BZ Reaction - Oscillating Reaction - Physical Chemistry

Purpose

1.     To understand the mechanism of Belousov–Zhabotinsky reaction.

2.     To determine the apparent activation energy of the reaction by potentiometry.

  

Computer simulation of the Belousov–Zhabotinsky 

reaction occurring in a Petri dish (From Wikipedia)

B-Z reaction with indicator

Principles

  Belousov–Zhabotinsky reaction is a very complex reaction and is thought to involve about 18 different steps. So it is difficult to use simple way to describe the reaction. The FKN mechanism is usually introduced to simplify the problem on description of mechanism.

FKN Mechanism

(R1)     HOBr + Br- + H+ → Br2 + H2O

(R2)     HBrO2 + Br- + H+ → 2HOBr

(R3)     BrO3- +Br- +2H+ → HBrO2 + HOBr

(R4)     2HBrO2 → BrO3- + HOBr + H+

(R5)     BrO3- + HBrO2 + H+ → 2BrO2 + H2O

(R6)     BrO2 + Ce3+ + H+ → HBrO2 + Ce4+

(R7)     BrO2 + Ce4+ + H2O → BrO3- + Ce3+ + 2H+

(R8)     Br2 + MA → BrMA + Br- + H+

(R9)     6Ce4+ + MA + 2H2O → 6Ce3+ + HCOOH + 2CO2 + 6H+

(R10)   4Ce4+ + BrMA + 2H2O → Br- + 4Ce3+ + HCOOH + 2CO2 + 5H+

                When the concentration of [Br^-] is “higher” the main reaction path is R1-R2-R3. The total reaction equation can be represented as follow：

BrO₃^- +5Br^- +6H⁺ → 3Br₂ + 3H₂O

       The product ,Br₂ , is consumed through R8. The route, R1-R2-R3-R8, is called Chain A, and its total reaction equation can be written as follow：

BrO₃^- + 2Br^- + 3CH₂(COOH)₂ + 3H⁺ → 3BrCH(COOH)₂ + 3H₂O

       

When the concentration of [Br^-] is “lower” the main reaction path is R5-R6. The total reaction equation can be represented as follow：

2Ce³⁺ + BrO₃^- + HBrO₂ + 3H⁺ → 2Ce⁴⁺ + 2BrO₂ + H₂O

The product HBrO₂ could autocatalysis the reaction，but the concentration of HBrO₂

is restricted with R4. We call the path, R4-R5-R6, Chain B. Its total reaction equation：

BrO₃^- + 4Ce³⁺ + 5H⁺ → HOBr + 4Ce⁴⁺ + 2H₂O

Finally, the route R9-R10 is called Chain C. Its total reaction equation is as follow：

HOBr + 4Ce⁴⁺ + 3BrCH(COOH)₂ + H₂O → 2Br^-+ 4Ce³⁺ +3CO₂ + 6H⁺

       

After the analysis, we notice the concentration of [Br^-], [HBrO₂] and [Ce⁴⁺]/[ Ce³⁺] are periodically changing according to time. So we can use ion selective electrode to determine the concentration of bromine ion [Br^-], and use platinum electrode with SCE (standard calomel electrode) to determine the ration of [Ce⁴⁺]/[ Ce³⁺]. The apparent activation energy of the reaction can be determined by the measurement of the length of induction time at different temperature.

  

Chemicals

1.     Cerium ammonium nitrate solution: 0.02M

2.     Malonic acid solution: 0.5M

3.     Sulfuric acid: 0.8M

4.     Potassium bromate: 0.2M

Apparatus

1.     Computer

2.     HS-4 Thermostatic water bath

3.     Reactor

4.     Platinum electrode

5.     Standard Calomel Electrode

6.     Washing bottle

Procedure

1.     Turn on the computer, the recorder and the circulating water of the thermostatic water bath.

2.     Set up the reactor as the following picture:

3.     Set the temperature of the circulating water at 20.00℃. Add 7mL of malonic acid solution,15 mL of potassium bromate solution, 18 mL sulfuric acid solution in the clean reactor. Turn on the stirrer and put the electrodes in the reacting mixture. After the read of the potential is stable, add 2 mL of cerium ammonium nitrate solution in to the reactant.

4.     Observe the color changing. After the oscillations appear 6~8 times, stop recording and save the data. Raise up the temperature of the circulating water for 3℃ and repeat the step 3~step 4 until finish records.

Experimental Record

Raw Data

Figure 1. 20.00℃

Figure 2. 23.00℃

Figure 3. 26.00℃

Figure 4. 29.00℃

Data Process

                        Table 1. Data Process

Reaction T(K)	Induction Time t(s)	Oscillating Period t’(s)
293.15	625.02	123.69
296.15	493.17	79.80
299.15	407.08	73.80
302.15	330.67	54.21

                Draw the diagrams of ln(1/t_induction)-1/T and ln(1/t_oscillating)-1/T and do linear fit to find the slopes. And then find the E_induction、E_oscillating from the slopes.

Figure 5. ln(1/t_induction)-1/T

Figure 6. ln(1/t_oscillating)-1/T

                According to Arrhenius equation, the apparent activation energy can be deduced：

                        

Table 2. Apparent Activation Energy

Slopeinduction	Slopeoscillating	Apparent Ea (induction) (kJ/mol )	Apparent Ea (oscillating) (kJ/mol )
-6207	-7545	51.605	62.729

References

[1]  傅献彩, 沈文霞, 姚天扬. 物理化学, 上册欧4 版. 北京:高等教育出版社, 1990:144.

