Chromium Questions Page 4
ADVERTISEMENT
1
2
3
4Chem Factsheet
75. The Chemistry of Chromium
Aqueous chemistry of chromium(VI)
Interconversion of Cr(VI) compounds
Access to the +6 state
In general, it is difficult to oxidise Cr(III) compounds to Cr(VI) ones for
Practical 8 - Preparation of sodium dichromate(VI)
two reasons.
Acidify the solution of sodium chromate(VI) prepared in Practical 7.
•
Most species containing Cr(VI) have a strong tendency to become reduced
Notice the colour change from yellow to orange.
back to the Cr(III) state. However, the CrO
2-
ion is not an oxidant, so
4
(aq) → Cr
2-
+
2-
this provides an obvious point of entry to the +6 oxidation level.
2CrO
(aq) + 2H
O
(aq) + H
O(l)
4
2
7
2
•
Most oxidising agents operate only in acidic solution and, under these
Now make the solution alkaline again and see the colour revert to
2-
2-
conditions, the CrO
ion is converted to Cr
O
.
4
2
7
yellow.
Oxidising agents which function in alkaline conditions and
(aq) → 2CrO
Cr
O
2-
(aq) + 2OH
-
2-
(aq) + H
O(l)
which are powerful enough to convert chromate(III) ions to
2
7
4
2
chromate(VI) ions include sodium peroxide, hydrogen peroxide and
Remember - Chromates(VI) are yellow but dichromates(VI) are
lead(IV) oxide.
orange.
Potassium dichromate(VI) is made by adding KCl(aq) to Na
Cr
O
(aq)
Practical 7 - Preparation of sodium chromate(VI)
2
2
7
and crystallising the solution. Adding KOH to K
Cr
O
will give
Pour a solution of a chromium(III) salt into a beaker and add NaOH(aq)
2
2
7
K
CrO
.
with stirring until the initial precipitate of Cr(OH)
dissolves. Add
2
4
3
Na
O
(s), stir and warm until the green solution changes colour to yellow.
2
2
Boil the solution to decompose excess Na
O
, then cool it and allow
2
2
Na
CrO
to crystallise out.
Answers
2
4
2-
1
Add zinc (or other suitable metal) and boil. The Cr
O
solution
(aq) → 2CrO
2
7
3+
2-
-
2-
2Cr
(aq) + 3O
(aq) + 4OH
(aq) + 2H
O(l)
+
changes colour to green and stays green. The VO
solution changes
2
4
2
2
colour to green and then to blue (see Factsheet 46).
Practice Questions
2-
1
Acidified solutions containing the dichromate(VI) ion, Cr
O
, and the
2
(a) Ammonium dichromate(VI), (NH
)
Cr
O
2
7
4
2
2
7
dioxovanadium(V)ion, VO
+
, are both orange and can both act as oxidising
(b) Chromium(III) oxide, Cr
O
2
2
3
agents. Suggest a chemical means of distinguishing between them. Choose
(c) C is green/blue-green/turquoise.
one test only, and describe what you would expect to see with each
3+
2+
It contains [Cr(H
O)
]
(also [Cr(OH)(H
O)
]
).
2
6
2
5
solution.
D is green.
-
2-
3-
It contains [Cr(OH)
(H
O)
]
, [Cr(OH)
(H
O)]
and [Cr(OH)
]
.
2
When crystals of an orange solid A were heated they decomposed
4
2
2
5
2
6
(aq) → 2CrO
(d) Cr
O
2-
(aq) + 2OH
-
2-
(aq) + H
O(l)
exothermically, leaving a green solid B which dissolved in nitric acid to
2
7
4
2
(aq) → NH
(e) NH
+
(aq) + OH
-
(g) + H
O(l)
give a solution C and also in concentrated sodium hydroxide solution
4
3
2
to give a solution D. When sodium hydroxide solution was added to A,
3
(a) From orange to dark green
it formed a yellow solution of E and gave off a pungent gas F which
(b) Fe → Fe
× 2
3+
+ 3e
-
add
turned moist red litmus paper blue.
→ 2Cr
2-
+
-
3+
Cr
O
+ 14H
+ 6e
+ 7H
O
2
7
2
(a) Write down the name and formula of A.
⇒ 2Fe + Cr
→ 2Fe
2-
+
3+
3+
O
+ 14H
+ 2Cr
+ 7H
O
2
7
2
(b) Identify B.
(c) Hydrogen
(c) State the colours of solutions C and D and write down the formulae
(d) Some of the electrons lost by Fe as it dissolves are transferred to
of their chromium containing ions.
→ H
hydrogen ions from the acid: 2H
+
(aq) + 2e
-
(g)
2
(d) Write an ionic equation for the formation of E.
(e) Write an ionic equation for the formation of F.
-
-
4
Only free Cl
ions will react with AgNO
(aq): not Cl
ions which are
3
coordinated to the central Cr
3+
ion.
3
When iron wire was added to an acidified solution of potassium
Weigh out equal masses of the three isomers, dissolve each in water,
dichromate(VI), the wire dissolved, the solution changed colour, and
acidify with dilute HNO
and add excess AgNO
(aq). Filter off the
an odourless, neutral gas was evolved.
3
3
precipitated AgCl, wash, dry and weigh.The masses will be in the ratio
(a) What colour change was observed?
3:2:1.
(b) Construct an ionic equation for the overall reaction between iron
3d
4s
and dichromate(VI) ions. (You may omit state symbols.)
5
(a)
(c) What gas was evolved?
2+
Cr
[Ar]
(d) In terms of electron transfer, how do you account for the formation
of this gas?
Cr
3+
[Ar]
4
There are three isomers of chromium(III) chloride-6-water, with the
following formulae:
(b) 3d sub-shell electrons are split by ligands into two sets of d-
[Cr(H
O)
]Cl
[CrCl(H
O)
]Cl
.H
O
[CrCl
(H
O)
]Cl.2H
O
2
6
3
2
5
2
2
2
2
4
2
orbitals. There are three orbitals in the lower energy set and two in
Suggest a chemical method for distinguishing between them.
3+
the higher energy set. Cr
can have all its 3d electrons in the lower
2+
set, but Cr
must have at least one 3d electron in the upper set.
5
(a) Using half-arrows to represent electrons, complete the following
electronic configurations of the Cr
2+
and Cr
3+
ions.
3d
4s
Acknowledgements: This Factsheet was researched and written by John
2+
Brockington. Curriculum Press, Bank House, 105 King Street, Wellington,
Cr
[Ar]
Shropshire, TF1 1NU. ChemistryFactsheets may be copied free of charge by
teaching staff or students, provided that their school is a registered subscriber.
Cr
3+
[Ar]
No part of these Factsheets may be reproduced, stored in a retrieval system, or
transmitted, in any other form or by any other means, without the prior
(b) Suggest why Cr
2+
ions are readily oxidised to Cr
3+
ions.
permission of the publisher. ISSN 1351-5136
4
ADVERTISEMENT
0 votes
Related Articles
Related forms
Related Categories
Parent category: Education