WJEC Chemistry 3.2 Redox Reactions

    Cards (10)

    • Potassium Dichromate (VI) and Potassium Manganate (VII) are oxidising agents.
    • Thiosulfate S2O3^2- is a reducing agent and donates electrons to become oxidised.
    • Ion-electron half equations:
      Acidified manganate (VII) ions-
      MnO4 – (aq) + 8H+ (aq) + 5e– ⇌ Mn2+(aq) + 4H2O(l)
      Colour change: purple to colourless
    • Ion-electron half equations:
      Acidified dichromate ions-
      Cr2O7 2–(aq) + 14H+ (aq) + 6e– ⇌ 2Cr3+(aq) + 7H2O(l)
      Colour change: orange to green
    • Ion-electron half equations:
      Iodine to Iodide ions-
      2(aq) + 2e– ⇌ 2I– (aq)
      Colour change: brown to colourless
    • Ion-electron half equations:
      Thiosulfate ions-
      2S2O3 2–(aq) ⇌ S4O6 2–(aq) + 2e–
    • Redox titration-Aqueous thiosulfate ions and aqueous iodine:
      1. Thiosulfate solution placed in the burette.
      2. Aqueous iodine pipetted into the conical flask.
      3. The S2O3 2– solution is run into the flask until the colour fades to a pale-yellow.
      4. Starch solution is added as an indicator, turning the mixture dark blue. The end point is when the blue colour is decolourised.
      2(aq) + 2S2O3 2–(aq) → S4O6 2–(aq) + 2I– (aq)
    • Redox titration-Aqueous iodide ions and aqueous copper(II) ions:
      1. The blue solution loses its colour.
      2. A white precipitate of CuI forms along with a brown solution of I2.
      3. The I2 can then be titrated with S2O3 2–.
      2Cu2+(aq) + 4I– (aq) → 2CuI(s) + I2(aq)
    • Redox titration-Aqueous dichromate ions and iron(II) ions:
      1. Aqueous Cr2O7 2– is placed in the burette and added to a conical flask where it oxidises aqueous Fe2+.
      2. An indicator is required.
      3. The colour change is orange to yellow.
    • Redox titration-Aqueous manganate(VII) ions and iron(II) ions:
      1. Potassium manganate(VII) solution must be standardised by titration with a reducing agent such as iron(II) ammonium sulfate.
      2. The aqueous MnO4 – is placed in the burette.
      3. Fe2+ solution pipetted into the conical flask with an excess of aqueous sulfuric acid.
      4. Keep adding Fe2+ until the solution turns a permanent pink colour.