Chromium Questions Page 3
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4Chem Factsheet
75. The Chemistry of Chromium
Oxides
Chromium forms three oxides.
•
Chromium(II) oxide, CrO, a little known basic oxide.
•
Chromium(III) oxide, Cr
O
, which is amphoteric, dissolving in acids to give chromium(III) salts and in alkalis to give chromates(III).
2
3
•
Chromium(VI) oxide, CrO
, which is acidic, dissolving in water to give chromic(VI) acid, H
CrO
, and in alkalis to give chromates(VI).
3
2
4
Chromium(III) hydroxide, Cr(OH)
, like the oxide, is amphoteric. The
The acidity of oxides increases with oxygen content.
3
nature of a hydroxide is usually the same as that of the corresponding
(Some people say that acidity increases with oxidation number.)
oxide.
Aqueous chemistry of chromium(III)
Practical 4 - Reverse changes
Hydrated chromium(III) ion
Add a dilute acid to a chromate(III) solution. Chromium(III) hydroxide
[Cr(H
O)
]
3+
, named systematically as the hexaaquachromium(III) ion, is
2
6
is precipitated at first, and then dissolves in excess acid to give a bluish-
purple, but gives a turquoise coloured acidic solution.
green solution of a chromium(III) salt.
+
-
H
(aq) ions from the acid react with OH
(aq) ions (to give H
O) so that
Practical 2 - Hydrolysis of chromium(III) salts
2
the various equilibria are disturbed to the left hand side.
Dissolve in water a purple chromium(III) salt, such as chrome alum, and
test the solution with pH paper. Note the change in colour and the pH
of the solution.
Practical 5 - Cr(III) salts with NH
(aq)
The hydrated chromium(III) ion can donate a proton from one of its
3
Add dilute NH
(aq) dropwise, with shaking, to a solution of a
coordinated water molecules to a molecule of solvent water:
3
chromium(III) salt. As with NaOH(aq), a grey-green precipitate of
O(l) ¾ [Cr(OH)(H
hydrated chromium(III) hydroxide is formed in an acid-base reaction:
3+
2+
+
[Cr(H
O)
]
+ H
O)
]
(aq) + H
O
(aq)
2
6
2
2
5
3
(aq) ¾ [Cr(OH)
An equilibrium is established containing both [Cr(H
O)
]
3+
, which is
2+
+
[Cr(OH)(H
O)
]
(aq) + 2NH
(H
O)
](s) + 2NH
(aq)
2
6
2
5
3
3
2
3
4
2+
purple, and [Cr(OH)(H
O)
]
, which is green. Consequently, the solution
2
5
Now add concentrated NH
(aq). Although the precipitate is insoluble in
is bluish-green. It is also acidic, because H
O
+
(aq) ions (“hydrogen
3
3
dilute NH
(aq), it will partially dissolve in the concentrated reagent in a
ions”) are formed, and the concentration of these is greater than that of
3
ligand substitution reaction to give a violet solution containing the
-
OH
(aq) ions.
hexaamminechromium(III) ion:
The reaction is known as an acid-base reaction or deprotonation
(aq) ¾ [Cr(NH
3+
-
[Cr(OH)
(H
O)
](s) + 6NH
)
]
(aq) + 3OH
(aq) + 3H
O(l)
3
2
3
3
3
6
2
3+
because [Cr(H
O)
]
behaves as an acid (proton donor) and H
O as
2
6
2
a base (proton acceptor).
-
2-
Other molecules or ions, notably OH
, NH
and CO
can also react
3
3
Practical 6 - Cr(III) salts with CO
2-
(aq)
3+
as bases with [Cr(H
O)
]
, but these species are stronger bases
3
2
6
Add Na
CO
(aq), with shaking, to a solution of a chromium(III) salt.
than H
O and further deprotonation can occur.
2
3
2
Notice the formation of a grey-green precipitate of hydrated chromium(III)
hydroxide, accompanied by effervescence. Collect the gas evolved in a
squeezed out teat pipette and bubble it through limewater.
-
Practical 3 - Cr(III) salts with OH
(aq)
2-
Again, there is an acid-base reaction but, when CO
ions accept protons,
3
Add dilute NaOH(aq) dropwise, with shaking, to a solution of a
carbonic acid is not formed because of its instability. Instead, its
chromium(III) salt. A grey-green precipitate of hydrated chromium(III)
decomposition products, carbon dioxide and water, are produced.
hydroxide is formed at first. This dissolves in excess NaOH(aq) to give
(aq) → [Cr(OH)
[Cr(OH)(H
O)
]
2+
(aq) + CO
2-
(H
O)
](s) + H
O(l) + CO
(g)
a grass-green solution of a chromate(III).
2
5
3
3
2
3
2
2
Exam Hint - Do not confuse a green chromate(III) with a yellow
The addition of NaOH(aq), NH
(aq) or Na
CO
(aq) to an
3
2
3
chromate(VI).
aqueous solution of a chromium(III) salt gives an identical grey-
green precipitate of hydrated chromium(III) hydroxide. In all cases,
Chromium(III) hydroxide is formed in an acid-base reaction in which
an acid-base (deprotonation) reaction occurs, in which the hydrated
three water molecules coordinated to Cr
3+
each lose a proton. This can be
chromium(III) ion acts as an acid on the Brønsted-Lowry theory, i.e.
represented by the equation:
a proton donor.
(aq) ¾ [Cr(OH)
2+
-
[Cr(OH)(H
O)
]
(aq) + 2OH
(H
O)
](s) + 2H
O(l)
2
5
3
2
3
2
Now add excess Na
CO
(aq) to the precipitate of chromium(III)
2
3
or
hydroxide. For two reasons it will not dissolve.
(aq) ¾ [Cr(OH)
3+
-
[Cr(H
O)
]
(aq) + 3OH
(H
O)
](s) + 3H
O(l)
2-
-
•
CO
is a weaker base than OH
, so a chromate(III) ion cannot be
2
6
3
2
3
2
3
formed.
The dissolving of chromium(III) hydroxide in excess NaOH(aq) occurs
2-
•
CO
, unlike NH
, is not a ligand, so there can be no ligand substitution
by a continuation of the same process:
3
3
reaction.
(aq) ¾ [Cr(OH)
[Cr(OH)
(H
O)
](s) + OH
-
(H
O)
]
-
(aq) + H
O(l)
3
2
3
4
2
2
2
2+
Remember - With Na
CO
(aq), M
ions form precipitates of metal
(aq) ¾ [Cr(OH)
2
3
-
-
2-
[Cr(OH)
(H
O)
]
(aq) + OH
(H
O)]
(aq) + H
O(l)
carbonates, MCO
, while M
3+
ions give metal hydroxides, M(OH)
,
4
2
2
5
2
2
3
3
plus CO
. Whatever the precipitate, it will never dissolve in excess
(aq) ¾ [Cr(OH)
2-
-
3-
[Cr(OH)
(H
O)]
(aq) + OH
]
(aq) + H
O(l)
2
5
2
6
2
reagent.
An equilibrium is established in which the dominant ion is
[Cr(OH)
(H
O)
]
-
at low concentrations of OH
-
, and [Cr(OH)
]
3-
at high
4
2
2
6
concentrations.
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