Vanadium ionic strength dependence

Rate law flooding. The second-order rate constant for the reaction between the hydrated ions of vanadium(3+) and chromium(2+) depends on [H+ ]. From the data given, which refer to T = 25.0 °C and a constant ionic strength of 0.500 M, formulate a two-parameter equation that describes the functional dependence. Evaluate the two constants. Compare your result to the one derived in to Problem 1 -2. 

Strongly acidic vanadium(V) oxidises bromide in a sulphate ion medium . The reaction is first-order in both oxidant and sulphuric acid. The dependence of the rate on bromide ion concentration is complex and a maximum is exhibited at certain acidities. A more satisfactory examination is that of Julian and Waters who employed a perchlorate ion medium and controlled the ionic strength. They used several organic substrates which acted as captors for bromine radical species. The rate of reduction of V(V) is independent of the substrate employed and almost independent of substrate concentration. At a given acidity the kinetics are... 

In aqueous solution both the tetramer and the pentamer have cyclical structures (Fig. 6) as shown by the narrow 51V NMR linewidths indicating high symmetry around vanadium (29). The relative amounts of the species are concentration dependent but under moderate concentrations the tetramer is the most important (Figs. 2 and 3). The hexamer [V60i8]6 is a very minor species under most conditions, but it becomes somewhat more prominent at high concentrations and high ionic strength (8, 10, 15, 32). Indications are that it has a cyclic structure in aqueous solution (10). 

The singly and doubly protonated decamers predominate under most conditions (10, 32). The percentage concentration of the unpro-tonated decavanadate ion is strongly dependent on the ionic strength. For instance, calculations show that its maximum concentration in ionic medium 0.15 M NaCl is less then 10% of the total vanadium concentration (0.02 M), but in 3.0 M NaC104 medium it amounts to about 50% (28). 

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