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Vanadium is not an abundant element – there are no concentrated ores – and it is chiefly used as an alloying metal for cutting and other tool steels, e.g. spanners. Its oxidation states of +5, +4, +3 and +2 are easily produced sequentially by reduction of solutions of vanadium(V) salts since the reduction potentials for the different states are more or less evenly and quite widely spaced.
Sulphur dioxide will reduce V(+5) to V(+4) only; zinc in hydrochloric acid will reduce V(+5) step by step to V(+2). The half equations and the overall equation is given for each of these reactions.
The aqueous chemistry of vanadium(V) is complex and depends
on pH. Here we take strongly acidic solutions in which the yellow ion VO2+
is the species present. The colour changes for the complete reduction
sequence are:
|
+5 |
+5 and +4 |
+4 |
+3 |
+2 |
|
yellow |
green |
blue |
green |
lavender |
|
VO2+ |
VO2+
& VO2+ |
VO2+ |
[V(H2O)6]3+ |
[V(H2O)6]2+ |
The water ligands are not shown in the equations below, but remember that they are there for V(+3) and V(+2).
The reduction equations.
|
+5 à
+4: |
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with aqueous SO2: |
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2VO2+(aq)
+ 4H+(aq)
+ 2e – |
à |
2VO2+ (aq) + 2H2O (l) |
|
SO32 – (aq) + H2O (l) |
à |
SO42 – (aq) + 2H+(aq) + 2e – (aq) |
|
2VO2+(aq)
+ SO32 – (aq) +
2H+(aq) |
à |
2VO2+ (aq)
+ SO42 – (aq)
+ H2O (l) |
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with zinc in HCl: |
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2VO2+(aq)
+ 4H+(aq)
+ 2e – |
à |
2VO2+ (aq) + 2H2O (l) |
|
Zn(s) |
à |
Zn2+ (aq) + 2e – |
|
2VO2+(aq)
+ 4H+(aq)
+ Zn(s) |
à |
2VO2+ (aq)
+ Zn2+ (aq)
+ 2H2O (l) |
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+4 à
+3 with zinc in HCl |
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|
2VO2+ + 2e – + 4H+ (aq) |
à |
2V3+ (aq) + 2H2O (l) |
|
Zn(s) |
à |
Zn2+ (aq) + 2e – |
|
2VO2+
+ Zn(s) +
4H+ (aq) |
à |
2V3+ (aq)
+ Zn2+ (aq)
+ 2H2O (l) |
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+3 à
+2 with zinc in HCl |
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2V3+(aq) + 2e – |
à |
2V2+ (aq) |
|
Zn(s) |
à |
Zn2+ (aq) + 2e – |
|
2V3+(aq) + Zn(s) |
à |
2V2+ (aq) + Zn2+ (aq) |
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