6 REDOX REACTIONS OF TRANSITION METALS

• What is the process of oxidation of Fe2+ to Fe3+ called?

  Redox titration
• In a redox titration, what is being titrated against what?

  The oxidising agent is being titrated against a reducing agent
• What happens during a redox reaction?

  Electrons are transferred from one species to another
• Why are indicators not necessary in redox titrations?

  Redox titrations using transition metal ions naturally change colour when changing oxidation state
• What is an example of a redox titration involving iron and manganate ions?

  • Oxidation: Fe2+ → Fe3+
  • Reduction: MnO4- → Mn2+
  • Reaction: MnO4- (aq) + 8H+ (aq) + 5Fe2+ → Mn2+ (aq) + 5Fe3+ + 4H2O (aq)
  • Colour change: purple to colourless
• What colour is potassium manganate(VII) in solution?

  Purple
• What colour change occurs when Fe3+ ions are reduced to Fe2+ ions?

  From orange-brown to pale green
• What is the reaction for the reduction of Fe3+ to Fe2+ using potassium iodide?

  • Reduction: Fe3+ → Fe2+
  • Oxidation: I- → I2
  • Reaction: 2Fe3+ (aq) + 2I- (aq) → 2Fe2+ + I2 (aq)
  • Colour change: orange-brown to pale-green and formation of brown iodine
• What colour are aqueous dichromate (VI) ions?

  Orange
• What colour are aqueous chromium (III) ions?

  Green
• How can dichromate (VI) ions be reduced to chromium (III) ions?

  By the addition of zinc
• What is the reaction for the reduction of Cr2O72- to Cr3+ using zinc?

  • Reaction: Cr2O72- (aq) + 14H+ (aq) + 3Zn (s) → 2Cr3+ (aq) + 7H2O (l) + 3Zn2+ (aq)
  • Colour change: orange to green
• What happens to chromium(III) ions in excess zinc?

  They are reduced to chromium(II) ions, which are pale blue
• What is the reaction for the oxidation of chromium(III) to chromium(VI) using hydrogen peroxide?

  • Reaction: 3H2O2 (aq) + 2Cr3+ (aq) + 10OH- (aq) → 2CrO42- (aq) + 8H2O (l)
  • Colour change: dark green to yellow
• What is the oxidation state change of chromium in the reaction from Cr3+ to CrO42-?

  From +3 in Cr3+ to +6 in CrO42-
• What is the reaction for the conversion of chromate (VI) to dichromate (VI)?

  • Reaction: 2CrO42- (aq) + 2H+ (aq) → Cr2O72- (aq) + H2O (l)
  • Colour change: yellow to orange
• What colour is copper(II) ions in solution?

  Pale blue
• What happens when Cu2+ ions are reduced to Cu+ ions?

  They form a white precipitate of CuI
• What is the reaction for the reduction of Cu2+ to Cu+ using iodide ions?

  • Reaction: 2Cu2+ (aq) + 4I- (aq) → 2CuI (s) + I2 (aq)
  • Colour change: pale blue to white precipitate and formation of brown iodine
• What occurs during the disproportionation of copper(I) ions?

  Copper(I) ions are both oxidized and reduced
• What is the reaction for the disproportionation of copper(I) oxide with dilute sulfuric acid?

  • Reaction: Cu2O (s) + H2SO4 (aq) → Cu (s) + CuSO4 (aq) + H2O (l)
  • Colour change: from brown copper(I) oxide to blue copper(II) sulfate
• What is the oxidation state change of copper in the reaction from Cu2O to Cu?

  From +1 in Cu2O to 0 in Cu
• What is the oxidation state change of copper in the reaction from Cu2O to CuSO4?

  From +1 in Cu2O to +2 in CuSO4