24.5 Redox and qualitative analysis

• Transition metals can form ions with different oxidation states
• Fe2+ ions are oxidised to Fe3+ ions by acidified manganate ions

  MnO4−​ + 5Fe2+ + 8H+ → Mn2+ + 5Fe3+ + 4H2​O

  solution changes from purple to colourless
• Fe3+ ions are reduced to Fe2+ ions by iodide ions

  2Fe3+ + 2I− → 2Fe2+ + I2​

  solution changes from yellow-orange to brown
• Acidified dichromate ions Cr2O7 2- are reduced to chromium ions Cr3+ by zinc metal

  Cr2​O72−​ + 14H+ + 3Zn → 2Cr3+ + 3Zn2+ + 7H2​O

  solution changes from orange to green
• Chromium ions are oxidised to chromate ions by hydrogen peroxide

  2Cr3+ + 3H2​O2​ + 10OH− → 2CrO42−​ + 8H2​O

  solution changes to yellow
• Copper ions are reduced to solid copper iodide by iodide ions

  2Cu2+ + 4I− → 2CuI + I2​

  Solution changes from light blue to a brown solution with a white precipitate
• Solid copper oxide reacts with dilute sulfuric acid to form a brown precipitate of copper metal and a blue solution of copper sulfate in a disproportionation reaction