July 5, 2021
In A-Level Chemistry, ooking at the reactions with NaOH, NH3, CO32- and Cl-, we can identify these transition metal ions.
Aluminium Al3+
- colourless as aqueous solution
- white precipitate when reacting with NaOH/NH3: [Al(H2O)6]3+ +3OH- ⇨ Al(H2O)3(OH)3 +3H2O // [Al(H2O)6]3++ 3NH3 ⇨ Al(H2O)3(OH)3 +2H2O+ 3NH4+
- Excess NaOH: white precipitate dissolves into colourless solution Al(H2O)3(OH)3 + OH- à [Al(OH)4]3-+ 3H2O
- Excess NH3: white precipitate remains
- white precipitate + effervescence with CO32-: 2[Al(H2O)6]3++ 3CO32- ⇨ 2[Al(H2O)3(OH)3]+ 3CO2 + 3H2O
Chromium III Cr3+
- purple as aqueous solution
- green precipitate when reacting with NaOH/NH3: [Cr(H2O)6]3++ 3OH- ⇨ Cr(H2O)3(OH)3 +3H2O // [Cr(H2O)6]3++ 3OH- ⇨ Cr(H2O)3(OH)3+ 2H2O+ 3NH4+
- Excess NaOH: green precipitate dissolves into green solution Cr(H2O)3(OH)3 + 3OH- ⇨ [Cr(OH)6]3-+ 3H2O
- Excess NH3: green precipitate dissolves into purple solution Cr(H2O)3(OH)3 + 6NH3 ⇨ [Cr(NH3)6]3++ 3H2O + 3OH-
- green precipitate + effervescence with CO32-: 2[Cr(H2O)6]3++ 3CO32- ⇨ 2[Cr(H2O)3(OH)3]+ 3CO2 + 3H2O
Cobalt II Co2+
- pink as aqueous solution
- blue precipitate when reacting with NaOH/NH3: [Co(H2O)6]2++ 2OH- ⇨ Co(H2O)4(OH)2 +2H2O // Co(H2O)6]2++ 2OH- ⇨ Co(H2O)4(OH)2 +2H2O+ 2NH4+
- Excess NaOH: blue precipitate remains
- Excess NH3: blue precipitate dissolves into pale yellow-brown solution Co(H2O)4(OH)2 + 6NH3 ⇨ [Co(NH3)6]2++ 4H2O + 2OH-
- pink precipitate with CO32-: [Co(H2O)6]2++ CO32- ⇨ CoCO3+ 6H2O
- blue solution with CL-: [Co(H2O)6]2++ 4Cl- ⇨ [CoCl4]2- + 6H2O
Copper II Cu2+
- pale blue as aqueous solution
- blue precipitate when reacting with NaOH/NH3: [Cu(H2O)6]2++ 2OH- ⇨ Cu(H2O)4(OH)2 +2H2O // [Cu(H2O)6]2++ 2OH- ⇨ Cu(H2O)4(OH)2 +2H2O+ 2NH4+
- Excess NaOH: blue precipitate remains
- Excess NH3: blue precipitate dissolves into deep blue solution Cu(H2O)4(OH)2 +4NH3 ⇨ [Cu(NH3)4(H2O)2]2++ 2H2O + 2OH-
- blue-green precipitate+ effervescence with CO32-: [Cu(H2O)6]2++ CO32- ⇨ CuCO3+ 6H2
- yellow-green solution with Cl-: [Cu(H2O)6]2++ 4Cl- ⇨ [CuCl4]2- + 6H2O
Iron II Fe2+
- pale green as aqueous solution
- green precipitate when reacting with NaOH/NH3: [Fe(H2O)6]2++ 2OH- ⇨ Fe(H2O)4(OH)2 +2H2O // [Fe(H2O)6]2++2OH- ⇨ Fe(H2O)4(OH)4 +2H2O+ 4NH4+
- Excess NaOH: green precipitate remains
- Excess NH3: green precipitate remains
- green precipitate + effervescence with CO32-: [Fe(H2O)6]2++ CO32- ⇨ FeCO3+ 6H2O
Iron III Fe3+
- purple as aqueous solution
- yellow-brown precipitate when reacting with NaOH/NH3: [Fe(H2O)6]3++ 3OH- ⇨ Fe(H2O)3(OH)3 +3H2O // [Fe(H2O)6]3++ 3OH ⇨ Fe(H2O)3(OH)3 +2H2O+ 3NH4+
- Excess NaOH: yellow-brown precipitate remains
- Excess NH3: yellow-brown precipitate remains
- yellow-brown precipitate + effervescence with CO32-: 2[Fe(H2O)6]3++ 3CO32- ⇨ 2[Fe(H2O)3(OH)3]+ 3CO2 + 3H2O
As a summary:
Drafted by Eunice Wong (Chemistry)