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8/2/2019 Auto Catalysis

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ASC08-Lab Practical Work 1

KINETICS OF AN AUTOCATALYTIC REACTION

The aim of this practical is to study the kinetics of the permanganate/oxalate reaction using a UV-

Visible spectrometer. The data obtained are processed with help of Excel to estimate the rate

constants of the reactions involved in the kinetic model.

Introduction

Potassium permanganate has been widely used as an oxidizing agent for over 100 years. It is a

reagent that is readily available, inexpensive, and requires no indicator unless very dilute solutionsare used. Permanganate undergoes a variety of chemical reactions, since manganese can exist in

oxidation states of +2, +3, +4, +6, and +7.

Sodium oxalate, Na2C2O4, is a good primary standard for permanganate in acid solution. It can be

obtained in a high degree of purity, is stable on drying, and is non-hygroscopic. Its reaction withpermanganate is somewhat complex, and even though many investigations have been made, the

exact mechanism is not clear. The reaction is slow at room temperature, and hence the solution is

normally heated to about 60 C. Even at an elevated temperature the reaction starts slowly, but therate increases as manganese (II) ion is formed. Manganese (II) acts as a catalyst, and the reaction

is termed autocatalytic, since the catalyst is produced in the reaction itself. The equation for the

reaction between oxalate and permanganate is:2 MnO4

- + 5 H2C2O4 + 6 H3O+ 2Mn2+ + 10 CO2 + 14 H2O

Experimental work

4 solutions were prepared as following:Solution 1: 200 cm3 distilled water.

Solution 2: In a 200 cm3 volumetric flask, 10 cm3 C2O42- (0.02 mol/L) were poured and 3 cm3

H3PO4 (2 mol/L), then the final volume of the solution was brought up to 200 cm3 with the addition

of distilled water.

Solution 3: In a 200 cm3 volumetric flask, 10 cm3 C2O42- (0.02 mol/L) were poured, 3 cm3 H3PO4

(2 mol/L) and 2 cm3 Mn2+, then the final volume of the solution was brought up to 200 cm 3 with

the addition of distilled water.Solution 4: approximately 150cm3 of a 0.01 mol/L MnO4

- solution.

1) First experiment: Acquisition of the absorption spectrum of MnO4-

The spectrum was acquired using the UV-Visible spectrometer in Spectrum mode and inthe wavelength range 390-600 nm. The solution was prepared adding 4 cm3 of solution 4into solution 1. To ensure reproducibility of the experiment the spectrum was recorded

three times and the average was used for the calculations.

2) Second experiment: Kinetics without Mn2+


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For this experiment the spectrometer was used in Fixed Wavelength mode to record

absorbance at three wavelengths: 400, 457 and 525 nm. The blank was done with solution

2 and for following the kinetics of the reaction 4 cm3 of solution 4 were added to solution2. The absorbance was measured at time zero and every 30 seconds until the there was no

absorption. The room temperature was 19 C.

3) Third experiment: Kinetics with Mn2+

The spectrometer was also used in Fixed Wavelength mode to record absorbance at 400,457 and 525 nm wavelengths. To observe if manganese in the oxidation state II has a

catalytic effect on the reaction, 4 cm3 of solution 4 were added to solution 3 and the blank

was recorded with solution 3.

Interpretation of the experimental results

The absorbance signal at every wavelength is the addition of the contributions of all the absorbing

species:

Given that

and with the help of the spectrum obtained from the first experiment, the absorbance coefficients

of these three species are achieved as in the following table 1:

Table 1. Absorbance coefficients of Mn(III), Mn(IV) and Mn(VII) at 400, 457 and 525 nm

(L.mol-1cm-1) 400(nm) 457(nm) 525(nm)

Mn(3) 45.59 150.00 63.03

Mn(4) 1585.50 525.00 0.00

Mn(7) 140.21 256.13 2250.00

The absorbance files from experiments 2 and 3 were then decomposed into concentration files of

Mn(III), Mn(IV) and Mn(VII). (see worksheets Abs&Conc_EXP2 and Abs&Conc_EXP3 in

the attached Spreadsheet)

Simulation of the experiments

Based on the following suggested three-step mechanism

and the Euler method for very small time change


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The spreadsheet built from simulation for experiment 2 and 3 were achieved. (see worksheet SIM-exp2 and SIM-exp3 in Spreadsheet). However, the Solver tool couldnt find the optimal k

values unexpectedly. So instead, we used trial-and-error method on k to get the best values which

could make the experimental and simulated curve for each species match each other well. Theresulting outcomes are shown:

Table 2. Experimental values of the rate constants

K1 K2 K3

Experiment2 50 0.005 0.004

Experiment3 450 0.012 0.023

Conclusion

In this practical, the absorbance of Mn(VII) was firstly measured and then the absorbance profiles

of Mn(III), Mn(IV) and Mn(VII) during reaction at fixed wavelength of 400, 457 and 525 nm were

recorded which were further decomposed into their concentration profiles. Lastly, the rate

constants involved in the suggested mechanism were determined.

Since the rate limiting step for the mechanism is usually the one with the lowest rate constant, the

rate limiting step in this suggested mechanism is step 3, namely the generation of Mn(II),

considering k1>>k2, k3 and k1(exp3)>k1(exp2)

Besides, this suggested three-step mechanism is quite good on the grounds that the concentrationevolution patterns of Mn(III), Mn(IV) and Mn(VII) from simulation are in good accordance withthose curves deduced from the experiments.

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