Oxidation of formic acid by bromine in aqueous, strongly acid media

✍ Scribed by Peter Herbine; James G. Brummer; Richard J. Field

• Publisher: John Wiley and Sons
• Year: 1980
• Tongue: English
• Weight: 444 KB
• Volume: 12
• Category: Article
• ISSN: 0538-8066
• DOI: 10.1002/kin.550120605

✦ Synopsis

Spectrophotometric methods were used to investigate the rate of the reaction of Br:! with HCOOH in aqueous, acidic media. The reaction products are Br-and COa. The kinetics of this reaction are complicated by both the formation of Br3-as Bris formed and the dissociation of HCOOH into HCOO-and H+. Previous work on this reaction was carried out a t acidities lower than the highest used here and led to the conclusion that only HCOO-reacts with Brz. It is agreed that this is by far the principal reaction. However, at the highest acidity experiments, an added small component of reaction was found, and it is suggested that it results from the direct reaction of Bra with HCOOH itself. On this assumption, values of the rate constants for both reactions are derived here. The rate constant for the reaction of HCOO-with Brz agrees with values previously reported, within a factor of 2 on the low side.

The reaction involving HCOOH is more than 2000 times slower than the reaction involving HCOO-, but it does contribute to the overall rate as [H+] approaches 1M. These derived rate constants are able to simulate quantitatively the authors' absorbance-versus-time data, demonstrating the validity of their data treatment methods, if not mechanistic assignments. Finally, activation parameters were determined for both rate constants. The values obtained are: AEt(HCO0H + Br2) = 13.3 f 1.1 kcal/mol, AS1 (HCOOH + Br:!) = -28 f 3 cal/deg mot, AEi (HCOO-+ Br2) = 13.1 f 0.9 kcal/mol, and AS(HCO0-+ Br2) = -12 f 1 cal/deg mol. That the activation energies of the two reactions turn out to be essentially identical does not support the authors' suggestion that both HCOOH and HCOO-react with Br:!.