Base decomposition of heptamolybdate

Abstract

The reaction of heptamolybdate, Mo₇O₂₄^6-, with excess hydroxide ion proceeds through base-independent, and base-dependent, pathways. The reaction is strongly dependent upon the salt chosen to maintain constant ionic strength. The values of the base-independent rate constant increase in the order K⁺(2.5 s^-1 at 2.0 mol l^-1 KCl and 25.0°C) < Na⁺ (9 s^-1) < Li⁺ (20 s^-1) and those of the base-dependent rate constant in the order K⁺ (8200 1. mol^-1 s^-1) < Na⁺ (10300 1. mol^-1 s^-1) < Li⁺ (19600 1. mol^-1 s^-1). Extensive measurements in mixed cation media show that the effect of ion pairing decreases in the order K⁺ > Na⁺ > Li⁺. The kinetic behaviour and activation parameters are contrasted with those for the base decomposition of decavanadate, V₁₀O₂₈^6-, and different mechanisms are proposed.