ISSN:
1434-1948
Keywords:
Heteropolyacids
;
Molybdovanadophosphates
;
Hydrazine hydrate
;
Reduction
;
Oxidations
;
Thermochemistry
;
Chemistry
;
General Chemistry
Source:
Wiley InterScience Backfile Collection 1832-2000
Topics:
Chemistry and Pharmacology
Notes:
Differential enthalpies of reduction, -ΔrH4j, have been found at 343 K for 0.2 m aqueous heteropolyacids H3+xPVxMo12-xO40 (HPA-x, where 1 ≤ x ≤ 6) and their salts by hydrazine hydrate (HH). The -ΔrH4j values depend on x and m, where m = [VIV]/[HPA-x]. At m ≈ 0, the initial -ΔrH4j value is 436 ± 14 kJ mol-1 HH for all HPA-x. For HPA-x with 3 ≤ x ≤ 6, the curves -ΔrH4j = f(mj) have maxima associated with formation and reduction of the outer-sphere cation VVVIVO33+. A similar maximum is observed on reduction of 0.1 M VO2ClO4 in 4 M HClO4. No maxima are observed for the reduction of HPA-1, HPA-2, and salts of HPA-3. For HPA-3 salts, -ΔrH4j = f(mj) depends primarily on the pH of the salt solution, rather than on the nature of the cation. Reduction and oxidation of HPA-x and their salts are reversible reactions. In HPA-x solutions, VO2+ in the outer sphere is reduced first, and VV in the inner coordination sphere of the heteropoly anion is reduced next. Differential enthalpies for the oxidation of the reduced HPA-x by dioxygen (-ΔrH2j) have been calculated. The sequence of the oxidation of different VIV forms in the reduced HmHPA-x solutions is the reverse of that for the reduction of the VV forms in the initial HPA-x solutions.
Additional Material:
4 Ill.
Type of Medium:
Electronic Resource
