exist in its compounds in at least 10 different oxidation states from —3 to +5 poses certain thermodynamic and mechanistic problems that invite systematic treatment. Thus, in several compounds N exists in more than one oxidation state, e.g. [N- "H4] + [N "02] , [N-" H4] + [N 03] , [N-"2H5] + [N 03]-, [N- "H4] + [N-3 3]-, etc. Furthermore, we have seen (p. 423) that, under appropriate conditions, NH3 can be oxidized by O2 to yield N2, NO or NO2, whereas oxidation by OCl yields N2H4 (p. 427). Likewise, using appropriate reagents, N2H4 can be oxidized either to N2 or to HN3 (in which the average oxidation number of N is — ). The thermodynamic relations between these various hydrido and 0x0 species containing N can be elegantly codified by means of their... 

When the desired halide is hydrolytically unstable then dry methods must be used, often at elevated temperatures. Pre-eminent amongst these methods is the oxidative halogenation of metals (or non-metals) with X2 or HX when more than one oxidation state is available X2 sometimes gives the higher and HX the lower, e.g. ... 

In 1826 J. J. Berzelius found that acidification of solutions containing both molybdate and phosphate produced a yellow crystalline precipitate. This was the first example of a heteropolyanion and it actually contains the phos-phomolybdate ion, [PMoi204o] , which can be used in the quantitative estimation of phosphate. Since its discovery a host of other heteropolyanions have been prepared, mostly with molybdenum and tungsten but with more than 50 different heteroatoms, which include many non-metals and most transition metals — often in more than one oxidation state. Unless the heteroatom contributes to the colour, the heteropoly-molybdates and -tungstates are generally of varying shades of yellow. The free acids and the salts of small cations are extremely soluble in water but the salts of large cations such as Cs, Ba" and Pb" are usually insoluble. The solid salts are noticeably more stable thermally than are the salts of isopolyanions. Heteropoly compounds have been applied extensively as catalysts in the petrochemicals industry, as precipitants for numerous dyes with which they form lakes and, in the case of the Mo compounds, as flame retardants. 

Figure 30.4 Volt-equivalent versus oxidation state for lanthanides with more than one oxidation state.

Sugars are easily oxidized. One oxidation reaction that shows this involves cupric hydroxide, O /... 

It is to.be noted that Na20 unites violently with w to form NaOH as does CaO to form Ca(OH)2, while Fe and PbO react very slowly. If a metal forms more than one oxide, the lower oxide may be more basic, for example FeO and Fe203, where Fe(OH)2 is the stronger base... 

Step 1 Identify the cation and the anion (see Table D.l or Appendix 3A, if necessary). To determine the oxidation number of the cation, decide what cation charge is required to cancel the total negative charge of the anions. Step 2 Name the cation. If the metal can have more than one oxidation number (most transition metals and some metals in Groups 12 through 15/V), give its charge as a Roman numeral. 

Except for mercury, the elements at the ends of each row of the d block occur in only one oxidation state other than 0. All the other elements of each row have at least two oxidation states and manganese, at the center of its row, has seven oxidation states. 

In the introductory chapter we stated that the formation of chemical compounds with the metal ion in a variety of formal oxidation states is a characteristic of transition metals. We also saw in Chapter 8 how we may quantify the thermodynamic stability of a coordination compound in terms of the stability constant K. It is convenient to be able to assess the relative ease by which a metal is transformed from one oxidation state to another, and you will recall that the standard electrode potential, E , is a convenient measure of this. Remember that the standard free energy change for a reaction, AG , is related both to the equilibrium constant (Eq. 9.1)... 

Metals exist in nature primarily in positive oxidation states, and many form stable com-poimds in more than one oxidation state. The formal oxidation number of the most common form can range from -t-1 to +6. The stable form in a given environment depends on the oxidation potential and chemical composition of that environment. Often the stable form at the Earth s surface in the presence of molecular... 

Table 16-3 Oxidation states of key elements. The oxidation state of an element is its charge or valence state in various chemical species. Many elements have more than one oxidation state. In nature you can usually determine the oxidation state of an element in a compound by assuming 0(—II) and H(I)...

The number of valence electrons in niobium oxychloride clusters decreases as the number of oxide hgands increases (Table 6.4). The compound Cs2LuNbi Cli70 has the VEC of 16 as found in most chloride clusters, suggesting that the presence of one oxide ligand per cluster does not tip the balance between the Nb-Nb bonding and Nb-L anti-bonding contributions to the a2u state. The VEC of most clusters... 

The cations in transition metal oxides often occur in more than one oxidation state. Molybdenum oxide is a good example, as the Mo cation may be in the 6-r, 5-r, and 4+ oxidation states. Oxide surfaces with the cation in the lower oxidation state are usually more reactive than those in the highest oxidation state. Such ions can engage in reactions that involve changes in valence state. 

A polarographic study revealed one oxidation process to occur in acetone solution (59). The reduction waves reported have to be ascribed to the reduction of mercury dithio-carbamates, formed by a reaction of the electrode material (60). 

Most transition metals have more than one oxidation number because —... 

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