15.10 - Reactions of Transition Metal Ions with Aqueous OH-

• reaction with sodium hydroxide and ammonia

  produces a precipitate of metal hydroxide which is amphotheric
• how to reverse reaction with sodium hydroxide or ammonia

  add acid to hydroxide precipitate - hydroxide ligands patronate and precipitate dissolves as solube metal-aqua ions are reformed
• what happens during the reaction with ammonia or sodium hydroxide

  water ligands are depatronated in an acid-base reaction producing a coloured hydroxide precipitate
• what's an amphotheric hydroxide?

  hydroxide that can dissolve in aqueous acid or alkali - acts both as a base and acid
• reaction of aqueous Co2+ with little ammonia

  produces Co(OH)2 precipiatate and ammonium ions

  pink solution to blue precipitate which turns to a pale brown solution that darkens on standing as it's oxidised
• reaction of aqueous Co2+ with excess ammonia

  produces [Co(NH3)6]2+ and hydroxide ions and water

  goes from a pink solution to a yellow-brown solution that forms a brown solution when left on standing as it's oxidised
• reaction of aqueous Co2+ with sodium hydroxide

  forms Co(OH)2 precipitate and water

  precipitate doesn't dissolve in excess sodium hydroxide

  pink solution to blue precipitate
• reaction of aqueous Fe2+ with sodium hydroxide

  forms Fe(OH)2 precipitate and water

  precipitate is insoluble in excess sodium hydroxide

  pale green solution to green precipitate which darkens on standing as it's oxidised (orange at the top)
• reaction of aqueous Fe2+ with ammonia

  forms Fe(OH)2, ammonium ions and water

  precipitate is insoluble in excess ammonia

  pale green solution to green precipiate that darkens on standing (forming orange at the top) as it's oxidised
• reaction of aqueous Fe3+ with sodium hydroxide

  forms Fe(OH)3 precipitate and water

  precipitate is incoluble in excess sodium hydroxide

  yellow solution to browny-red precipitate
• reaction of Fe3+ with ammonia

  forms Fe(OH)3 and ammonium ions

  precipitate is insoluble in excess ammonia

  yellow solution to browny-red precipitate
• colour of Cr2+ in acidic, aqueous solution

  blue
• colour of Cr+3 in acidic, aqueous solution

  violet when surrounded by 6 water ligands but they're usually substituted by impurities (like Cl-) so makes it green
• Cr2O7-2 in acidic, aqueous conditions

  orange
• [Cr(OH)6]-3 in alkali solution


  green
• CrO4- in alkali solution

  yellow
• reaction of Cr+3 with OH- not in excess

  acid-base reaction that forms a Cr(OH)3 precipitate and water

  green or violet solution to grey-green precipitate

  reverse by adding acid (hydroxide is amphotheric)
• Cr+3 with excess OH-

  acid-base reaction that forms [Cr(OH)6]-3

  green or violet solution to dark green solution

  reverse by adding acid - chromium hydroxide is amphotheric
• Cr+3 with ammonia not in excesss

  acid-base reaction that forms a Cr(OH) precipitate and ammonium ions

  goes from green or violet solution to grey-green precipitate
• Cr+3 with excess ammonia

  ligand exchange reaction that forms [Cr(NH3)6]+3, hydroxide ions and water

  goes from green or violet solution to purple solution
• conditions for reduction of Cr2O7-2 to Cr+3

  zinc in acidic conditions (dilute acid)
• equation for reduction of Cr2O7-2 to Cr+3

  Cr2O7-2 +14H+ + 3Zn -> 3Zn+2 + 2Cr+3 + 7H2O
• electric potential of reduction of Cr2O7-2 to Cr+3

  +2.09 V
• further reduction of Cr2O7-2 to Cr+2 conditions

  zinc in acidic conditions, inert atmosphere - Cr+2 is so unstable it oxidises back to Cr+3 in air
• equation for further reduction of Cr2O7-2 to Cr+2

  2Cr+3 + Zn -> Zn+2 + Cr+2
• electric potential of further reduction of Cr2O7-2 to Cr+2

  +0.35 V
• conditions for oxidation of Cr+3 to form Cr2O7-2

  hydrogen peroxide in alkaline solutions, then acidification
• electric potential for oxidation of Cr+3

  +1.08 V
• equation for oxidation of Cr+3 to form Cr2O7-2

  2Cr+3 + 10OH- +3H2O2 -> 2CrO4-2 + 8H2O

  2CrO4-2 <=> Cr2O7-2
• 2CrO4-2 <=> Cr2O7-2

  add H+ for forward reaction

  add OH- for backward reaction

  CrO4-2 is yellow

  Cr2O7-2 is orange