Question

what is the main purpose of this experiment? and how to analyze the data, what are we trying to get? and how this experiment is working?

Kinetics of the Oxidation of Benzyl Alcohol

Objective: To study the kinetics of a reaction by a spectrometric method and how this relates to the dynamics of the system.

Theory

The oxidation of benzyl alcohol by the permanganate ion in acidic media is: 2MnO4 + 5C6H5CH2OH + 6H → 2Mn + 5C6H5CHO + 8H2O

The balanced equation does not provide any information concerning the reaction kinetics or dynamics. Indeed, the reaction does not progress to products by the simultaneous combination of all the reactants in their stoichiometric ratios. Rather, the reaction progresses through several simpler steps where the stoichiomentric coefficients indicate the order of the reaction with respect to that reactant. The mechanism describing the reaction often involves reactants not accounted for in the overall reaction. A typical mechanism1 for this oxidation involves a hydride transfer from the alcohol:

H+ + MnO4 ↔ HMnO4 {fast equilibrium}

HMnO4 + C6H5CH2OH → H2MnO4- + C6H5CHOH+ {slow}

C6H5CHOH+ → C6H5CHO + H+ {fast}

H2MnO4- → products {fast}

Notice the speed at which a step occurs is indicated and that the slowest step determines the overall rate of the reaction.

The above mechanism is conjecture until it is supported through experiment. Support for this mechanism is achieved by measuring the orders with respect to the reactants and demonstrating that these orders, in conjunction with the proposed rate determining step, agree with the suggested dynamics.

Experimental Setup

The chemical reaction studied will be the oxidation of benzyl alcohol by acid permanganate with the formation of benzaldehyde. According to the proposed mechanism, the kinetics should be of first order with respect to each reactant and will be determined by the spectrometric method using a Varian Spectrometer. A general schematic UV-Vis spectrometer is shown in figure 1. The basic parts of a spectrophotometer are a light source (often an incandescent bulb for the visible wavelengths, or a deuterium arc lamp in the ultraviolet), a holder for the sample, a diffraction grating or monochromator to separate the different wavelengths of light, and a detector. The method used in this experiment is a modification of the method proposed by Liu2.

The samples that you will prepare hold both the acid and alcohol concentrations in such excess that the pseudo first order kinetics is observed. You will be able to determine the order with respect to the remaining reactants from analysis of the rate constants as a function of concentration of the remaining reactants.

The samples that you will prepare hold both the acid and alcohol concentrations in such excess that the pseudo first order kinetics is observed. You will be able to determine the order with respect to the remaining reactants from analysis of the rate constants as a function of concentration of the remaining reactants.

Figure 1. General Schematic UV-Vis Spectrometer.

Procedure

Preparing the solution. Prepare 50 ml of 0.002 M potassium permanganate solution and store it in a brown bottle. The other solutions necessary for this experiment is a 0.04 M Benzyl alcohol and 1.4 M Sulfuric Acid, both of which are already prepared.

Preparing the sample. Absorption/time profiles on seven samples will be measured using the Varian UV-vis spectrometer. The concentrations of those samples are:

Sample	Volume of KMnO4 (ml)	Volume of C6H5CH2OH (ml)	Volume of H2SO4 (ml)	Volume of H2O (ml)
1	5	10	10	-
2	5	7	10	3
3	5	5	10	5

Data Analysis

Plot the Absorption/time profiles of your data. Manipulate the data so that you may acquire the rate constant associated with each sample run. It should be obvious that the reaction is first order with respect to the permanganate ion. Determine the orders with respect to the remaining reactants and verify the proposed mechanism.

Answer 1

The main purpose of this experiment is to study the kinetics of a reaction ,using spectrometry.During the experiment you can analyse the rate of the reaction, determine the order of the reaction(first order as it is expected to be) and also be able t calculate the rate constant for the reaction.

Rate of a reaction is calculated using the equation,

Rate=change of concentration of reactant or product/change of time

or,Rate= delta C/delta t

As it is expected to be a first order reaction ,so the rate will be dependent only on one of the main reactant ,other reactants may be taken in fixed amounts so that their change of concentrations with time is constant.

From the data ,you can see that only the initial concentration of benzyl alcohol is changing for three different sets of the experiment,so rate will depend on change of concentration of benzyl alcohol.The concentration of other reactants are fixed or unchanging for all the sets.

By using spectrometric method one can visually detect the change of concentration of the reactants after a fixed time interval.The reactant KMnO4 is colored with its color intensity decreasing, as the reaction proceeds and it getting used up.

The lUV-visible light from the spectrometer gets absorbed by the reaction solution, giving an Absorbance value,A..This absorbance value is directly proportional to the amount of KMnO4 present in the solution.As the amount of KMnO4 decreases ,intensity of the color of the solution decreases and also the Absorbance value.

Beer's law-

A=elc

where e=absorptivity of the colored species in the solution

c=concentration the colored species in the solution

l=path length of light (fixed)

Thus , Absorbance gives a good estimate about the change in concentration of KMnO4 ,which is related to the change in cncentration of benzyl alcohol.

To find out the order of the reaction three plots are to be made using the same data set, one each for zero order,first order ,second order as there are two main reactans.

1) First order plot

A plot of Absorbances ln (A) vs time ,t is to be made ,to work out the equation of Integrated law for first order reaction

ln[A]=-kt +ln[A] where[ A]=absorbance at time t, [A]o =initial value of absorbance at t=0

k=rate constant

the plot will be a straight line with slope=-k

Thus ,rate constant can be calculated.

2)ntegrated rate equation for zero order reaction:

[A]=-Kt +[A]o

plot of [A[ vs t is a straight line of the form y=mx+c ,m=slope=-k where k=rate contant

2)

Integrated rate equation for second order reaction: 1/[A]=kt + 1/[A]o

plot of 1/[A[ vs t is a straight line of the form y=mx+c ,m=slope=-k where k=rate contant