CHEM 443L
Inorganic Chemistry Laboratory
Revision 1.1
Two Interesting Copper Compounds
In this laboratory exercise we will synthesize two interesting Copper containing compounds.
The first is Potassium Trichlorocuprate (II); KCuCl3. This compound has an unusual red color
and crystallizes as a chain structure. The other is Tris(Thiourea)Copper (I) Chloride;
[Cu(SC(NH2)2)3]Cl. The Thiourea ligands complex with the Cu+ ion and stabilize the Copper in
the less than usual +1 oxidation state. Hence both of these compounds exhibit peculiarities that
make them worth studying.
Copper metal is one of nine Elements known since antiquity. This is due to the fact that the
smelting of Copper containing ores to isolate the pure metal was one of the first smelting
processes to be discovered. Early Romans referred to the metal as aes Cyprium; aes being a
generic term for alloys of Copper and Cyprium because most of the Copper ores were mined in
Cyprus. This was later shortened to cuprum and then Anglicized into Copper.
The chemical and physical properties of Copper, like those of the other Transition Metals, are
heavily influenced by the presence of d-subshell electrons in its outer orbitals; Cu having an
electron configuration of:
Cu
=
1s2 2s22p6 3s23p63d10 4s1
=
[Ar] 3d10 4s1
This is consistent with Copper's placement in Group 1B of the Periodic Table of the Elements,
suggesting it has but a single valence electron. In fact, Mendeleev's original Periodic Table had
Copper, Silver and Gold placed within the same chemical group as the Group 1A elements (Li,
Na, K, Rb, Cs). This suggests its salts should involve Copper in the +1 oxidation state.
Cu+ =
[Ar] 3d10
And, this is sometimes the case. Salts of Cu+ are reasonably common, often water insoluble and
mostly white in color. However, Cu+ is readily oxidized to Cu2+ under most aqueous solution
conditions. So, Cu2+ salts are much more common than the Cu+ salts. Many of these salts are
water soluble and are usually colored a bright blue or green. (See, for example, Malachite and
Azurite.)
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Malachite
http://en.wikipedia.org/wiki/Malachite
Azurite
http://en.wikipedia.org/wiki/Azurite
Our first compound to synthesize is KCuCl3; prepared by mixing CuCl2 and KCl, followed by
the precipitation of the desired product. This salt is a complex halide and not a double salt. (A
double salt contains multiple cations/anions and will dissolve in Water with complete
dissociation of its simple ions. A classic example of a double salt is Alum; KAl(SO4)2. This
compound will dissociate into K+, Al3+ and SO42- in Water. Our complex chloride can be
thought of as K+ and CuCl3-; although this is not quite correct either.) A number of these
complex halides are known: KCuCl3, KMgF3, NH4[CdCl3], NaSbF6, etc. Although these
compounds have similar chemical formulas, their structures can be quite different. Some form
lattice structures, much like simpler salts, and others form layer or chain structures. For
example, NaSbF6 adopts the face-centered cubic structure of NaCl; with ions Na+ and SbF6-.
In constrast, our compound, KCuCl3, consists of infinite chains of distorted octahedra of CuCl6
sharing opposite and adjacent edges. This forms a double chain of octahedra held together by
the positive K+ ions. The octahedra of Chlorines about each Copper is not symmetrical. Four
chlorines are at equivalent distances and two are slightly further away. The double chains of
octahedra can be pictured as a portion of the CdCl2 layer structure.
… CdCl2 possesses a lattice of face centered cubic Cl- ions with half of the octahedral holes
occupied, as required by the stoichiometry. The interesting feature, however, is that only alternate
layers of holes are occupied by the Cd2+ ions.
… Careful inspection of the fcc lattice of anions shows that this structure can be drawn as parallel
sheets of anions. [The figure below] shows two different perspectives of the fcc structure…
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… the MX2 stoichiometry [of the CdCl2 structure] requires that only half of the octahedral holes
be filled by Cd2+ ions. This could be done by Cd2+ ions occupying half of the octahedral holes in
each sheet of holes; but, in fact, CdCl2 has the layer structure in which all the holes in alternate
hole-sheets are occupied. This, of course, means that the structure may be thought of as a sheet of
cations sandwiched between two sheets of anions [pictured below], and this three-layer
arrangement repeats itself perpendicular to the page.
An Introduction to Inorganic Chemistry
Keith F. Purcell and John C. Kotz
Of further note, KCuCl3, although containing Cu2+, is an unusual dark red color.
