Chromium
1
A case study
14
15
Li
Be
B
C
N
O
F
Ne
Na
Mg
2
3
4
5
6
7
8
9
10
11
12
Al
13
Si
P
S
16
Cl
17
Ar
Br
Kr
H
K
Rb
Cs
Chromium ore
18
He
Ca
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Ti
V
Cr
Mn
Fe
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Y
Zr
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Mo
Tc
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I
Xe
Ba
*
La
Hf
Ta
W
Re
Os
Ir
Pt
Au
Hg
Tl
Pb
Bi
Po
At
Rn
•Element no. 24 of the periodic table
Fr
Ra
**
Ac
*
Ce
Pr
Nd
Pm
Sm
Eu
Gd
Tb
Dy
Ho
Er
Tm
Yb
Lu
Th
Pa
U
Np
Pu
Am
Cm
Bk
Cf
Es
Fm
Md
No
Lr
**
•Atomic symbol Cr
•”Chromium” from ”colour” κρωμα: skin, colour
•d-block metal
•Many co-ordination compounds
Appears most often as Cr(III)
Chromite (FeCr2O4)
[CrCl2(H2O)4]Cl·2H2O
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Inorganic Chemistry
- large deposits in South Africa
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Isolation of chromium metal
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Chromium nuggets
•
•
•
•
• FeCr2O4 +NaOH + O2→ Na2CrO4 + Fe2O3 +H2O
• Na2CrO4(aq) + C → Cr2O3 + Na2CO3(aq)
Hard, gray metal
Density 7.2 g·ml-1
Melting point 1890 oC
Corrosion resistant
• Cr2O3 + Al → Cr + Al2O3
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Metal + acid
•
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+
2H+
2Craq3+
Composition of ”green chromium(III) chloride”
→
+ H2(g)
= -0.41V
green solution in hydrochloric acid
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[CrCl2(H2O)4]Cl·2H2O
• Cr + 2H+ → Craq2+ + H2(g)
Eo(Cr2+/Cr) = -0.91V
light blue solution in hydrochloric acid
2Craq2+
Inorganic Chemistry
Eo(Cr3+/Cr2+)
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1.
Chromium analysis
•
2Craq3+ + 3S2O82- + 7H2O → Cr2O72- + 6HSO4- + 8H+
•
Excess 2S2O82- + 2H2O heated → 4HSO4- + O2
•
Cr2O72- + 6Fe2+ + 14H+ → 2Cr3+ + 6Fe3+ + 7H2O
Excess Fe2+ determined by Cr2O72-
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1
[CrCl2(H2O)4]Cl·2H2O
[CrCl2(H2O)4]Cl·2H2O
Stabilty
Cr(H2O)63+ + nCl- → Cr(H2O)6-nCl(3-n)+
logK1 = -1.2 , logβ2 = -3.3
Composition of ”green chromium(III) chloride”
2.
Chloride analysis
a. Dissolution of solid in nitric acid and addition of
silver nitrate; weighing heated silver chloride :
M1 =m1/n1
b. Dissolution of solid in nitric acid, boiling of solution
and addition of silver nitrate; weighing heated silver
chloride
M2 =m2/n2
M1 =3 M2
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Kinetics slow
Cr(H2O)63+ + H2O* → Cr(H2O)5H2O*3+ + H2O
k = ~10-5 sec-1
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New yellow compond
Synthesis of new compound
from green chromium(III) chloride
• Recrystallised from dilute HCl
(separation by filtration)
–
–
–
–
green solution in DMSO
destillation of water + DMSO azeotropic mixture
to give a violet solution of [CrCl3(DMSO)3]
added to an excess liquid 1,2-ethanediamine gives an
brown mass
– mixture dissolves in dilute hydrochloric acid with a
yellowish brown colour
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Inorganic Chemistry
1,2-ethanediamine abbreviated
as ”en”
[Cren3]Cl3·3H2O
– Light sensitive
– Soluble in water
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Absorption spectrum
Inorganic Chemistry
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Octahedral Geometry
A regular octahedron with
tris(1,2- ethanediamine)chromium(III)
Absoptivity
80
70
• Cr(III) ion (the central atom)
in the centre and
• Three 1,2-ethanediamines as
bidentate ligands
• 6 N-atoms in the 6 corners of the octahedron in
the same distance from the centre
60
50
40
30
20
10
0
300
350
400
450
500
550
Wavelength
(λ, ε)max = (457 , 75.0), (351 , 60.5) og (λ, ε)min = (394 , 14.5).
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2
Ligand field model
Symmetry
• ν1(4A2g(F) →4T2g(F)=
21600 cm-1
•
ν1(4A2g(F)→4T1g(F)=
28500 cm-1
• ∆=10Dq = 21600 cm-1
• B
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= 650 cm-1
Inorganic Chemistry
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Quantum chemical calculation
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Thermodynamics
• Equilibrium:
[Cr(H2O)6]3+ + 3en ⇔ [Cren3]3+ + 6H2O
3+
β3 =
[Cren3 ]
3+
[Cr ( H 2O) 6 ] ⋅ [en]3
logβ3= 19.4 (24 oC; 1.00M NaCl) ; logK = 6.43
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Magnetic behaviour
Crystal form
Distances and angles between atoms
IR spectrum
Reactivity
Redox behaviour
and so on……..
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Dimensions in Chemistry
Other properties
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