Chemistry of Oscillating Color-Changing Reactions

1. Chemistry of Oscillating Color-Changing Reactions Andrew Aspaas and Levi Stanley Augustana College Advanced Inorganic Chemistry December 1, 2000

2. Contents • Introduction • History • Mechanisms • Briggs-Rauscher Reaction • Belousov-Zhabotinsky Reaction • Demonstrations • References

3. 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.

4. 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.

5. 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

6. 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

7. 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

8. 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

9. 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

10. 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)

11. 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

12. 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

13. Briggs-Rauscher Reaction • A summation of this complicated process is diagrammed below:

14. 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

15. 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

16. 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

17. 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)

18. 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

19. 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)

20. 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

21. 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

22. 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

23. 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.