practical 11

• TEST 1A - sodium hydroxide

  add dropwise and shake until it is in excess

  results:

  Cr3+ = dark green to green ppt to green solution

  Fe3+ = yellow solution to brown ppt

  Cu2+ = blue solution to blue ppt

  provides OH- for hydrolysis and then further substitution with Cr3+
• TEST 1B - leave to sit

  leave to sit in a beaker of hot water

  results:

  Cr3+ = no change

  Fe3+ = brown/red solid (Fe2O3)

  Cu2+ = black solid (CuO)

  change is due to thermal decomposition
• TEST 2 - sodium carbonate

  add 10 drops and shake

  results:

  Cr3+ = dark green solution to green ppt (effervescence)

  Fe3+ = yellow solution to brown ppt (effervescence)

  Cu2+ = pale blue solution to green/blue ppt

  Cr3+ and Fe3+ undergo substitution reactions as they cause the OH bond to become acidic (increases polarity), whereas Cu2+ only undergoes hydrolysis as it is not electron withdrawing enough
• TEST 3 - silver nitrate

  add 10 drops and shake, then leave for 10 minutes

  results:

  Cr3+ = dark green solution to dark green solution and white ppt

  Fe3+ = no change

  Cu2+ = pale blue solution to pale blue solution and white ppt

  undergoes substitution reactions to form silver-containing precipitates and transition metal complexes with different anion ligands