Chemistry of Oscillating Color-Changing Reactions

Andrew Aspaas and Levi Stanley

Augustana College

Advanced Inorganic Chemistry

December 1, 2000

Contents

• Introduction

• History

• Mechanisms– Briggs-Rauscher Reaction– Belousov-Zhabotinsky Reaction

• Demonstrations

• References

Introduction

• The Briggs-Rauscher and the Belousov-Zhabotinsky (BZ) reactions are two of the most commonly demonstrated oscillating reactions.

• The demonstrations are not difficult to put together, but the chemistry behind the color changes is often neglected.

Introduction

• Briggs-Rauscher reaction - colorless liquids are mixed to form an amber solution which turns a blue-black color, and cycles between the two colors.

• BZ reaction - the reaction mixture cycles through steps of green, blue, purple, and red colors.

The Chemists Behind the Reactions

• B.P. Belousov - Russian Chemist– 1950s - attempting to simulate

Krebs cycle– Discovered his reaction oscillated

– Submitted manuscript to a journal, rejected on

the grounds that "it's impossible"

– Conventional opinion of the time - all reactions

should go to thermodynamic equilibrium

smoothly

The Chemists Behind the Reactions

• Belousov was so offended by this rejection that he decided to leave science forever!

• Biochemistry professor S.E. Schnoll took an interest in Belousov's reaction

• Under Schnoll, A.M. Zhabotinsky investigated the mechanism and spatial distribution patterns of the reaction

The Chemists Behind the Reactions

• W.C. Bray and H. Leibhafsky, U.C. Berkeley– Described first homogeneous isothermal chemical

oscillator in 1921, ignored by scientific journals for many years

• T.S. Briggs and W.C. Rauscher - Galileo High School in San Francisco– Investigated Bray reaction at U.C. Berkeley– Combined parts of the Bray reaction with parts of

the BZ reaction to form their BR reaction

Briggs-Rauscher Reaction

• 3 solutions are mixed:– A: 4.0M H2O2

– B: 0.20M KIO3 and 0.077M H2SO4

– C: 0.15M malonic acid and 0.020M MnSO4

– Spiked with 1% starch solution

Briggs-Rauscher Reaction

• Non-radical reduction of iodate to hypoiodous acid:• If there is an abundance of I- ions:

– IO3- + I- + 2H+ HIO2 + HOI (1)

– HIO2 + I- + H+ 2 HOI (2)

– HOI + H2O2 I- + O2 +H+ + H2O (3)

• The net result of which is:

– IO3- + 2 H2O2 + H+ HOI + 2 O2 +2 H2O (4)

• Hypoiodous acid reacts with malonic acid:

– HOI + CH2(CO2H)2 ICH(CO2H)2 + H2O (5)

• As HOI is used up in rxn 5, the I- concentration from rxn 3 decreases, and rxns 1 and 2 slow greatly

Briggs-Rauscher Reaction• When I- concentration is low, the radical reduction of iodate takes

over, but has the same end result of the non-radical reduction

– IO3- + HIO2 + H+ 2 IO2• + H2O (6)

– IO2• + Mn2+ + H2O HIO2 + Mn(OH)2+ (7)

• Since more HIO2 is produced than consumed, the rate of the above steps increases as they occur, and therefore much more HOI is formed from rxn 8:

– 2 HIO2 IO3- + HOI + H+ (8)

• Mn is oxidized in rxn 7,providing a means for the reduction of IO2• to HIO2, and Mn is reduced in rxn 9:

– Mn(OH)2+ + H2O2 Mn2+ + H2O + HOO• (9)

– 2 HOO• H2O2 + O2 (10)

Briggs-Rauscher Reaction

• The HOI produced in the radical process reacts with malonic acid, as in rxn 5 of the non-radical process.

• But the production of HOI from the radical process is faster than its rxn with malonic acid, and an excess of HOI builds up

• This excess HOI reacts with H2O2 as in rxn 3 to produce I-

• The buildup of I- shuts off the radical process and returns the system to the slow, nonradical process we saw first

Briggs-Rauscher Reaction

• Where does the color change come from?– The reaction of HOI and malonic acid is actually a sequence:

– I- + HOI + H+ I2 + H2O (11)

– I2 + CH2(CO2H)2 ICH(CO2H)2 + H+ + I- (12)

• The amber color is the I2 produced in rxn 11– [HOI] > [I-] from radical process

