Discuss the activity
What is the identity of the precipitates formed when sodium hydroxide solution is added to the two iron solutions?
The precipitates formed are iron hydroxide. The green precipitate is iron(II) hydroxide, Fe(OH)2 (s), and the brown precipitate is iron(III) hydroxide, Fe(OH)3 (s).
Fe2+ (aq) + 2OH- (aq) Fe(OH)2 (s)
Fe3+ (aq) + 3OH- (aq) Fe(OH)3 (s)
Why does the green iron(II) hydroxide precipitate turn brown on standing in air?
On standing in air, the iron(II) hydroxide is oxidised to iron(III) hydroxide.
4Fe(OH)2 (s) + O2 (g) + 2H2O (l) 4Fe(OH)3 (s)
(Working out the equation above from two ionic equations is a good problem to stretch more able students).
Why does the iron(III) solution give a dark red colour with potassium thiocyanate?
A ligand exchange reaction takes place in which a water ligand is exchanged for the thiocyanate ligand.
[Fe(H2O)6]3+ + SCN- (aq) [Fe(SCN)(H2O)5]2+ + H2O (l)
The [Fe(SCN)(H2O)5]2+ complex has a deep red colour. The ligand exchange reaction with iron(II) does not result in a colour change. This means adding thiocyanate ions can be used as a chemcial test for iron(III).
On addition of potassium iodide and starch solution, why is the blue-back colour seen with iron(III) solution and not the iron(II) solution?
Iron(III) oxidises the iodide ions to form iodine, which gives the characteristic blue-black colour with starch solution.
2Fe3+ (aq) + 2I- (aq) 2Fe2+ (aq) + I2 (aq)
The Fe2+ (aq) ions cannot oxidise the iodide ions and so no blue-black colour is seen.
Why does the deep colour disappear when potassium manganate(VII) solution is added to the iron(II) solution?
The manganate(VII) ion is powerful oxidising agent. It oxidises the Fe2+ ions to form Fe3+ ions.
MnO4- (aq) + 5Fe2+ (aq) + 8H+ Mn2+ (aq) + 5Fe3+ (aq) + 4H2O (l)
As the Fe2+ (aq) ions are oxidised, the MnO4- (aq) ion is reduced to Mn2+ (aq) and so the deep purple colour is lost.
The manganate(VII) ion is not able to oxidise the Fe3+ (aq) ions to Fe4+ (aq) ions in the iron(III) solution.
On addition of silver nitrate solution, why is silver metal seen with the iron(II) solution and not the iron(III) solution?
The Fe2+ions are oxidised to form Fe3+ ions and the silver (Ag+) ions are reduced to form silver.
Fe2+ (aq) + Ag+ (aq) Fe3+ (aq) + Ag (s)
This reaction does not take place with the Fe3+ ions.