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2 O 4. The conversion of chromite to chromia proceeds via Na 2 Cr 2 O 7, which is reduced with sulfur at high temperatures: [8] Na 2 Cr 2 O 7 + S → Na 2 SO 4 + Cr 2 O 3. The oxide is also formed by the decomposition of chromium salts such as chromium nitrate, or by the exothermic decomposition of ammonium dichromate. (NH 4) 2 Cr 2 O 7 → Cr ...
Compounds of chromium(V) are rather rare; the oxidation state +5 is only realized in few compounds but are intermediates in many reactions involving oxidations by chromate. The only binary compound is the volatile chromium(V) fluoride (CrF 5). This red solid has a melting point of 30 °C and a boiling point of 117 °C.
Chromium trioxide is generated by treating sodium dichromate with sulfuric acid: [6] H 2 SO 4 + Na 2 Cr 2 O 7 → 2 CrO 3 + Na 2 SO 4 + H 2 O. Approximately 100,000 tonnes are produced annually by this or similar routes. [7] The solid consists of chains of tetrahedrally coordinated chromium atoms that share vertices.
The +3 and +6 states occur most commonly within chromium compounds, followed by +2; charges of +1, +4 and +5 for chromium are rare, but do nevertheless occasionally exist. [ 29 ] [ 30 ] Common oxidation states
Commonly three electrons are added to a chromium atom, reducing it to oxidation state +3. In acid solution the aquated Cr 3+ ion is produced. Cr 2 O 2− 7 + 14 H + + 6 e − → 2 Cr 3+ + 7 H 2 O ε 0 = 1.33 V. In alkaline solution chromium(III) hydroxide is produced.
Chromium is a very common naturally occurring element. It is the 21st most abundant element in the Earth's crust with an average concentration of 100 ppm. The most common oxidation states for chromium are zero, trivalent, and hexavalent states. Most naturally occurring chromium is in the hexavalent state. [20]
The oxidation states are also maintained in articles of the elements (of course), and systematically in the table {{Infobox element/symbol-to-oxidation-state}} See also [ edit ]
Chromium oxide may refer to: Chromium(II) oxide, CrO; Chromium(III) oxide, Cr 2 O 3; Chromium dioxide (chromium(IV) oxide), CrO 2, which includes the hypothetical compound chromium(II) chromate; Chromium trioxide (chromium(VI) oxide), CrO 3; Chromium(VI) oxide peroxide, CrO 5; Mixed valence species, such as Cr 8 O 21