Unit 5 Section 3 Other - Variable Oxidation States

Fe2+/Fe3+

Readily oxidised in solution: Fe2+(aq) + reducing agent  ---> Fe3+(aq) + e-

• Mn,2+: Pale Pink
• Mn,4+: Purple
• Mn,7+: Purple

Mn7+ reduced to Mn2+ by Fe2+

• MnO4,-(aq) + 8H-(aq) + 5e- >>> Mn,2+(aq) + 4H2O(l)
• Fe,2+(aq) >>> Fe,3+(aq) + e-

Overall:

• MnO4,-(aq) + 8H+(aq) + 5Fe,2+ >>> Mn,2+(aq) + 4H2O(l) + 5Fe,3+(aq)

• V,+5: Yellow - VO2,+ Ion
• V,+4: Blue - VO,2+ Ion
• V,+3: Green - V,3+ Ion
• V,+2: Violet - V,2+ Ion

• Strong reducing agents (Zn/acid) will reduce +5 ions all the way to +2 ions

e.g. 2VO2,+(Yellow, aq) + 8H+(aq) + 3Zn(s) >>> 2V,2+(violet, aq) + 3Zn,2+(aq) + 4H2O(l)

• Weak reducing agents will only reduce +5 to +4
• Air will oxidise +2 to +3
• Colour of complex formed depends on the ligand - e.g. If HCl is the acid used, the +3 complex is [V(H2O)4],+, which is green, but if H2SO4 is used, the +3 complex is [V(H2O)6],3+, which is grey-blue

In aqueous/acid solution:

• Cr,+6: Orange - Cr2O7,2- Ion
• Cr,+3: Green - Cr,3+ Ion
• Cr,+2: Blue - Cr,2+ Ion

In alkaline solution:

• Cr,+6: Yellow - CrO4,2- Ion
• Cr,+3: Green - Cr(OH)6,3- Ion

The +6 chromium complexes can be readily interconverted using acid and alkali:

• 2CrO4,2-(aq, Yellow) + 2H+(aq) >>> Cr2O7,2-(aq, Orange) + H2O(l)
• Cr2O7,2-(aq, Orange) + 2OH,-(aq) >>> 2CrO4,2-(aq, Yellow) + H2O(l)

NOTE: Oxidation is favoured by alkaline conditions and reduction is favoured by acidic conditions.

Zn/acid can reduce +6 to +3 and +2

• The chromium is first reduced to the +3 oxidation state:Cr2O7,2-(aq) + 14H+(aq) + 6e- >>> 2Cr,3+(aq) + 7H2O(l) and Zn(s) >>> Zn,2+(aq) + 2e-Overall: Cr2O7,2-(aq) + 14H+(aq) + 3Zn(s) >>> 2Cr,3+(aq) + 7H2O(l) + 3Zn,2+(aq)
• It is then further reduced:Cr,3+(aq) + e- >>>Cr,2+(aq) and Zn(s) >>> Zn,2+(aq) + 2e-
• Overall: Zn(s) + 2Cr,3+(aq) >>> Zn,2+(aq) + 2Cr,2+(aq)

Colour change observed is orange(Cr2O7,2-) to green(Cr,3+) to blue(Cr2+).

Milder reducing agents(Fe,2+) can only reduce to 3+

+6 oxidation state to be oxidised to +3 by H2O2

[Cr(OH)6],3- complex(+6) is formed in alkaline conditions allowing H2O2 to oxidise it.

(hydrogen peroxide is a reducing agent in acidic solution but is an oxidising agent in an alkaline solution)

[Cr(OH)6],3-(aq) + 2OH-(aq) >>> CrO4,2- (aq) + 4H2O(l) + 3e-

H2O2(aq) + 2e- >>> 2OH-(aq)

Overall:

2[Cr(OH)6],3-(aq, Green) + 3H2O2(aq) >>> 2CrO4,2-(aq, Yellow) + 8H2O(l) + 2OH-(aq)

Cobolt: in acid/neutral conditions

• Co,+3: Yellow - Co,3+ Ion
• Co,+2: Pink - Co,2+ Ion
• Difficult to oxidise cobolt from 2+ to 3+

• Co,+3: Brown - Co(OH)3 Ion
• Co,+2: Blue - Co(OH)2 Ion

Easier to oxidise Co +2 to +3 using H2O2

Co(OH)2(s) + OH-(aq) >>> Co(OH)3(s) + e-

H2O2(aq) + 2e- >>> 2OH-(aq)

Overall:

2Co(OH)2(s, Blue) + H2O2(aq) >>> 2Co(OH)3(s, Brown)

• Co,+3: Brown - Co(NH3)6,3+ Ion
• Co,+2: Straw - Co(NH3)6,2+ Ion

Very easy to oxidise in air

Co(NH3)6,2+(aq) <<<>>> Co(NH3)6,3+(aq) + e-

O2(g) + 2H2O(l) + 4e- >>> 4OH-(aq)

Overall:

4Co(NH3)6,2+(aq, Straw) + O2(g) + 2H2O(l) >>> 4Co(NH3)6,3+(aq, Brown) + 4OH-(aq)