Chemistry at and near the surface of liquid sulfuric acid: A kinetic, thermodynamic, and mechanistic analysis of heterogeneous reactions of acetone

The interactions of gas-phase acetone with liquid sulfuric acid solutions a re described. The solutions were prepared as 0.05 -0.10 mu m thick films de posited on single-crystal metal substrates. Experiments were carried out ov er bread ranges of acid composition (70 - >96 wt % H2SO4), temperature (180 -220 K), and acetone pressure (10(-7)-10(-3) Pa). Two types of measurements are reported: the time-dependent acetone uptake probability, and the infra red spectra of absorbed acetone and its reaction products. From the infrare d measurements, a reaction scheme is identified in which gas-phase acetone is taken up by sulfuric acid to form protonated acetone. In solutions conta ining more than 70 wt % H2SO4, protonated acetone undergoes a self-condensa tion/dehydration reaction to form mesityl oxide. In films that contain 85 w t % or more H-2 SO4, a second reaction sequence occurs, ultimately resultin g in the formation of trimethylbenzene. The uptake probability measurements are consistent with the infrared data. In 70 wt % H2SO4, the acetone uptak e probability rapidly decreases from an initial value near unity to a stead y-state value of zero, due to the formation of a saturated acetone + sulfur ic acid solution. The Henry's law solubility constants of acetone in 70 wt ro H2SO4 were obtained from the integrated uptake measurements. The tempera ture dependence of the measurements implies that the standard-state enthalp y and entropy changes of acetone solution in 70 wt % sulfuric acid are -66 kJ mol(-1) and -249 J mol(-1) K-1, respectively. In the more concentrated f ilms, the steady state uptake probability is never measured to be zero, sin ce absorbed acetone goes on to form the condensation/dehydration products. A two-step kinetic scheme is proposed to account for the reactions of aceto ne in sulfuric acid. By fitting the data to the model predictions, the Henr y's law solubility constants and the reaction rate constants may be estimat ed.