• The dark blue color is from an I3- complex with starch

– When [I-] > [HOI] from rxn 3 with H2O2

– Causes slower non-radical process to take over

– Color fades as I2 is consumed by rxn 12

– Cycle repeats once radical process starts again

Briggs-Rauscher Reaction• A summation of this complicated process is

diagrammed below:

B-Z Reaction

• 3 Solutions are combined– A: 0.23M KBrO3

– B: 0.31M malonic acid and 0.059M KBr– C: 0.019M cerium(IV) ammonium nitrate

and 2.7M H2SO4

– Spiked with ferroin solution • 1,10-phenanthroline, ferrous sulfate

B-Z Reaction

• Belousov originally used citric acid instead of malonic acid

• It was later found that a number of different carboxylic acids could be used

• Also, manganese ions could be used instead of cerium ions

• This overall rxn is cerium-catalyzed oxidation of malonic acid by bromate ions in dilute sulfuric acid

B-Z Reaction

• The overall rxn is represented by this eqn:

– 3 CH2(CO2H)2 + 4 BrO3- 4 Br- + 9 CO2 + 6 H2O (1)

– But, this equation does not account for the color changes seen in the reaction, the catalytic effect of cerium, or the role of bromide ions

• Again, the mechanism involves 2 different processes

– Process A: ions and 2-electron transfers

– Process B: radicals and 1-electron transfers

• Concentration of Br- ions determines which process is dominant.

– Process A: High Br- concentration

– Process B: Low Br- concentration

B-Z Reaction• Process A net reaction:

– BrO3- + 5 Br- + 6 H+ 3 Br2 + 3 H2O (2)

– This is a reduction of bromate ions by bromide ions in 2 electron transfers

– This occurs when solutions A and B are mixed• This process occurs through these three steps:

– BrO3- + Br- +2 H+ HBrO2 + HOBr (3)

– HBrO2 + Br- + H+ 2 HOBr (4)

– HOBr +Br- +H+ Br2 + H2O (5)

B-Z Reaction

• Malonic acid exists mainly in the diacid form but slowly enolizes..

• The bromine reacts with the enol form of malonic acid:– Br2 + CH2(CO2H)2 BrCH(CO2H)2 + Br- + H+

(6)

• These reactions together reduce the concentration of Br- in the solution

C

H

2

O

O

H

O

H

O

C

H

O

H

O

H

O

H

O

B-Z Reaction

• Once process A has consumed most of the Br-, process B takes over

• The overall reaction of process B is represented by:

– 2 BrO3- + 12 H+ + 10 Ce3+ Br2 + 6 H2O 10 Ce4+ (7)

• This process is composed the following rxns:

– BrO3- + HBrO2 + H+ 2 BrO2• + H2O (8)

– BrO2• + Ce3+ + H+ HBrO2 + Ce4+ (9)

– 2 HBrO2 HOBr + BrO3- + H+ (10)

– 2 HOBr HBrO2 + Br- + H+ (11)

– HOBr + Br- + H+ Br2 + H2O (12)

B-Z Reaction

• The net result of eqn 8 + 2 (eqn 9) is:– 2 Ce3+ + BrO3

- + HBrO2 + 3 H+ 2 Ce4+ + H2O + 2HBrO2

(13)

• This sequence produces HBrO2 autocatalytically

• Autocatalysis is an essential feature of this reaction, but does not continue until the reactants are depleted, because there is a 2nd-order destruction of HBrO2 (rxn 10)

• Rxns 11 and 12 represent the disproportionation of hyperbromous acid (HOBr) to bromous acid (HBrO2) and Br2

B-Z Reaction

• Ce4+ and Br2 oxidize malonic acid to form bromide ions

• Once this concentration of Br- becomes high enough, process A is reactivated

B-Z Reaction

• Ferroin provides two of the colors seen in this reaction– As [Ce(IV)] increases, it oxidizes the iron in

ferroin from iron(II) (red) to iron(III) (blue)• Cerium(III) is colorless, and Ce (IV) is yellow

– The combination of Ce(IV) and Fe(III) make the green color

• The oscillating concentrations and oxidation states of Fe and Ce account for the color changes

References

• "B.P. Belousov: The Way To Discovery," Available: [online] 11/28/2000 http://www.musc.edu/~alievr/belous.html

• Shakhashiri, B.Z (1985), Chemical Demonstrations, 2: 232-261.

• Winfree, A.T. (1984), "The Prehistory of the Belousov-Zhabotinsky Oscillator," Journal of Chemical Education, 61: 661-663.