1
2
3
4
5
Chromium
6
7
8
9
10
11
12
13
14
15
16
17
18
H
He
Li
Be
Na
Mg
24Cr
B
C
N
O
F
Ne
Al
Si
P
S
Cl
Ar
Kr
K
Ca
Sc
Ti
V
Cr
Mn
Fe
Co
Ni
Cu
Zn
Ga
Ge
As
Se
Br
Rb
Sr
Y
Zr
Nb
Mo
Tc
Ru
Rh
Pd
Ag
Cd
In
Sn
Sb
Te
I
Xe
Cs
Ba
La
Hf
Ta
W
Re
Os
Ir
Pt
Au
Hg
Tl
Pb
Bi
Po
At
Rn
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
*
*
**
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Inorganic Chemistry 1
2
Chromium
A case study
14
15
Li
Be
B
C
N
O
F
Ne
Na
1
Mg
2
3
4
5
6
7
8
9
10
11
12
13
Al
Si
P
S
16
Cl
17
Ar
K
Ca
Br
Kr
H
18
He
Sc
Rb
Sr
Y
Cs
Ba
La
Ti
V
Cr
Zr
Nb
Mo
Hf
Ta
W
Mn
Fe
Co
Ni
Cu
Zn
Ga
Ge
As
Se
Ru
Rh
Pd
Ag
Cd
In
Sn
Sb
Te
I
Xe
Os
Ir
Pt
Au
Hg
Tl
Pb
Bi
Po
At
Rn
*
Re
•Element no. 24 of the periodic table
Fr
Ra
**
Ac
*
Ce
Pr
Nd
Th
Pa
U
Sm
Eu
Gd
Tb
Dy
Ho
Er
Tm
Yb
Lu
**
•Atomic symbol Cr
•”Chromium” from ”colour” κρωμα: skin, colour
•d-block metal
•Appears most often as Cr(III)
•Many co-ordination compounds
[CrCl2(H2O)4]Cl·2H2O
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Inorganic Chemistry
3
Electrolytic chromium-plating
In acidic chromate(VI)-baths chromium metal is
deposited on the cathode
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CrO3 +6H+ + 6e- → Cr + 3H2O
Inorganic Chemistry
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1
Chromium ore
Chromite (FeCr2O4)
- large deposits in South Africa
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Inorganic Chemistry
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Isolation of chromium metal
• FeCr2O4 +NaOH + O2→ Na2CrO4 + Fe2O3 +H2O
• Na2CrO4(aq) + C → Cr2O3 + Na2CO3(aq)
• Cr2O3 + Al → Cr + Al2O3
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Inorganic Chemistry
6
Chromium nuggets
•
•
•
•
Hard, gray metal
Density 7.2 g·ml-1
Melting point 1890 oC
Corrosion resistant
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Inorganic Chemistry
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2
Metal + acid
• Cr + 2H+ → Craq2+ + H2(g)
Eo(Cr2+/Cr) = -0.91V
light blue solution in hydrochloric acid
• 2Craq2+ + 2H+ → 2Craq3+ + H2(g) Eo(Cr3+/Cr2+) = -0.41V
green solution in hydrochloric acid
green solution:(λmax = 635 nm ; 445 nm)
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Inorganic Chemistry
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[CrCl2(H2O)4]Cl·2H2O
Composition of ”green chromium(III) chloride”
CrCl3(H2O)6
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 standard Cr2O72-
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Inorganic Chemistry
9
Safety: Hazard symbols for
K2Cr2O7
New symbols in EU since 2010
19.12.2013
Inorganic Chemistry
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3
Cr(IV)
CrCl3(H2O)6 + O2H- → CrO422CrO3 + H2O → Cr2O72- +2H+
Cr2O72- + 2OH- → 2CrO42-+ H2O
PbCrO4, BaCrO4 (Hac)
Cr2O72- +2H+ + H2O2 → Cr(O2)2O
less polar solvent
water
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Inorganic Chemistry
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Cr(H2O)63+
Cr(H2O)63+ → Cr(OH)(H2O)52+ + H+
pK~4
+OH- → Cr(OH)3(s) → Cr(OH)4+SH- → Cr(OH)3(s)
+NH3 → Cr(OH)3(s) → {Cr(NH3)n(H2O)x(OH)y}m
NH4+,slow
+O2H- → CrO4228-01-2017
Inorganic Chemistry
12
[CrCl2(H2O)4]Cl·2H2O
Composition of
”green chromium(III) chloride”
2.
