According to reference1 the power to which the concentration of a species (product or reactant) is raised in a rate law of this nature is the order of the reaction with respect to that species. In equation (1) first order with respect to [A] and first order with respect to [B]; however, the overall reaction is the sum of the individual orders. Thus we have a second order reaction. In this experiment a hexacyanoferrate(III) ion ([Fe(CN)6]3-) oxidizes ascorbic acid (C6H8O6) by the following reaction: (2) 2[Fe(CN)6]3- + C6H8O6 = 2[Fe(CN)6]4- + C6H6O6 + 2H+
The reaction above is of a first order reaction at room temperature with respect to individual reactants; therefore the reaction stoichiometry and rate law at time t are: (3) aA + bBproducts and (4) -d[A] = k[A] [B]
where [A] represents the concentration of ascorbic acid and [B] represents the concentrations of [Fe(CN)6]3- at time t. For this experiment we will use an integrated rate law in the form of: (5) ln [A] = b [A]0 a [B]0 kt + ln [A]0
where [A]0 and [B]0 are the initial concentrations of C6H8O6 and [Fe(CN)6]3- and a=1 and b=2. From equation (5), it is possible to calculate the second-order rate constant k by plotting ln [A]/[B] against time (find slope of line where b=2 and a=1). EDTA in this experiment is used as a masking agent to hide metal ions that would normally interfere with the analysis in this reaction. Thus the absorbance of [Fe(CN)6]3- at time t is given by: (6) Absorbance = 1012 [Fe(CN)6]3-
The oxidation of C6H8O6 by [Fe(CN)6]3- involves a mechanism that consists of 3 steps.2 In the first step, the ascorbate ion (AH-) is rapidly formed by ionization of the ascorbic acid. (7) AH2 AH + H+
Following the ionization is the slow rate-determining step, the oxidation of the ascorbate ion to an ascorbate free radical (AHâˆ™): (8) [Fe(CN)6]3- + AH-[Fe(CN)6]4- + AHâˆ™ During the final step, an electron is rapidly transferred from the ascorbate free radical to the hexacyanoferrate(III) anion, producing dehydroascorbic acid (A): (9) [Fe(CN)6]3- + AH- [Fe(CN)6]4- + A + H+
The slow rate-determining step is an ionic reaction between [Fe(CN)6]3- and AH-. According to reference3, the specific rate constant of an ionic reaction in aqueous solution depends on two factors: the ionic strength I of the solution and on the charges ZA and ZB of the ionic species reacting to for the activated complex. (10) log k = log k0 + 1.02ZAZB I1/2
All reagents in this experiment were of reagent grade. Mass measurements were taken on a Shimadzu Libror AEG-120 analytical scale with an uncertainty of ±0.0001. Manual data acquisition was taken with a Barnstead/Turner SP-830 spectrophotometer and a stopwatch. The computerized data acquisition was completed by a Cary 50 Bio. The experiment began by preparing four solutions of 1 x 10-3 M of K3Fe(CN)6 with varied concentrations of NaNO3: 0.025 M, 0.05 M, 0.1 M and 0.2 M.
This was completed by dissolving 0.0329245 (±0.001) g of K3Fe(CN)6 with the specified concentrations of NaNO3 and deionized water in a 100 mL volumetric flask. A 25 mL aliquot of each solution was transferred into a 250 mL Erlenmeyer flask and the temperature of the aliquot was recorded. Next, a 500 mL 2.5 x 10-4 M solution of ascorbic acid was prepared by using a standardized 0.01 M HNO3 solution dissolved in 0.005 g of EDTA and deionized water. A 25 mL aliquot was transferred into each of the four 100 mL beakers by using a 25 mL pipet.
The spectrophotometer was set to 418 nm and the absorbance reading was zeroed by using deionized water as a standard. The ascorbic acid in the beaker was poured into the K3Fe(CN)6 solution and the timer was immediately started. The Erlenmeyer flask was swirled for 2-3 seconds before pouring the reacting mixture into a 1-cm cuvette. The cuvette was conditioned with the reacting solution 4 times before being placed into the sample holder of the spectrophotometer. An absorbance reading was taken at 30 seconds and every 30 seconds thereafter for a total of 6 minutes. The same process was implemented with the Cary 50 Bio except that each sample was analyzed by the computer for 7 minutes and 53 seconds. Data/Results