Autocatalytic reaction is a chemical process in which at least one of the reactants turns out to be — Physical Chemistry — Thermodynamics Chemistry Question

Autocatalytic reaction is a chemical process in which at least one of the reactants turns out to be also a product. The rate equations for autocatalytic reactions are fundamentally nonlinear. Oxidation of oxalic acid with permanganate is one of famous examples of such reactions. When no manganese(II) ions are added into the system, the reaction initially proceeds slowly. Still, at a particular moment the rate gradually increases because the product autocatalyzes the reaction. Addition of a few crystals of MnSO₄ greatly accelerates the reduction of purple permanganate affording a grayish-brown complex ion A. This complex ion is characterized by an extremely low stability in water and practically immediately starts decomposing via disproportionation giving colorless products.

The progress of the disproportionation reaction can be followed photometrically at the wavelength of 405 nm corresponding to the maximum absorbance of the trioxalomanganesate ion. Since potassium permanganate is rapidly and quantitatively consumed in the reaction and all the rest substances found in the reaction mixture are colorless, the measured value of the absorbance (A) is proportional to the complex ion concentration (c) according to Beer^-Lambert^-Bouguer law:

A = ε c l

where ε is the molar extinction coefficient and l is the optical path length.

Chemicals and reagents
* Potassium permanganate,
* Manganese(II) sulphate,
* Oxalic acid,

Equipment and glassware
* Analytical balance (± 0.0001 g),
* Visible spectrophotometer (or photometer with fixed wavelength of 405 nm) equipped with thermostated cell holder,
* Spectrophotometer cells,
* Tissue to clear cell surfaces,
* Thermostat,
* Thermometer,
* PC (or other computing techniques) with Microsoft Excel software (English version),
* Volumetric flask, 100 cm₃, with a tight stopper (4 ea.),
* Graduated cylinder, 100 cm₃,
* Volumetric pipettes, 5 and 10 cm₃,
* Pipette filler,
* Spatula.

Procedure

A. Preparation of stock solutions
Read the procedure to the end and calculate the concentrations of stock solutions of potassium permanganate, manganese(II) sulfate, and oxalic acid that will allow easy preparation of the reaction mixture to be analyzed. Prepare the stock solutions of potassium permanganate, manganese(II) sulfate, and oxalic acid in the volumetric flasks according to standard procedures.

B. General design of the task
The reactions are carried out at five different temperatures, always with the same concentrations of the reagents manganese(II) sulfate (0.012 mol dm^-3), oxalic acid (0.085 mol dm^-3), and potassium permanganate (0.002 mol dm^-3). The disproportionation progress is always followed photometrically at the wavelength of 405 nm by detecting the decrease of the complex ion concentration. The reactions are first conducted at a temperature between 20 and 25 oC. When changing to new conditions, the temperature is always increased by about 10 oC. Carefully fix the actual temperatures studied. This will be of importance in order to get reliable results when analyzing the experimental data.

C. Determination of the temperature dependence of the reaction rate
1. Prepare the mixture containing MnSO₄ and oxalic acid in the required concentrations in the volumetric flask.
2. Adjust the temperature of the thermostated cell unit and that of the thermostat to the required temperature. Place the flasks with the reaction mixture and potassium permanganate stock solution in the thermostat and let the mixture and the solution attain the desired temperature. Check the temperatures inside the flasks from time to time with the thermometer. Always carefully wash the thermometer with water before placing it in the next flask.
3. When ready with temperatures, add the required amount of potassium permanganate stock solution and mix well for 2 - 3 s. Transfer the mixture promptly to the photometer cell, place the cell into the cell holder and immediately start recording the absorbance.
Note. The thermostated cell holder is desirable, still can be omitted, since the reaction is completed within a short period of time. As a result, the temperature alterations will produce only slight effect on the results of kinetic studies.
4. Continue recording the absorbance till the reaction mixture turns colorless.
5. Carefully wash the cell with plenty of water, dry the cell and wipe the walls with clean tissue.
6. Repeat the steps 2-5 for the other four temperatures.