Chromium Questions
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1
2
3
4C
F
hem
actsheet
Number 75
The Chemistry of Chromium
To succeed in this topic you need to understand:
The +6 state
•
the basics of transition metals (covered in Factsheet 28);
This is described as an ‘oxidising state’ because most species (i.e. molecules
•
the reactions of transition metal ions with hydroxides and ammonia
and ions) of chromium in this state have a tendency to become reduced to
3+
2-
(covered in Factsheet 46);
more stable chromium(III) species, usually the Cr
ion. The Cr
O
ion is
2
7
•
the principles of oxidation and reduction (covered in Factsheet 11).
particularly important: in acidic solution (dilute H
SO
is usually used), its
2
4
orange solution is reduced to a green solution of Cr
3+
(aq):
After working through this Factsheet you will:
→ 2Cr
2-
+
-
3+
Cr
O
(aq) + 14H
(aq) + 6e
(aq) + 7H
O(l)
•
know the oxidation states of chromium;
2
7
2
•
know that the +3 state is stable, while +6 is an oxidising state and +2
2+
Reducing agents which can be oxidised by dichromates(VI) include Fe
is a reducing state;
salts (to Fe
3+
salts), sulphites (to sulphates) and ethanol (to ethanal).
•
understand the relationships between the principal compounds of
chromium;
Remember - Not all chromium(VI) species are oxidants. (A reaction
•
know the acid-base character of the oxides;
will occur only if a mechanism is available.) In particular, the
•
understand the reactions of Cr
3+
(aq) with OH
-
(aq), NH
(aq) and CO
2-
(aq);
chromate(VI) ion, CrO
2-
, is not an oxidant.
3
3
4
•
understand how to access the +6 oxidation state;
•
know the interconversion of chromium(VI) compounds.
The +2 state
This is a reducing state - a remarkably powerful one.
2+
Practical 1 - The action of air on Cr
The element
Prepare a chromium(II) compound in the absence of air by boiling a
Chromium (atomic number = 24) is the fourth member of the first series of
solution of potassium dichromate(VI) with zinc and dilute hydrochloric
d-block transition elements.
acid in a conical flask fitted with a bunsen valve (Fig 1).
2
2
6
2
6
5
1
•
Its electronic configuration is 1s
, 2s
, 2p
, 3s
, 3p
, 3d
, 4s
. This
Fig 1. A conical flask fitted with a bunsen valve
arrangement, in which each of the 3d orbitals is singly occupied, is
4
2
more stable than the alternative [Ar] 3d
, 4s
.
•
It is a hard metal with a bluish tinge, familiar as chromium plating on
glass rod as a stopper
furniture and the trim of classic cars. The melting point is very high
(1890
o
C).
rubber tubing with a slit
rubber bung
Exam Hint - Although chromium itself is not in A-level specifications,
glass tubing
its aqueous chemistry is a popular topic.
Oxidation states
The oxidation states of chromium are +2, +3 and +6, of which
only +3 is stable.
Principal compounds and ions are as follows.
The solution changes colour from orange to green and then to sky blue as
II
2+
2
2
•
Cr
The chromium(II) ion, Cr
. Its electronic configuration is 1s
, 2s
,
chromium is reduced from +6 to +3 and then to +2. Afterwards, remove
2p
6
, 3s
2
, 3p
6
, 3d
4
.
the bunsen valve and shake the flask. The solution immediately changes
•
Cr
III
Chromium(III) oxide, Cr
O
, chromium(III) hydroxide, Cr(OH)
,
from blue to green as oxygen from the air oxidises chromium(II) to
2
3
3
the chromium(III) ion, Cr
3+
, and various complex ions such as the
chromium(III).
3+
hydrated chromium(III) ion, [Cr(H
O)
]
.
2
6
3+
2
2
6
2
6
The electronic configuration of the Cr
ion is 1s
, 2s
, 2p
, 3s
, 3p
,
3d
3
.
Remember - The most stable oxidation state of chromium is +3.
Remember - Whenever ions are formed from atoms of transition
Redox chart
elements, it is always the outer 4s electrons that are lost first.
On the redox chart of chromium (p2), the principal compounds and ions
have been arranged in order of oxidation number.
VI
2-
•
Cr
Chromium(VI) oxide, CrO
, the chromate(VI) ion, CrO
, and the
•
Any movement up the chart represents an oxidation process, for which
3
4
dichromate(VI) ion, Cr
O
2-
.
an oxidising agent is required.
2
7
•
Movement down the chart corresponds to reduction. For this, a reducing
Exam Hint - Never say that a chromate(VI) or a dichromate(VI)
agent is needed, such as Zn and HCl(aq) in Practical 1.
6+
contains Cr
because this symbolises a non-existent ion. The
•
Movement across the chart is neither oxidation nor reduction, and is
correct abbreviation for chromium(VI), i.e. chromium in the +6
VI
achieved with reagents such as water, dilute acids, NaOH(aq), NH
(aq)
oxidation state, is Cr
.
3
or Na
CO
(aq).
2
3
1
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