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Zinc is not a transition element, so aqueous solutions of its tetraquo ion, [Zn(H2O)4]2+ are colourless. |
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On addition of sodium hydroxide solution a white precipitate of zinc hydroxide is formed. This can be represented as a simple precipitation reaction or as the deprotonation of the original tetraquo complex: |
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Zn2+(aq) + 2OH-(aq) ⇒ Zn(OH)2(s) |
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or [Zn(H2O)4]2+(aq) + 2OH-(aq) ⇒ Zn(H2O)2(OH)2(s) + 2H2O(l) |
This precipitate is dissolves in excess sodium hydroxide solution due to the formation of a soluble zincate:
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Zn(H2O)2(OH)2(s) + 2OH-(aq) ⇒ [Zn(OH)4]2-(aq) + 2H2O(l) |
The same precipitate of zinc hydroxide forms on addition of ammonia solution (ammonia solution is alkaline as it produces hydroxide ions in aqueous solution). However, in excess ammonia it dissolves due to the formation of the tetrammino zinc ion by ligand exchange.
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Zn(H2O)2(OH)2(s) + 4NH3(aq) ⇒ [Zn(NH3)4]2+(aq) + 2H2O(l) + 2OH-(aq) |
Zinc only has one oxidation state (+2), so is not affected by oxidizing agents.
