Oxidation State of Reduced Chromium

The chromium leaves, taking the bonding electron pair with it. Overall, 2-propanol has been oxidized to acetone, and the chromium has been reduced from the +6 oxidation state in H2C1O4 to the +4 oxidation state in H2OO3. The mechanism has additional steps that result in the final oxidation state of the chromium being +3. 

The oxidation state of the chromium ions in the dichromate ions is +6, so six moles of electrons will be required to reduce 1 mole of Cr6+ ions ... 

The standard free energies of formation of the metal oxides in kJ moF are adjusted for fair comparison by dividing them by n, the total decrease in oxidation state required to reduce the metal atoms contained in the oxide to oxidation states of 0. Thus, the reduction of Cr203 involves a change in the oxidation state of two chromium atoms from + 3 to 0, so n = 2 X 3 = 6. 

The mechanism involves the formation of a chromate ester, and at the end of the reaction, the oxidation state of the chromium changes from +6 (orange colour) to +3 (green colour), i.e. the chromium is reduced. 

Ghromium(III) Compounds. Chromium (ITT) is the most stable and most important oxidation state of the element. The E° values (Table 2) show that both the oxidation of Cr(II) to Cr(III) and the reduction of Cr(VI) to Cr(III) are favored in acidic aqueous solutions. The preparation of trivalent chromium compounds from either state presents few difficulties and does not require special conditions. In basic solutions, the oxidation of Cr(II) to Cr(III) is still favored. However, the oxidation of Cr(III) to Cr(VI) by oxidants such as peroxides and hypohaUtes occurs with ease. The preparation of Cr(III) from Cr(VI) ia basic solutions requires the use of powerful reducing agents such as hydra2ine, hydrosulfite, and borohydrides, but Fe(II), thiosulfate, and sugars can be employed in acid solution. Cr(III) compounds having identical counterions but very different chemical and physical properties can be produced by controlling the conditions of synthesis. 

The determination of oxidation states of transition metals such as vanadium, chromium, or molybdenum in supported oxides is difficult, because many of the reduced phases are weak Raman scatterers. In such cases, the combination of UV—vis DRS and Raman spectroscopy is essential for identifying the reduced oxidation states and the extent of reduction. UV—vis DRS provides information about the changes in oxidation state (Weckhuysen, 2003) and the extent of reduction (Gao et al., 1999), and it permits determination of the specific wavelengths at which it is possible to resonantly enhance the Raman signal. 

The two main oxidation states of chromium usually display opposite charges in their compounds. Chromium(VI) is particularly toxic and all operations are subject to stringent legislation in most countries with significant process industries. Chromium(III) is oxidized by hydrogen peroxide to chromium(VI) in neutral or alkaline conditions, but reduced from chromium (IV) to (III) in acid (Figure 6.4). 

Fortunately, it has been established that the reduction of the oxidized precursors can also be performed with a simpler reductant such as CO, with formation of a single oxidation product (CO2) that is not adsorbed on the sample (3,182,185,195-197). This simpler reduction procedure allows one to obtain a simplified version of the catalyst, whereby the oxidation state of chromium and the surface hydroxylation are much better controlled. According to the literature data, no significant difference in the polymerization products has been found between the CO-reduced system and the ethene-reduced one (3,182,198). Therefore, this CO-reduced catalyst, containing predominantly anchored Cr(II), has been considered as a model catalyst. ... 

Chromium commonly forms compounds in which it has an oxidation state of +2, +3, or +6, as shown in Table 20.5. The Cr2+ (chromous) ion is a powerful reducing agent in aqueous solution. In fact, traces of 02 in other gases can be removed from other gases by bubbling the gaseous mixture through a Cr2+ solution ... 

From this reaction, chromium as reduced from an oxidation state of -1-6 to an oxidation state of -i-3. The oxidation products are carbon dioxide and water. The oxidation state is a measure of the degree of affinity of the atom to the electrons it shares with other atoms. A negative oxidation state of an atom indicates that the electrons spend more time with the atom, while a positive oxidation state indicates that the electrons spend more time with the other atom. 

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