### PREPARATORY PROBLEM 31 (PRACTICAL) **Oscillating reactions** #### Introduction In 1921 W. Bray p — Analytical Chemistry Chemistry Question
Oscillating reactions
### PREPARATORY PROBLEM 31 (PRACTICAL)
Oscillating reactions
#### Introduction
In 1921 W. Bray published an article describing the oscillating reaction of oxidation of hydrogen peroxide with potassium iodate. However thorough investigation of oscillating reaction mechanisms has begun only in 1951, when B.P. Belousov discovered oscillations of concentrations of reduced and oxidized forms of cerium catalyzing oxidation of citric acid by bromate-ion. Later it was shown that oscillating reactions are possible in other redox systems. A.M. Zhabotinsky investigated the oxidation of malonic acid by bromate-ion in the presence of manganese ions. This reaction mechanism is very sophisticated and includes dozens of intermediate compounds.
We will investigate an oscillating reaction taking place in the malonic acid-iodate ion system in the presence of manganese salt and hydrogen peroxide.
#### Reagents and equipment
* KIO3 reagent grade, solid,
* 40 % H2O2,
* conc. H2SO4,
* C3H4O4, malonic acid,
* MnSO4·5 H2O,
* starch,
* KI, solution,
* AgNO3, solution,
* analytical balance,
* weighing dishes,
* flat-bottom flasks or beakers (250 - 500 cm3), 4 pcs,
* stop-watch.
#### Procedure
Prepare three solutions (may be prepared in advance):
1) solution of 80 cm3 40 % H2O2 in 120 cm3 of water,
2) solution of 8.7 g KIO3 and 0.9 cm3 conc. H2SO4 in 190 cm3 of water,
3) solution of 3 g C3H4O4, 2.4 g MnSO4 · 5 H2O and 0.06 g starch in 195 cm3 of water.
Mix the solutions in the same vessel and observe the oscillating process. Evaluate the oscillation period and its change in time.
Divide the mixture into two parts and place them into beakers.
To one of the parts add AgNO3 solution (first – several drops, then ~3 cm3). Observe changes of the oscillation period. Note the color of the solution upon completion of the oscillation reaction.
To the other part add KI solution (several drops). Observe changes of the oscillation period.
Oxidation of malonic acid by potassium iodate is an autocatalytic process. Write down the net equation of the reaction. Which product is the catalyst of the oscillating process? Explain the effect of silver nitrate.
Model Answer
The reaction mechanism is very complex and consists of many steps and parallel ways. The net equation of the reaction is
KIO3 + CH2(COOH)2 → KI + HCOOH + 2 CO2 + H2O, (1)
The steps are:
IO3– + 5 I– + 6 H+ → 3 I2 + 3 H2O (2)
I2 + 5 H2O2 → 2 HIO3 + 4 H2O (3)
Thus, iodine derivatives are the catalysts of the oscillating process.
The addition of AgNO3 eliminates I– ion from reaction, so oscillations become slower and then stop.
B. P. Belousov used bromate-ion as an oxidizing agent. Suggest what would happen if we substitute iodate-ion by bromate-ion in the reaction with malonic acid. What role does hydrogen peroxide play in the oxidation of malonic acid with iodate-ion?
Model Answer
BrO3– is a stronger oxidizing agent than IO3–, so oscillation frequency will increase and visual observation would become more difficult. H2O2 oxidizes I2 to IO3– ion.
It is well known, that one of the stages of the oscillating process is formation of iodomalonic acid with its subsequent decomposition. How can we explain the fact that potassium iodide inhibits the reaction?
Model Answer
I– ion interacts with one of the reagents according to equation (2), so adding I– will decrease the oscillation frequency and increase the oscillation period.
B. P. Belousov used the Ce4+/Ce3+ redox couple to study oscillating reactions. Is it possible to use the following transient metal redox couples as a catalyst: Co3+/Co2+, Fe3+/Fe2+, Tl3+/Tl1+?
Eo(Co3+/Co2) = 1.81 V, Eo(Ce4+/Ce3+) = 1.61 V,
Eo(Mn3+/Mn2+) = 1.51 V, Eo(Fe3+/Fe2+) = 0.77 V?
Model Answer
Transition metal ions participate in oscillation reaction:
10 Mn2+ + 2 IO3– + 12 H+ → I2 + 10 Mn3+ + 6 H2O (4)
and next
6 Mn3+ + CH2(COOH)2 + 2 H2O → 6 Mn3+ + HCOOH + 2 CO2 + 6 H+ (5)
or
4 Mn3+ + ICH(COOH)2 + 2 H2O → 4 Mn3+ + HCOOH + 2 CO2 + 5 H+ + I– (6).
Co2+ is not oxidized by iodate-ion, Fe3+ is not a strong oxidizing agent and does not oxidize malonic acid. Redox process Tl3+ → Tl+ involves two-electron transfer so it is very slow.