The homogeneous decomposition reactions of gaseous formic acid

By use of reaction vessels with specially treated surfaces the homogeneous decomposition of formic acid has been studied kinetically in the range 436 to 532°C. Neither of the two simultaneous reactions (a) HCOOH = CO2 + H2, (b) HCOOH = CO + H2O, is retarded by the usual inhibitors of chain processes. Each appears to be molecular. Reaction (a) is of the first order in the range 3 to 650 mm, the first-order rate constant being given by kCO2 = 104⋅8 exp (–30600/RT)s-1. It is suggested tentatively that the abnormal kinetic parameters might be explained by regarding the reaction as a decarboxylation of (H+) (HCOO¯) ion pairs present in minute concentration. Reaction (b) shows a pressure dependence most simply explained by a superposition of a predominant second-order component with a small first-order component. The most satisfactory interpretation of the second-order reaction is that it represents the unimolecular decomposition of dimer molecules, known to be present in formic-acid vapour. On this basis the rate constant is given by kCOdimer = 1013⋅58 exp (–42600/RT)s-1, the kinetic parameters thus being in the normal range. The various alternative interpretations are discussed.