Reactions Of Inorganic Compounds In Aqueous Solution
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22.6. Reactions of Inorganic Compounds in Aqueous Solution
Lewis acids and bases
In the formation of complex ions the ligand is the Lewis
Definitions: Lewis acid: electron pair acceptor
base because it is donating a pair of electrons in the
Lewis base: electron pair donator
dative covalent bond and the metal ion is the Lewis acid.
Metal-aqua ions
Metal aqua ions are formed in aqueous solution.
In solution and Fe(III) appears
yellow/brown due to hydrolysis reactions.
2+
[M(H
O)
]
, limited to M = Fe (green) and Cu (blue);
The violet colour is only really seen in
2
6
3+
[M(H
O)
]
, limited to M = Al (colourless), and Fe (violet)
solid hydrated salts that contain these
2
6
complexes.
Acidity or hydrolysis reactions
The following equilibria happen in aqueous solutions of metal ions.
The equilibria lead to generation of acidic
3+
solutions with M
ions, and very weakly
2+
+
+
[M(H
O)
]
+ H
O
[M(H
O)
(OH)]
+ H
O
2
6
2
2
5
3
2+
+
acidic solutions with M
ions. The 3
ions
are noticeably more acidic.
3+
2+
+
[M(H
O)
]
+ H
O
[M(H
O)
(OH)]
+ H
O
2
6
2
2
5
3
3+
2+
The acidity of [M(H
O)
]
is greater than that of [M(H
O)
]
in terms of the greater polarising power
2
6
2
6
(charge/size ratio) of the 3+ metal ion. The greater the polarising power, the more strongly it attracts the water
molecule. This weakens the O-H bond so it breaks more easily.
-
Reaction with limited OH
and limited NH
3
-
The bases OH
and ammonia when in limited
M(OH)
(H
O)
: Cu blue ppt, Fe (II) green ppt
2
2
4 (s)
amounts form the same hydroxide
M(OH)
(H
O)
: Fe(III) brown ppt, Al white ppt
3
2
3 (s)
precipitates. They form in deprotonation
acid base reactions
2+
-
2+
+
[Cu(H
O)
]
+ 2OH
Cu(H
O)
(OH)
+ 2H
O
[Fe(H
O)
]
+ 2NH
Fe(H
O)
(OH)
+ 2NH
2
6
(aq)
(aq)
2
4
2 (s)
(l)
2
6
(aq)
3 (aq)
2
4
2 (s)
4
(aq)
2
3+
-
[Al(H
O)
]
+ 3OH
Al(H
O)
(OH)
+ 3H
O
3+
+
[Fe(H
O)
]
+ 3NH
Fe(H
O)
(OH)
+ 3NH
2
6
(aq)
(aq)
2
3
3 (s)
(l)
2
2
6
(aq)
3 (aq)
2
3
3 (s)
4
(aq)
This process can happen step wise removing one proton at a time. Be able to write equations for this too.
3+
-
2+
e.g. [Al(H
O)
]
+ OH
[Al(H
O)
(OH)]
+ H
O
2
6
(aq)
(aq)
2
5
(aq)
(l)
2
-
Here the NH
and OH
ions are acting as
3
2+
-
+
[Al(H
O)
(OH)]
+ OH
[Al(H
O)
(OH)
]
+ H
O
Bronsted-Lowry bases accepting a proton
2
5
(aq)
(aq)
2
4
2
(aq)
(l)
2
-
Reaction with excess OH
This hydroxides is classed as amphoteric
With excess NaOH the Al hydroxide dissolves.
because it reacts and dissolves in both acids and
-
Al becomes [Al(OH)
]
(aq) colourless solution.
4
bases.
-
-
+
3+
Al(H
O)
(OH)
+ OH
[Al(OH)
]
+ 3H
O
Al(H
O)
(OH)
+ 3H
[Al(H
O)
]
2
3
3 (s)
(aq )
4
(aq)
(l)
2
3
3 (s)
(aq )
2
6
(aq)
2
Reaction with excess NH
3
With excess NH
a ligand substitution reaction occurs with Cu and its precipitate dissolve
3
2+
Cu becomes [Cu(NH
)
(H
O)
]
deep blue solution
This substitution is incomplete with Cu.
3
4
2
2
2+
-
Cu(OH)
(H
O)
+ 4NH
[Cu(NH
)
(H
O)
]
+ 2H
O
+ 2OH
In this reactions NH
is
2
2
4(s)
3 (aq)
(aq)
2
(l)
(aq)
3
3
4
2
2
acting as a Lewis base
donating an electron pair.
1
N Goalby
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