10.Calculate the rate constant using the equation (31), which is modified in accordance with the experimental relation between the resistance and the concentration of the solution

compare this value with the average value of the rate constant (kaver.) calculated by the equation (32).

12. Calculate the relative error in the determination of the constants from

the equation ∆k = ∆t + 2∆Rs k t Rs

Resistance is measured with an accuracy of 1%, t = 1 min.

4.2.TEST QUESTIONS

1.What kind of processes is called heterogeneous?

2.From what stages comprise heterogeneous reactions?

3.What is a limiting stage?

4.What is the difference between reactions occurring in the diffusion and kinetic areas?

5.What is the diffusion process?

6.What is the First Fick 's law?

7.Which conditions hold the Second Fick's law?

8.What is the steady-state diffusion?

9.Write Shchukarev`s equation.

10.What is the diffusion layer?

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11.How changes the character of diffusion with stirring?

12.How is it possible to determine experimentally the rate of diffusion?

13.Which procedure should be expected for reactions in the diffusion limitations?

14.How affects the temperature on the diffusion processes?

15.What factors effect on the rate of dissolution of solids?

LABORATORY EXERCISE 10.

KINETICS AND CATALYSIS OF HYDROGEN PEROXIDE

DECOMPOSITION

Purpose. In this lab you will study a rate of the hydrogen peroxide decomposition at the presence of catalyst.

Background. Hydrogen peroxide in aqueous solution spontaneously decomposes slowly according to the equation

In the presence of cations and anions of certain organic compounds, as well as a number of solid substances (glass, metals, carbon, salts, metal oxides) hydrogen peroxide decomposition is greatly accelerated. Depending on the catalyst taken for the reaction can be homogeneous or heterogeneous catalytic reaction.

Measurement of the rate of hydrogen peroxide decomposition is based on determination of the amount of oxygen liberated in the reaction. It is necessary to take into account that the total volume of oxygen evolved during the reaction (V∞) corresponds to the amount of hydrogen peroxide having at the beginning of the reaction (c0), while the difference between the total volume of evolved oxygen and its volume at the moment (V∞–Vt) corresponds to the amount of hydrogen peroxide (c) that is still undecomposed at this moment of time t. In order to calculate the rate constants of the first order reaction we replace the concentration ratio c0/c in the eq.(4.6) by the proportional values of V∞ and (V∞ – Vt) and obtain

The V∞ value (volume of overall the oxygen released during the decomposition) can be found by determination of the hydrogen peroxide concentration in the initial solution using titration method or gasometric

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method by fully decomposing of hydrogen peroxide. The choice of method is carried out as directed by the teacher.

In this lab, the process of decomposition of the Н2О2 with given concentration are studied in the presence of activated carbon or other solid catalyst. For this purpose we use gasometric method which allows to control with sufficient accuracy the reaction rate of hydrogen peroxide decomposition. Amount of the Н2О2 decomposed at a given moment is proportional to the volume of the evolved oxygen. The equipment setup for measurements of the oxygen volume is shown in Fig. 4.5. The equipment consists of a reaction vessel (1), a gas burette (2) with a valve (3), and the equalizing vessel (4).

Certain parts of the equipment are interconnected by rubber tubes. Burette and the equalizing vessel are filled with water. The volume of oxygen is determined by the volume of water displaced from the burette into the equalizing vessel at equal liquid levels in both of them. Equal levels are achieved by moving the vessel (4) vertically that provides equal pressure from atmospheric within the system.

Fig. 4.5. The experimental setup for the study of the catalytic decomposition of hydrogen peroxide

Procedure

1.Prepare a sample of the hydrogen peroxide solution by diluting (as directed by the teacher) of 1-3 mL concentrated hydrogen peroxide solution (perhydrol) with water in a volumetric flask to 50 mL and mix it thoroughly.

2.Determination of the oxygen volume released at full decomposition of the peroxide solution (V∞) by volumetric titration. In three conical flask

add 20 mL of the 0.15 M sulfuric acid solution. Pipette 2 mL of the hydrogen peroxide solution. Add 2-3 drops of mixture of the MnSO4 in

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concentrated H2SO4 (to accelerate the reaction). First mix gently and then titrate each sample by the 0.02 M KMnO4 solution until the pink color not disappearing within 1-2 minutes. Find the arithmetic average of the three values of the potassium permanganate volume. Stoichiometric equation for the reaction is

Thus, it follows that 1 mol of KMnO4 corresponds to 2.5 moles of H2O2. Calculation of the V∞ value obtained by titration method is given in

paragraph 6.

