2803 words - 12 pages

Second Order Reaction Kinetics

Abstract

The objective of this experiment was to determine if the specific rate constant of the reduction reaction of hexacyanoferrate (III) ion with ascorbic acid (C6H8O6) is affected by the ionic strength of the solution and the charges of the ion species within the solution. A Cary Bio 50 Spectrophotometer and its associated software was used to measure the absorbance of the solution at a wavelength of 418 nm. Analysis of the data collected supports the conclusion that the ionic strength of the solution and the charges of ions on the activated complex have a direct relation to the rate constant. The experimental value of 2.237 for the ionic strength of ...view middle of the document...

The course of the reaction was followed spectrophotometrically since the hexacyanoferrate (III) ion is colored yellow in an aqueous solution with an absorption coefficient of 1012 M-1 cm-1.The absorbance of hexacyanoferrate is given by the equation

[5]

absorbance=1012 FeCN63-

[6]

which was used to calculate the concentration of hexacyanoferrate (III). From the stoichiometry of the reaction, the concentration of ascorbic acid at a given time is:

C6H8O6=C6H8O60-12FeCN603--FeCN63-

Once the initial concentrations are known for ascorbic acid and hexacyanoferrate (III) ion were calculated, the following equation 7 could be used to calculate for the rate constant k using the slope of ln[C6H8O6] /[Fe(CN)] vs time

[7]

slope m=k2C6H8O60-FeCN60

The rate of a reaction refers to the change in concentration of a component within a reaction per unit time. The rate is determined through stoichiometry and uses both the reactants and products of the reaction. The rate can also be expressed through a rate law, which is also determined through stoichiometry but only takes into account the reactants. It shows the relationship of the rate of a reaction to the rate constant, k, and the concentrations of the reactants, A and B, raised to a powers, x and y respectively, which are determined experimentally and do not depend on stoichiometric coefficients from the balanced equation.

The order of a reaction refers to the sum of the experimentally determined powers that the concentrations of reactants are raised to in order to determine the rate of the reaction. Reactions are classified as zero order, first order, second order, or third order based on the sum of the powers of the reactant concentrations.

In this experiment’s reaction with the hexacyanoferrate (III) ion and ascorbic acid, the reaction stoichiometry is of the form

aA + bB → products [8]

[9]

and therefore the rate law is

-d[A]dt=kA[B]

where [A] and [B] are the concentrations of reactants A and B and k is the rate constant. Equation 9 integrates into the equation

ln[A][B]=b[A]0-a[B]0akt+ln[A]0[B]0

[10]

Where a and b represent constants and [A]0 and [B]0 represent the initial concentrations of A and B. The a and b values for this experiment are 1 and 2, respectively (Sime, 1990).

In this particular reaction, the orders x and y of the general rate law equation are both 1 because the reaction is first order with respect to hexacyanoferrate (III) and first order with respect to ascorbic acid. Therefore, overall, the reduction reaction is referred to as a second order reacti0on.

The stock solution of ascorbic acid contains disodium EDTA which prevents the ascorbic acid solution from oxidizing at room temperature (Harris, 2010).

[11]

Ionic strength (I) of a solution is the measure of the total concentration of ions in solution and is given by the equation

I=12icizi2

Where ci is the concentration of the ith species and zi is the charge. The ionic strength of a...

Find the perfect research document on any subject