Upon heating, it reversibly loses oxygen, successively forming V2O4, V2O3, VO and the metal.
Acid-base reactions
Unlike most metal oxides, it dissolves slightly in water to give a pale yellow, acidic solution. When this compound is formed by V2O5 it is an amphoteric oxide. Thus :V2O5 reacts with strong non-reducing acids to form solutions containing the pale yellow salts containing dioxovanadium(V) centers:
V2O5 + 2 HNO3 → 2 VO2(NO3) + H2O
It also reacts with strong alkali to form polyoxovanadates, which have a complex structure that depends on pH.[6] If excess aqueous sodium hydroxide is used, the product is a colourless salt, sodium orthovanadate, Na3VO4. If acid is slowly added to a solution of Na3VO4, the colour gradually deepens through orange to red before brown hydrated V2O5 precipitates around pH 2. These solutions contain mainly the ions HVO42− and V2O74− between pH 9 and pH 13, but below pH 9 more exotic species such as V4O124− and HV10O285− (decavanadate) predominate.
Upon treatment with thionyl chloride, it converts to vanadium oxychloride, VOCl3:
V2O5 + 3 SOCl2 → 2 VOCl3 + 3 SO2
Redox reactions
V2O5 is easily reduced in acidic media to the stable vanadium(IV) species, the blue vanadyl ion (VO(H2O)52+). This conversion illustrates the redox properties of V2O5. For example, hydrochloric acid and hydrobromic acid are oxidised to the corresponding halogen, e.g.,
V2O5 + 6HCl + 7H2O → 2[VO(H2O)5]2+ + 4Cl− + Cl2
Solid V2O5 is reduced by oxalic acid, carbon monoxide, and sulfur dioxide to give vanadium(IV) oxide, VO2 as a deep-blue solid. Further reduction using hydrogen or excess CO can lead to complex mixtures of oxides such as V4O7 and V5O9 before black V2O3 is reached. Vanadates or vanadyl(V) compounds in acid solution are reduced by zinc amalgam through the interestingly colourful pathway:
VO2+ → VO2+ → V3+ → V2+
yellow blue green purple
Vanadium oxidation states.jpg
The ions are, of course, all hydrated to varying degrees.