3.Install the equipment (Fig. 4.5.). Place weighed (1 tablet) of activated carbon in offshoot of clean and dry reaction vessel (marked by 1 on the Fig.5). When using manganese dioxide as a catalyst it is necessary to take a small amount of catalyst (about 0.1 g or spatula tip per 1 mL of the perhydrol). Carefully pour 40 mL of prepared hydrogen peroxide solution into the reactor using a special funnel avoiding contact with the catalyst. Close the reaction vessel by the plug with inserted connecting tube.

4.Check the system for leaks. Open the valve 3, lift the vessel 4, and equalize the liquid meniscus in the vessel 4 with the liquid meniscus in the measuring burette 2 at the “zero” level. Close the valve 3 and move down the equalizing vessel 4 to quarter of the burettes height. If there is adequate tightness, the steady level moves down a bit and stop. In the case of leakage system, check tightness of individual parts of the equipment, check the integrity of the rubber tubes, and grease valve 3 by vacuum lubricant.

5.To initiate the reaction, without separating of the equipment units, tilt the reactor so that the coal tablet (catalyst) slipped into solution. The time of contact between catalyst and solution is considered as the beginning of the reaction. After 3-5 minutes make a first measurement of the evolved oxygen volume (at the time of the measurement liquid levels in the burette 2 and in the vessel 4 must be equalized). Measure a liquid level

in the burette every 5-10 minutes. More the rate of the gas, more often make the measurements. With further lowering of the liquid level in the burette continue alignment of liquid levels in both legs of the instrument; experiment should be completed when the amount of evolved oxygen will be slightly more than a half of the volume that can be released at full decomposition of H2O2. If the capacity of the gas burette is insufficient, it is necessary to pinch the hose connecting the reaction vessel 1 with a gas

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burette 2, and quickly (lifting vessel 4) displace oxygen through valve 3 into the atmosphere. Then set the level at zero, close the valve and continue measurements summing the results with previous ones. The measured volume values during the reaction correspond to the Vt' values in the eq. (36). With the influence of the water vapor pressure the volume of oxygen is equal to

pressure, mm Hg (see appendix).

Determination of the oxygen volume released at full decomposition of the peroxide solution (V∞) by gasometric technique. At the end of the experiment put the vessel 1 with a reaction mixture in a cup with hot water (70 ºC) without disassembling of the equipment. Higher temperatures accelerate decomposition of hydrogen peroxide. After completion of the reaction (when oxygen stops stand out) remove the vessel from the cup with hot water, let it cool to room temperature. Measure the liquid level in the burette. V∞ must be corrected with the influence of the water vapor pressure (36).

6. To determine the V∞ value by the titration method we calculate the concentration of the solution on the base of equation

where VKMnO4 is the volume of potassium permanganate with a concentration of СKMnO4 spent on the titration of the hydrogen peroxide solution, VH2O2 is the sample volume of the hydrogen peroxide solution

separated for the titration.

Number of the H2O2 moles in the solution used for the reaction is given by equation

where Vs is volume of the solution in the reaction vessel.

On the base of the stoichiometric equation we find the amount of oxygen released during the decomposition of the full amount of the taken H2O2

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The oxygen volume released at full decomposition of the peroxide solution (V∞) with volume Vs, can be calculated by the MendeleevClapeyron equation

O2

where nO2 is a number of the oxygen moles;

R = 0.082 L∙atm/deg∙mol;

PO2 = 1 atm;

Т is temperature (К).

Equations 37, 38, 39, and 40 can be combined into one

7.Record results of the experiment in the following form. Temperature of the experiment is ______º C;

The volume of solution taken for the reaction Vs = 40 mL; Barometric pressure Pb is ______________ mmHg;

The sample volume (hydrogen peroxide solution) taken for titration is 2 mL;

Potassium permanganate solution concentration is 0.02 M;

Volume of potassium permanganate spent for the titration, is ________

mL;

Catalyst is _______________________________________.

Volume of oxygen corresponding to complete decomposition of hydrogen peroxide (V∞) is _________mL.

8.Fill the Table 2 on the base of the data obtained. Calculate the rate constant of the decomposition of hydrogen peroxide according to the

equation (34) for each report and find the average value of kaver..

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