Chloride analysis
a. Dissolve solid in nitric acid and add silver
nitrate; weigh dry silver chloride :
M1 =m1/n1
b. Dissolve solid in nitric acid, boil the
solution and add silver nitrate; weigh dry
silver chloride
M2 =m2/n2
M1 =3 M2
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Inorganic Chemistry
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4
[CrCl2(H2O)4]Cl·2H2O
Composition of ”green chromium(III) chloride”
3.
Thermogravimetric analysis
Loss of mass at t>100 oC
Water: n(H2O)/n(Cr) = 2.0
Stoichiometry: CrCla2Clb(H2Oa)4(H2Ob)2
4 water molecules and 2 Cl- make stronger bonds
to Cr3+ than 1 Cl- and 2 water molecules
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Inorganic Chemistry
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[CrCl2(H2O)4]Cl·2H2O
Stability
Cr(H2O)63+ + nCl- → Cr(H2O)6-nCln(3-n)+
logK1 = -1.20 , logβ2 = -3.27 , log β3= -6.43
(logK2 = -2.07 , logK3= -3.16)
Kinetics slow
Cr(H2O)63+ + H2O* → Cr(H2O)5H2O*3+ + H2O
k = ~10-5 sec-1
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Cr(H2O)6-nCln(3-n)+
heat solution → approach equilibrium
Cr(H2O)63+
Cr(H2O)5Cl2+
Cr(H2O)4Cl2+
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x-H+
x-H+
x-H+
x-H+
x-H+
x-H+
x-H+
x-H+
x-H+
x-H+
x-H+
x-H+
x-Cr(H2O)6-nCln(3-n)+ +H+↔
x-H+ + Cr(H2O)6-nCln(3-n)+
x- (3-n)+
Kn < Kn+1
Inorganic Chemistry
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?
5
Cr(H2O)63+
Violet (λmax = 575, 409, 261 nm)
in crystals and in solution
4P
4F
4T
1
4T
1
4T
2
4A
as nitrate
or
2
perchlorate
low intensity (ε ~15 (Ml)-1)
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Inorganic Chemistry
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Synthesis of new compound
from green chromium(III) chloride
–
–
–
–
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 a
brown mass
– mixture dissolves in dilute hydrochloric acid with a
yellowish brown colour
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Inorganic Chemistry
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New yellow compound
• Recrystallised from dilute HCl (separation by filtration)
1,2-ethanediamine abbreviated
as ”en”
• Formula: [Cren3]Cl3·3H2O
• Name: tris(1,2-ethanediamine)chromium(III) chloride trihydrate
• Two properties
– Light sensitive (darkens and degrades)
– Soluble in water
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Inorganic Chemistry
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6
Absorption spectrum
aqueous solution
tris(1,2- ethanediamine)chromium(III)
Molar absorptivity
80
70
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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Inorganic Chemistry
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Octahedral Geometry
A regular octahedron with
• 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
with equal distances from the centre
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Inorganic Chemistry
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Symmetry
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Inorganic Chemistry
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7
Ligand field model
(understanding of absorption spectrum)
• ν1(4A2g(F) →4T2g(F)=
21600 cm-1
•
ν2(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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Thermodynamics
• Equilibrium:
[Cr(H2O)6]3+ + 3en  [Cren3]3+ + 6H2O
3
3 
[Cren3 ]
3
[Cr (H 2O) 6 ]  [en]3
logβ3= 19.4 (24 oC; 1.00M NaCl) ; logK1 = 6.43
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Inorganic Chemistry
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Other properties
•
•
•
•
•
•
•
Magnetic behaviour
Crystal form
Distances and angles between atoms
IR spectrum
Reactivity
Redox behaviour
and so on……..
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Inorganic Chemistry
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8