Kinetics f the dissociation of hydrogen sulphide by irn films

Hydrogen sulphide is chemisorbed dissociatively at -80°C attaining a maximum coverage of θ_s≃1.6. The hydrogen adatoms at -80°C are metastable with respect to H₂(g) for θ_s, values ≥ 0.7. Above 0°C, incorporation of sulphur into the metal occurs and the kinetics of dissociation conform to the following equation: -(dP_H2S/dt)θ_s,T = A_θs P_H2S/(2πmkT)^1/2exp(-E_θs/RT) The condensation coefficient A_θs, and activation energy E _θs, increase from 10^-7 to 10¹ and from 5 to 20 kcal mole^-1 respectively as θ_s increases from 1.5 to 4.0. A linear relationship exists between log Aθ_s, and Eθ_s,. The slow-step is considered to be dissociation at the sulphide surface and the variation in A_θs, is interpreted in terms of the absolute rate theory where the transition state complex gains mobility during sulphidation. The entropy of activation ΔS^≠ increased from +5 cal mole^-1 deg.^-1≃1.5 to +30 cal mole^-1 deg.^-1 at θ_s≃4.0.