Redox

• Transition metal redox reactions

  • Transition elements can exist in many different oxidation states.
  • They can change oxidation state by gaining or losing electrons in redox reactions. This often brings about a colour change.
• The oxidation of Fe2+ to Fe3+ by Mn(VII) ions in acid solution

  
• The reduction of Fe3+ to Fe2+ by iodide ions in solution

  
• The oxidation of Cr3+ to dichromate(VI) Cr2O72-

  
• The oxidation of Cr3+ to dichromate(VI) Cr2O72- (2)

  
• The reduction of Cr2O72- to Cr3+ by acidified zinc

  
• Cu2+ ions can be reduced to Cu+, giving an oxidation state change of +2 to +1.

  
• Disproportionation of Cu+ ions

  
• Test for carbonates


  • To test an unknown solution for carbonates (CO3^2-), add a dilute strong acid (e.g. dilute nitric acid or dilute hydrochloric acid).
  • If carbonates are present then carbon dioxide will be released.
  • To see if carbon dioxide is produced, you need to bubble any gas created through limewater. If carbon dioxide is present, then it'll turn the limewater cloudy

  
• Test for sulfates


  • To test for sulfate ions (SO4^2-), add a dilute strong acid.
  • You need to add the dilute acid to make sure you don't end up with a false positive due to carbonate or sulfite ions, which also produce a white precipitate when reacted with barium nitrate. Adding the acid removes any carbonates and sulfites present that might interfere with the test.
  • Add a few drops of barium nitrate solution, Ba(NO3)2, to a solution of the unknown substance.
  • If you get a white precipitate it'll be barium sulfate, which tells you that there are sulfate ions in the solution.

  
• Test for halides

  • To test for halide ions just add nitric acid, then silver nitrate solution.
  • If chloride, bromide or iodide is present, a precipitate will form.
  • The colour of the precipitate depends on the halide present.
  • Silver chloride (AgCI) is a white precipitate,
  • Silver bromide (AgBr) is a cream precipitate
  • Silver iodide (Agl) is a yellow precipitate.
• Test for ammonium ions


  • You'll also be able to smell ammonia — it has a distinctive, pungent smell.
  • Ammonia gas (NH3) is alkaline, so you can check for it using a damp piece of red litmus paper.
  • If there's ammonia present, the paper will turn blue.
• Test for ammonium ions (2)


  • You can use this to test whether a substance contains ammonium ions (NH4+).
  • Add a few drops of aqueous sodium hydroxide to your unknown substance in a test tube and warm the mixture.
  • Hold a piece of damp litmus paper near the top of the test tube. If the paper turns blue it means ammonia is being given off and there are ammonium ions in your substance.