3.2 REDOX REACTIONS

• Oxidation Number Rules

  
• What is a redox reaction?

  an oxidation reaction paired with a reduction reaction

  
• Example of reduction: Fe(III) salts to Fe(II) salts [1]


  Fe3+ + e- -> Fe2+
• Example of oxidising agents being reduced: iodine to iodide ions [+COLOUR CHANGE]

  I2 + 2e- -> 2I-

  BROWN TO COLOURLESS
• Example of oxidation: sulfites to sulfates


  SO3^2- + H2O -> SO4^2- + 2H+ + 2e-
• Example of an oxidising agent being reduced: Potassium Manganate (VII) reduced to Manganese salt in acid solution [+COLOUR CHANGE]


  MnO4- + 8H+ + 5e- -> Mn2+ + 4H2O

  PURPLE TO COLOURLESS
• Example of an oxidising agent being reduced: Potassium dichromate (VI) reduced to Chromium salt in acid solution [+COLOUR CHANGE]


  Cr2O7^2- + 14H+ + 6e- -> 2Cr3+ + 7H2O

  ORANGE TO GREEN
• Order how to balance charge in redox equation [3]

  1. H2O

  2. H+

  3. e-
• Redox titration of aqueous thiosulfate ions (S2O3^2-) and aqueous Iodine (I2) [6 + EQN]

  -Aqueous S2O3 2- ions are oxidised by aqueous I2.

  -The S2O3 2- solution is placed in the burette

  -The I2 is pipetted into the conical flask.

  -The S2O3 2- solution is run into the flask until the colour, due to I2, fades to a pale-yellow colour.

  -At this point starch solution is added as an indicator, turning the mixture dark blue.

  -The end point is when the blue colour is decolourised.

  
  

  I2(aq) + 2S2O3 2-(aq) → S4O6 2-(aq) + 2I- (aq)
• Redox titration of aqueous dichromate ions and iron(III) ions [3+EQN]

  -Aqueous Cr2O7 2- is placed in the burette

  -added to a conical flask where it oxidises aqueous Fe2+.

  -An indicator is required.

  Cr2O7 2-(aq) + 14H+ (aq) + 6Fe2+(aq) ⇌ 2Cr3+(aq) + 7H2O(l) + 6Fe3+(aq)
• Redox Titration of aqueous manganate (VII) ions and iron(II) ions [5+EQN]

  -The potassium manganate(VII) solution must be standardised by titration with a standard solution of a reducing agent like iron(II) ammonium sulfate.

  -The aqueous MnO4 - is placed in the burette

  - Fe2+ solution pipetted into the conical flask with an excess of aqueous sulfuric acid.

  -requires no indicator.

  -As soon as all the Fe2+ has been oxidised, the next drop of MnO4 gives a pink colour

  MnO4 - (aq) + 8H+ (aq) + 5Fe2+(aq) ⇌ Mn2+(aq) + 4H2O(l) + 5Fe2+(aq)
• Redox titration of aqueous iodide ions and aqueous copper (II) ions [3+EQN]


  -The blue solution loses its colour.

  -A white precipitate of CuI forms along with a brown solution of I2.

  -The I2 can then be titrated with S2O3 2-

  2Cu2+(aq) + 4I- (aq) → 2CuI(s) + I2(aq)
• Why isn't the interconversion of dichromate ions (Cr2O7^2-) and chromate (VI) ions (CrO4^2-) a redox reaction?[3]


  -Chromium oxidation number unchanged

  -Cr2O7 2-(aq) + OH- (aq) ⇌ 2CrO4 2- (aq) + H+ (aq)

  -ORANGE TO YELLOW