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.