Reactions of Inorganic Compounds in Aqueous Solution

Hydroysis Reaction

Hexaaquametal contains 2 type of bond: polar O-H bonds, and co-ordinate bonds between metal ion and water ligands
Hydrolysis breaks O-H bond 'heterolytically' (both bonding electrons remain with one of the two fragments between which the bond is broken)
M(II) hexaaqua cations, equilibrium lies to the left so solutions are weakly acidic
M(III) hexaaqua ions, equilibrium lies to the right so solutions are strongly acidic
Hydrolysis equilibria depends on the charge/size ratio
The greater the charge/size ration of the ion --> the more the O-H bonds are polarized --> therefore M(III) produce more acidic solutions because the ion is smaller and more highly charged than M(II)
LILAC ----------------------> INTENSE BROWN

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If a solution of NaOH or any other alkali is added to a solution of a transition metal (or hexaaquaaluminium), a precipitate of the metal hydroxide is produced.
Reactions:
[Fe(H2O)6]2+(aq) + 2OH-(aq) --> [Fe(H2O)4(OH)2](s) + 2H2O(l) iron (II) hydroxide produced
GREEN ----> BROWN
[Cu(H2O)6]2+(aq) + 2OH-(aq) --> [Cu(H2O)4(OH)2](s) + 2H2O(l) copper (II) hydroxide produced (pale blue precipitate)
[Co(H2O)6]2+(aq) + 2OH-(aq) --> [Co(H2O)4(OH)2](s) + 2H2O(l) cobalt (II) hydroxide produced (blue precipitate turning pink on standing)
[Al(H2O)6]3+(aq) + 3OH-(aq) --> [Al(H2O)3(OH)3](s) + 3H2O(l) aluminium hydroxide produced (white precipitate)
[Cr(H2O)6]3+(aq) + 3OH-(aq) --> [Cr(H2O)3(OH)3](s) + 3H2O(l) chromium (III) hydroxide (dark green precipitate)
Both aluminium hydroxide and chromium (III) hydroxide react with both acid and alkali
Aluminium hydroxide goes from white solid to colourless solution in both acid and alkali
Chromium (III) hydroxide goes from green solid to dark green solution in both acid and alkali

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The ammonia molecules behave in the same way as the hydroxide ions
The iron, copper and cobalt hexaaqua-ions react with 2NH3 to give their corresponding hydroxides and 2NH4+
The chromium, iron and aluminium hexaaqua-ions react with 3NH3 to give their corresponding hydroxides and 3NH4+

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[M(H2O)6)]3+ solutions are acidic enough to provide the hydrated protons for the following reactions:
CO32-(aq) + H3O+(aq) ---> HCO3-(aq) + H2O(l) then...
HCO3-(aq) + H3O+(aq) ---> 2H2O(l) + CO2 (g)

The [M(H2O)6]2+ ions do not behave as acids. Instead they for a precipitate directly with the carbonate ion
[Fe(H2O)6)]2+(aq) + CO32-(aq) ---> FeCO3(s) (green precipitate) + 6H2O
[Co(H2O)6)]2+(aq) + CO32-(aq) ---> CoCO3(s) (light purple precipitate) + 6H2O
[Cu(H2O)6)]2+(aq) + CO32-(aq) ---> CuCO3(s) (blue precipitate) + 6H2O

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Ammonia molecules are very similar in size to water molecules
On slow addition of ammonia to hexaaqua complexes, a precipitate is formed of the hydroxide, which redissolves in excess ammonia
For hexaaquacopper (pale blue solution) with ammonia, copper (II) hydroxide is produced and 2NH4+
Upon the addition of more ammonia solution the precipitate dissolves forming a dark blue solution of tetraaminebisaquacopper(II) ion [Cu(NH3)4(H2O)2]2+, water and OH- ions
In aqueous solution the substitution is incomplete and that only 4 of the water molecules are replaced
For hexaaquachromium (green solution) when reacted with ammonia, produces chromium (III) hydroxide (green precipitate) and 3NH4+
Chromium hydroxide then reacts with excess ammonia to produce [Cr(NH3)6]3+ (aq) (lilac solution) and water and OH- ions
For hexaaquacobalt (pink solution) with ammonia, cobalt hydroxide is formed (blue precipitate) and 2NH4+
In excess ammonia [Co(NH3)6]2+ (straw coloured solution) is formed and water and OH- ions
Co(II) is much more stable than Co(III) so the ion is oxidised in the air and the solution turns dark brown

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Hexaaquacobalt (pink) reacts with conc. HCl to produce [CoCl4]2- (dark blue) and water
Conc. HCl is used because it has a high concentration of chloride ions
The high chloride ion concentration pushes the position of equilibrium to the right
Chloride ions are bigger than water molecules so the co-ordination number changes from 6 to 4
Hexaaquacopper (pale blue) reacts with conc. HCl to produce [CuCl4]2- (olive-green) and water

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The chelate effect occurs when you replace water ligands around the central metal ions by multidentate ligands
Chelates are much more stable than complex ions formed from unidentate ligands
The chelate effect occurs because of entropy change
Any change which increases the amount of disorder increases the tendency of a reaction to happen
In the case of EDTA4- the number of species present is being increased from 2 on the left to 7 on the right, so disorder increases making it more likely to happen

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