Zinc is a good reducing agent.
Zn
Zn2+ + 2eThis occurs when it reacts with mineral acids
If we add Zn to and acidified transition Metal ion
we can often see colour changes.
These are due to changes in oxidation numbers
Vanadium and Chromium are 2 good examples
51
V
23
Common
Ox states
+2
+3
+4
+5
1s2 2s2 2p6 3s2 3p6 3d3 4s2
52
Cr
24
Common
Ox states
+2
+3
+6
1s2 2s2 2p6 3s2 3p6 3d5 4s1
Vanadium
Ammonium Vanadate (V) (white solid) can be added to dilute HCl
to give the orange Dioxovanadium (V) ion
VO3- + 2H+
VO2+ + H2O
If granulated zinc is now added it reduces the vanadium over a
period of several minutes and gives several colour changes
Blue [VO(H2O)5]2+ Vanadium (IV)
Green
[VCl2(H2O)4] + Vanadium (III)
Violet [V(H2O)6]2+ Vanadium (II)
Chromium
Chromium in yellow Chromate (VI) is in its highest oxidation state.
If we add acid the solution goes orange
2CrO42- + 2H+
Cr2O72- + H2O
The Dichromate ion will also be reduced by zinc and HCl
Orange
Green
Cr2O72- Cr (VI)
[CrCl2(H2O)4]+ Cr (III)
Blue [Cr(H2O)6]2+ Cr (II)
Preparation of Ions in solution
In redox titrations we saw a reduction of metal in a high
oxidation state to a low one
MnO4-
H+
Mn2+
Cr2O72-
H+
2Cr3+
In alkaline solutions the opposite is possible (oxidation of metals
in low oxidation states).
If we wish to store solutions such as Fe2+ ions we must store
them in acid conditions . In alkaline conditions the ion will
precipitate out as the hydroxide.
Preparation of metal complexes with high oxidation numbers
is done by :i) adding an alkali
ii) adding an oxidising agent
Preparation of Chromate (VI) in Alkaline Conditions
We know addition of NaOH to a Chromium(III) salt will give
a green precipitate.
Precipitate
Cr(H2O)63+
[Cr(H2O)3(OH)3]
Excess NaOH cause this green precipitate to dissolve to give
a green solution of [Cr(OH)6]3If H2O2 is added to this a yellow solution of Chromate (VI)
is produced
2 [Cr(OH)6]3- + 3 H2O2
2CrO42- + 2OH- + 8H2O
Needs :- alkaline conditions & oxidising agent H2O2
Oxidation of Complex ions
Often the oxygen in air is enough to oxidise species
eg.
Fe(OH)2
Co(OH)2
Fe(OH)3
Co(OH)3
Metal 2+ ions can be stabilized against this by keeping them
in acid conditions.
[Co(H2O)6]2+
NH3(aq)
Shaken in
air
[Co(H2O)4(OH)2]
Precipitate
[Co(H2O)3(OH)3]
Blue / green precipitate
NH3 Conc
[Co(H2O)6
]2+
[Co(NH3)6]2+
Pale brown precipitate
O2
air
[Co(NH3)6]3+