Kinetics and mechanism of osmium catalyzed reduction of tellurium(VI) with arsenic(III)

The kinetics and mechanism of osmium catalyzed reduction of tellurium(VI) by arsenic(III) have been studied in sulphuric acid medium. The reaction is zero order in arsenic(III), fractional order in tellurium(VI) and first order in osmium. While the products arsenic(V) and tellurium(IV) and ionic strength have negligible effect, the rate increases linearly with increasing [H3O+]. The proposed mechanism leads to the rate equation, rate = -d[As(III)]/dt = kK[Te(VI)][H3O+][Os(IV)](t)/1 + K[Te(VI)] where K is the formation constant for osmium(IV) - tellurium(VI) complex and k is second order rate constant for the rate limiting step. Delta H* and Delta S-308* of the rate determining step are found to be 66.3 +/- 0.1 kJ mol(-1) and - 22.8 +/- 0.3 JK(-1) mol(-1) respectively.