Lecture 19. The d-Block Elements.
IV-VI B groups
PhD. Halina Falfushynska
d-Block Transition Elements
IIIB
IVB
VB
Sc
Ti
V
Y
La
VIIIB
VIB VIIB
IB
IIB
Cr
Mn
Fe
Co
Ni
Cu
Zn
Zr
Nb Mo
Tc
Ru
Rh
Pd Ag
Cd
Hf
Ta
Re
Os
Ir
Pt
Hg
W
Au
4p
3d
4s
3p
3s
2p
2s
1s
Cr
1s2 2s2 2p6 3s2 3p6 3d5 4s1
SS CI 11.5 The d block
3
Extraction of chromium
• Chromite, FeCr2O4, is the most commercially
useful ore.
• Chromium ore is processed and purified into
chromium(III) oxide. This is reacted, very
exothermically, in a thermit style reaction, with
aluminium or silicon to free the chromium metal.
• Cr2O3(s) + 2Al(s) ==> Al2O3(s) + 2Cr(s)
• 2Cr2O3 + 3Si → 4Cr + 3SiO2
Chemical reactivity and trends
• Chromium reacts more readily with acids then
does either molybdenum or tungsten though
its reactivity depends on its purity and it can
easily be rendered passive.
Reactivity
towards halogens
CrX3 are prepared from Cr
with X2, dehydration of
CrCl3.6H2O requires SOCl2 at
650C.
Cr (s) + Cl2 (g) → CrCl3 (s)
Reduction of CrX3 with
H2/HX gives CrX2.
anhydrous CrCl3 and hydrated "CrCl3.6H2O",
Reactivity of chromium towards
oxygen and preparation
chromium oxides
Chromium reacts with
oxygen to form oxides
2Cr + 3О2  Cr2О3 (400 oC)
2Cr + 3Н2О  Cr2О3 + 3Н2
(600 oC)
(NH4)2Cr2O7  Cr2O3 + N2 +
4H2O
non-metals and metals
Cr(s) + S8(s) → Cr2S3(s)
Nitrides: CrN, Cr2N;
Carbydes: Cr4С, Cr7С3,
Cr3С2
Intermetalides: FeCr2,
CrMn3
Pb(NO3)2 and K2Cr4 reaction
Sodium Hydroxide To Chromic
Nitrate
Reactivity of chromium and elements of its
group towards acids
It dissolves readily in dil HCl but, if very
pure, will often resist dil H2SO4; again,
HNO3, whether dilute or concentrated.
Cr + 2HCl  2CrCl2 + H2
2CrCl2 + 1/2O2 + 2HCl  2CrCl3 + H2O.
2Сr + 6H2SO4 (d) Cr2(SO4)3 + 3SO2 + 6H2O
Cr + HNO3  no reaction
Decomposition of chromium
compounds
•
•
•
•
(NH4)2Cr2O7  Cr2O3 + N2 + 4H2O
4Na2Cr2O7  4Na2CrO4 + 2Cr2O3 + 3O2
Cr2(CO3)3 + heat  Cr2O3 + CO2
Cr2(NO3)3  Cr2O3 + NO2 + O2
A Conceptual Example
Write a plausible equation to explain the reaction shown
in Figure, in which pure ammonium dichromate ignited
with a match produces pure chromium(III) oxide.
Chromium oxides properties
• Cr2O3 + 2Al  Al2O3 + 2Cr
• Cr2O3 + 3K2S2O7  Cr2(SO4)3 + 3K2SO4;
• Cr2O3 + 3NaNO3 + 2Na2CO3  2Na2CrO4 +
3NaNO2 + 2CO2;
• 5Cr2O3 + 6NaBrO3 + 10NaOH  10Na2CrO4 + 3Br2
+ 7H2O;
Chromate and Dichromate Ions
In acid
In base
Cr(OH)3 Is Amphoteric
CHROMIUM(VI) oxidation state
chemistry
• When hydrogen peroxide is added to an alkaline
chromium(III) solution, oxidation occurs to give the
yellow chromate(VI) ion CrO42- .
– 2Cr3+(aq) + 3H2O2(aq) + 10OH-(aq) ==> 2CrO42-(aq) + 8H2O(l)
– Both H2O2 and Cr(VI) compounds are oxidising agents but in
alkaline solution H2O2 is the stronger oxidising agent.
