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7. Homogeneous catalysts
7.2 The reaction between hydrogen peroxide and 2,3-dihydroxybutanedioate ions
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Hydrogen peroxide oxidises the 2,3-dihydroxybutanedioate ions in potassium sodium 2,3–dihydroxybutanedioate (also known as potassium sodium tartrate or Rochelle salt) to carbon dioxide, methanoate ions and water.
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Speeding up the reaction
The reaction is slow, even when heated but it can be speeded up using cobalt(II) ions. The catalysed mixture starts off pink due to the presence of cobalt(II) ions. During the reaction the mixture turns green, indicating the presence of cobalt(III) ions. When the reaction is complete the mixture returns to a pink colour.

Reaction flow chart
2,3-dihydroxybutanedioate
Hydrogen peroxide
Co2+
arrow Co2+ reduces
hydrogen peroxide
and gets oxidised to
Co3+
Intermediates
Co3+
arrow Co3+ oxidises
2,3-dihydroxybutanedioate
and gets reduced back to
Co2+
Carbon dioxide
methanoate
water
Co2+
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Question 27

If you carried out this reaction in a beaker you would see the colour changes described above. What else would you see during the reaction?