[2]  清华大学化学系物理化学实验编写组. 物理化学实验. 北京：清华大学出版社, 1991.

[3]  Robert C. Wcast Handbook of Chemistry and Physics. Physics. 58^th ed. Ohio: CRC Press, 1977.

[4]  朱文涛. 物理化学. 北京：清华大学出版社，1995.

Acid Catalyzed Iodination of Acetone - Physical Chemistry - Lu Le Laboratory

Purpose

1.     To determine the order of the reaction of iodine-acetone.

2.     To determine the rate constant at an assigned temperature

  

Principles

        Acid catalyzed iodination of acetone is a complex reaction. The rate law for overall reaction cannot be determined from the balanced equation for the reaction but from experiments.

        When an aqueous iodine solution is reacted with acetone in the prescence of an acid, the yellow color slowly fades as the iodine, I₂, is consumed. The products of the reaction are iodoacetone and hydrogen iodide. Hydrogen ion is a catalyst for this reaction. The mechanism of the reaction is as follow:

Hydrogen ion participate in the reaction as a catalyst in step on and step two and also be produced as a product in step three. This kind of reaction is called an autocatalysis reaction. The rate equation can be represented as follow：

               

  

  The reaction progress can be tracked by the determination of the concentration of iodine and triiodide ion.

  For the reaction, the K^- =700 and the absorbance of the solution can be represented as follow： 

A = A(I₃^-)+A(I₂) = ε(I₃^-)L[I₃^-] +ε(I₂)L[I₂]

When we set the wavelength of the light source of the spectrometer at 565nm，the molar absorbance of I₂ and I₃^- is equal: ε(I₃^-) = ε(I₂)：

Absorbance = ε(I₃^-)L[I₃^- + I₂]

    Since the concentration of acetone and hydrochloric acid is much larger than the concentration of iodine/triiodide ion so we can assume the concentration of acetone and acid as a constant at the beginning of the reaction:

r = k[A]^α[I₃^-]^β[H⁺]^δ = k[A]^α[H⁺]^δ = constant

    Finally, we can figure out the reaction order and the activation energy of the reaction from Arrhenius equation:

                                                               

                           

Chemicals

1.     Iodine/KI solution (standardized): 0.02134M

2.     Acetone aqueous: 3.3738M

3.     Hydrochloric acid: 1.436M

4.     Distilled water

Apparatus

1.     Computer

2.     722S Spectrometer

3.     Cuvette

4.     Thermostatic water bath

5.     Pipette

6.     Dropper

7.     Washing bottle

Procedure

1.     Calibration the spectrometer with distilled water before use.

2.     Turn on the thermostatic water bath and set the temperature at 25℃.

Put the vessels with distilled water, acetone aqueous, hydrochloric acid, iodine solution in the water bath for at least 10 minutes.

3.     Measurement the εL value of iodine solution:

    Turn on the 722S spectrometer and warm it up for at least 10 minutes. Put the cuvette with d.d. water into the spectrometer as a blank. Pour 25.00mL iodine  solution into a 25mL volumetric flask and dilute to the mark line with distilled water. Rinse the cuvette with the solution for twice, and add the solution to the two-third full of the cuvette, and determine the absorbance with the spectrometer.

4.     Mix the reactants in a volumetric flask as follow and dilute to the mark line then put into the spectrometer:

Sample	Iodine solution (mL)	Acetone aqueous (mL)	Hydrochloric acid(mL)
1	5.00	5.00	5.00
2	5.00	2.50	5.00
3	5.00	5.00	2.50
4	7.50	5.00	5.00
5	7.50	5.00	5.00

Experimental Record

Table 1. Concentration of Reagents

Reagent	Concentration
Iodine/KI solution	0.02134 M
Acetone aqueous	3.3738  M
Hydrochloric acid	1.436   M

Table 2. The εL value

The εL value of the diluted iodine solution (λ=565nm)

0.3636

Data Process

                First, do linear fitting for each diagram.

Take 30~300s for sample 1.

Take 30~300s for sample 2.

Take 30~300s for sample 3.

Take 30~300s for sample 4.

        Take 30~150s for sample 5.

        Second, figure out the absorbance constant of iodine solution by the equation as follow:

Abs. x constant = Concentration

=> Abs.=0.3636, concentration = (0.02134*2.5/25)=0.002134M

=> constant= 5.869x10^-3_.

        Third, multiply the constant with each slopes which we get from the figure above and the rates of reactions are as follow:

  

Table 3. Rate of Reaction

Sample	rate
1	-4.4796x10-6
2	-2.1737 x10-6
3	-2.4358 x10-6
4	-4.3071 x10-6
5	-9.1560 x10-6

        The reaction orders can be found by the calculations as follow:

                                                   

=>   β= 1.0420

=>   δ= 0.8781

=>   α= -0.0953

        The rate equation can be represented as follow:

        Then the reaction constant k can also be found:

Table 4. Reaction constant k

Sample	k
1	1.199X10-5
2	1.199X10-5
3	1.199X10-5
4	1.199X10-5
5	2.549X10-5

        Finally the activation energy can be fiqure out from Arrhenius equation:

References

[1]  傅献彩, 沈文霞, 姚天扬. 物理化学, 上册欧4 版. 北京:高等教育出版社, 1990:144.

[2]  清华大学化学系物理化学实验编写组. 物理化学实验. 北京：清华大学出版社, 1991.

[3]  Robert C. Wcast Handbook of Chemistry and Physics. Physics. 58^th ed. Ohio: CRC Press, 1977.

[4]  朱文涛. 物理化学. 北京：清华大学出版社，1995.