This compound can be readily hydrolyzed in moist Air:
CuCl2•2H2O(s) + K2CuCl4•2H2O(s)
2 KCuCl3(s) + 4 H2O(g)
Both of the Copper containing products contain Copper in the 2+ oxidation state and are a typical
blue-green color.
Our second compound to be synthesized, [Cu(SC(NH2)2)3]Cl, is a pale white structure with
Copper in the +1 oxidation state. This is accomplished by initially treating Copper turnings with
hot, concentrated HCl, resulting in the oxidation of the Copper to Cu+.
2 Cu(s) + 2 HCl(aq)
2 Cu+(aq) + 2 Cl-(aq) + H2(g)
This, in and of itself, is unfavorable; note the unfavorable negative oxidation potential.
Cu(s)
Cu+(aq) + e-
Eo = - 0.518 V
(The reduction of H+ to H0 is not sufficiently favorable, Eo = 0.00 V, to drive this oxidation
forward. However, you should note the production of H2 gas during this phase of the synthesis.)
To stabilize the Copper in the +1 oxidation state, it is treated with Thiourea. The formation
constant for the complexation of Cu+ with Thiourea is large enough to drive the reaction to
completion.
Cu+(aq) + 3 Thiourea(aq)
Cu(Thiourea)3+(aq)
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A stabilization of uncommon metal oxidation states by complexation with an appropriate ligand
is a common trick.
Our compound will have a white color, characteristic of Copper compounds in this oxidation
state.
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Procedure
Preparation of Potassium Trichlorocuprate (II)
Dissolve 1.0g of Potassium Chloride (KCl) in 3.7 mL of distilled Water. In a fume hood,
dissolve 5.5g of Cupric Chloride Dihydrate (CuCl2•2H2O) in 50 mL of concentrated HCl in a
250 mL Erlenmeyer flask. Stir the mixture until it is dissolved; this may take several minutes.
Do not heat the mixture as this will drive off the HCl. Essentially dropwise, pour the KCl
solution into the CuCl2 solution slowly and with rapid stirring. Place the Erlenmeyer in an Ice
bath for half an hour. Vacuum filter the red, needle-like crystals in a medium porosity sintered
glass crucible. Spread the damp precipitate onto a watch glass and dry them in an oven that is
not any hotter than 85oC. Once dry, weigh the product and then store the sample in a 3 dram vial
flushed with Nitrogen and sealed with Parafilm wax.
Preparation of Tris(Thiourea) Copper (I) Chloride
Dissolve 5.0g of Thiourea in 25 mL of hot Water, add 1.0g of Copper Turnings. In a fume
hood, add 5 mL of concentrated HCl to this mixture. Heat this on a Steam bath while the
Copper dissolves. Do not boil. Add Water if needed to maintain the liquid at a constant level.
Filter the hot solution and allow to cool slowly. White opaque crystals will form as the solution
cools. Filter the crystals and wash them with Acetone. Allow the crystals to Air dry. Weigh the
product and store in a labelled 3 dram vial.
Model Structure of CdCl2
Use the solid state model kit to prepare a models of NaCl and the CdCl2 structure. Note both the
face center cubic structure and the filling of the octahedral holes for both of these crystal
structures. Note the layered structure of the CdCl2 model. Instructions for building this model
can be found on page 34 of the model kit's instruction manual.
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Post-Lab Questions
1.
Write balanced chemical reactions describing each of the above preparative schemes.
2.
Calculate the Percentage Yield for each product.
3.
Sketch the double octahedral chain of KCuCl3.
4.
If KCuCl3 were a double salt, instead of a complex chloride, what ions would it dissociate
into when dissolved in Water?
5.
According the Purcell and Kotz, CuCl2, one of the hydrolysis products of KCuCl3, form
chains that can be "viewed as edge-sharing squares of MCl4 subunits." Sketch this
structure.
6.
Cu can be readily oxidized to Cu2+ by concentrated Nitric Acid. Write a chemical equation
for this oxidation reaction. Notoriously, Gold cannot be oxidized by Nitric Acid.
However, it will dissolve in Aqua Regia. What is Aqua Regia and how does it act to
dissolve Gold. Write appropriate chemical equations. How does this relate to the curent
lab excise?
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References
Purcell, Keith F. and Kotz, John C. "An Introduction to Inorganic Chemistry." Saunders
College Publishing, Philadelphia, 1980.
Tanaka, J. and Suib, S.L. "Experimental Methods in Inorganic Chemistry." Prentice Hall, New
Jersey, 1999.
Wells, A.F. "Structural Inorganic Chemistry," 5th Ed. Clarendon Press, Oxford, 1984.