Introduction

Three white solids are added to a colorless stirred solution. The solution turns orange and after about 75 seconds, colorless. The oscillation continues with an initial period of about 20 seconds at 20 oC and continues for approximately 10 minutes.

The overall reaction is usually given as:

3CH2(COOH)2(aq) + 4BrO3–(aq) → 4Br–(aq) + 9CO2(g) + 6H2O(l)

The oxidation of the malonic acid by the bromate(V) ions is catalyzed by manganese(II) ions, and manganese(III) ions are assumed to be produced as intermediates. However, detectable levels of Mn(III) have not been detected in the reaction mixture.1

Some references claim that the red color is due to molecular bromine which could be produced via the following two steps:

Br–(aq) + BrO3–(aq) + 2H+(aq)  + 4 Mn(II)  →  HOBr(aq)  + 4 Mn(III)(aq)  +  2H2O

Br–(aq) + HOBr(aq) + H+(aq) → Br2(aq) + H2O(l)

However, other detailed studies of the processes occurring give a variety of colorless bromate ions and bromic acid molecules as intermediates, rather than bromine itself, so it is therefore possible that the red color is due to something else, maybe the transient existence of Mn3+ions which are known to be red/purple in color.

The HOBr reacts with malonic acid to from bromomalonic acid:

CH2(CO2H)2  +  HOBr  --à  BrCH(CO2H)2  +  H2O

Bromomalonic acid is oxidized by Mn(III) produced in the previous reaction, releasing bromide ions:

BrCH(CO2H)2  +  4Mn(III)  +  2H2O  --à  Br-  +  HCO2H  + CO2   + 4 Mn(II)  + 5 H+

To Conduct Demonstration

  1. Stir the sulfuric acid solution and add 9 g. malonic acid. Once this has dissolved add 8 g. potassium bromate. Finally add 1.8 g manganese(II)sulfate monohydrate. (0.1M malonic acid, 0.06M BrO3-, 0.013 Mn2+, and 1.6 M H2SO4)

Notes: Chloride ion contamination

Chloride ions, via the addition of a pinch of potassium chloride or dilute hydrochloric acid, will immediately stop the oscillations. The use of clean apparatus is therefore essential. The reaction will not work if chlorinated tap water is used.

Safety and

Malonic acid is a strong irritant. Sulfuric acid is a strong acid and powerful dehydrating reagent and can cause burns. Neutralize spills with NaHCO3. Bromates are strong oxidizing agents. When mixed with finely divided organic materials, metals or other combustible materials are easily ignited, sometimes explosively.

Reference:

  1.  B.Z. Shakhashiri, Chemical Demonstrations:  A Handbook for Teachers of Chemistry, Vol. 2, pp. 273-275, 1983.
  2. C.S. Reddy, Indian Journal of Chemistry, Vol 46A, pp.1737-1746,  2007.