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Chemistry LibreTexts

The Blue/Amber/Colorless Oscillating Reaction

Chemical Concepts Demonstrated

  • Reversible reactions
  • Equilibrium
  • Complex reaction mechanisms

Demonstration

Three solutions are combined in alphabetical order.
  • Solution A: 30% H2O2 diluted in water.
  • Solution B: KIO3 dissolved in water, H2SO4 added, and diluted.
  • Solution C: Malonic acid and MnSO4 * H2O dissolved in water, starch solution added, and diluted.
beaker.gif

Observations

The resulting solution turns fron amber to blue-black.  The blue slowly fades to colorless, and the reaction begins again.  During the course of the reaction, bubbles are evolved.

Explanations (including important chemical equations)

Although the complete mechanism is uncertain, the reaction is believed to be an indirect route to decomposing H2O2into H2O and O2, related to the H2O2/IO3- oscillating reaction.  If so, the amber color results from the production of Iin the absence of I -, while the blue color results from the simultaneous presence of both Iand I - leading to the blue starch-I3- complex.  The bubbles are oxygen gas.  The following steps have been proposed:

5 H2O(aq) + I(aq) --> 2 HIO3 (aq) + 4 H2O (l)

5 H2O(aq) + 2 HIO3 (aq) --> 5 O2 (g) + I(aq) + 6 H2O (l)

The functions of the malonic acid and the manganese ion may be to increase the size and frequency of the Iand I -fluctuations.

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