• When the resulting solution from above is acidified
with dilute sulphuric acid, the orange dichromate(VI)
ion Cr2O72- is formed.
• 2CrO42-(aq) + 2H+(aq) Cr2O72-(aq) + H2O(l) (no change in ox.
state)
CHROMIUM(VI) oxidation state
chemistry
• The dichromate(VI) ion is reduced in two stages
by a zinc/dilute sulphuric acid mixture. Cr(VI, +6)
==> Cr(III, +3)
Cr2O72-(aq) + 14H+(aq) + 6e- 2Cr3+(aq) + 7H2O(l)
• orange (+6) ==> green (+3), EØ = +1.33V
– Cr(III, +3) ==> Cr(II, +2): Cr3+(aq) + e- Cr2+(aq)
• green (+3) ==> blue (+2), so Cr(II) is readily oxidised.
Cr2O72-(aq) + 3Zn(s) + 14H+(aq) 2Cr3+(aq) + 3Zn2+(aq) + 7H2O(l)
2Cr3+(aq) + Zn(s) 2Cr2+(aq) + Zn2+(aq)
Potassium dichromate(VI), K2Cr2O7
• It can be crystallised to high purity standard
without water of crystallisation, and is a
valuable 'standard' redox volumetric reagent.
– It can used to titrate iron(II) ions in solution
acidified with dilute sulphuric acid, using a
redox indicator like barium diphenylamine
sulphonate(blue colour).
Cr2O72-(aq) + 14H+(aq) + 6Fe2+(aq) ==> 2Cr3+(aq) +
6Fe3+(aq) + 7H2O(l)
Potassium dichromate(VI), K2Cr2O7
The dichromate(VI) ion is a strong oxidising agent examples of oxidising action. It oxidises iodide
ions to iodine.
• Cr2O72-(aq) + 14H+(aq) + 6I-(aq) ==> 2Cr3+(aq) +
3I2(aq) + 7H2O(l)
– The released iodine can be titrated with standard
sodium thiosulphate solution using starch
indicator.
• 2S2O32-(aq) + I2(aq) ==> S4O62-(aq) + 2I(aq) (black/brown ==> colourless endpoint)
CHROMIUM(VI) oxidation state chemistry
• Dichromate and chromate equilibria is pH
dependent:
• HCrO4- → CrO42- + H+ K=10-5.9
H2CrO4 → HCrO4- + H+ K=10+0.26
Cr2O72- + H2O → 2HCrO4- K=10-2.2
HCr2O7- → Cr2O72- + H+ K=10+0.85
• Hence the variation found for solutions of CrO3 are:
• pH > 8 CrO42- yellow
pH 2-6 HCrO4- and Cr2O72- orange-red
pH < 1 H2Cr2O7
Test reaction for Chromium
• Cr2(SO4)3 + 3(NH4)2S + 6H2O  2Cr(OH)3 +
3(NH4)2SO4 + 3H2O – grey-green ppt;
• 3Na2CO3 + 2CrCl3 + 3H2O  2Cr(OH)3 + 3CO2
+ 6NaCl
• Na2CrO4 + AgNO3  Ag2CrO4 + 2NaNO3 –
brown-reddish ppt;
• Na2CrO4 + (CH3COO)2Pb  PbCrO4 +
2CH3COONa – yellow ppt
Chromium usage
Biological role of chromium
• Chromium is an essential trace element in
mammalian metabolism. In addition to
insulin, it is responsible for reducing blood
glucose levels, and is used to control certain
cases of diabetes. It has also been found to
reduce blood cholesterol levels by diminishing
the concentration of (bad) low density
lipoproteins "LDLs" in the blood.
Biological role of chromium and its
uses in pharmacy
• Chromium is an essential trace element.
Chromium plays a role in the metabolism of
glucose, and is necessary for energy production
• Chromium picolinate or Chromium Nicotinate
Complex influences carbohydrate, fat and
protein metabolism. It assists in the
management of fluctuating blood sugar levels in
healthy people. A higher dietary intake of
refined carbohydrates needs more chromium.
Source of chromium