Download the general features of transition metal chemistry

A2 CHEMISTRY NOTES – Transition Metals
WHAT IS A TRANSITION METAL?
The terms transition metal (or element) and __ block element are sometimes used as if
they mean the same thing. They don't:
d block elements
The 3s and 3p levels are full, but next the ___ level fills instead to give potassium and
then calcium. Only after that do the 3d levels fill.
The elements in the Periodic Table which
correspond to the d levels filling are
called _ ______ ________.
Sc
Ti
V
Cr
Mn
Fe
Co
Ni
Cu
Zn
[Ar] 3d14s2
You will notice that the pattern of filling
isn't entirely tidy! It is broken at both
chromium and copper.
[Ar] 3d104s2
A transition metal is one, which forms one or more stable ions, which have
_______________ __________ __ __________
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A2 CHEMISTRY NOTES – Transition Metals
Transition metal ions
 The ___ orbital is filled _______ the 3d orbitals as it is initially lower in energy.
 The 4s electrons then become higher in energy and are the _________ electrons
4s
3d
4s
3d
Not all d block elements count as transition metals!
Scandium and _____ are d-block metals but not transition metals.
Scandium has the electronic structure [Ar] ________. When it forms ions, it always loses
the __ ________ electrons and ends up with an argon structure. The Sc3+ ion has __ __
__________ and so doesn't meet the definition.
Zinc has the electronic structure [Ar] ________. When it forms ions, it always loses the two
4s electrons to give a 2+ ion with the electronic structure [Ar] ____. The zinc ion has ___
__ _________ and doesn't meet the definition either.
By contrast, copper, [Ar] ______, forms two ions. In the Cu+ ion the electronic structure is
[Ar] _____. However, the more common Cu2+ ion has the structure [Ar] ____. Copper is
definitely a ____________ metal because the Cu2+ ion has an __________ d shell.
Remember this:
When d-block elements form ions, the 4s electrons are lost ______.
To write the electronic structure for Co2+:
Co
Co2+
The 2+ ion is formed by the loss
of the two 4s electrons
To write the electronic structure for V3+:
V
V3+
The 4s electrons are lost first
followed by one of the 3d electrons
Transition metals have specific properties and these are:
1. Variable oxidation state (number)
2. The formation of complex ions
3. The formation of coloured compounds
4. Catalytic activity
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A2 CHEMISTRY NOTES – Transition Metals
VARIABLE OXIDATION STATE (NUMBER)
Transition metal chemistry show a wide range of oxidation numbers. Other than transition
metals only LEAD and TIN show variable oxidation state to any extent
Sc
Ti
V
Cr
Mn
Fe
Co
Ni
Cu
Zn
Explaining the variable oxidation states in the transition metals
Usually when a metal forms the compound forms the most stable compound which is the one
in which most _______ is released. The more energy released, the more ______ the
compound. The key energy changes are:
 The amount of energy needed to _______ the metal
 The amount of energy released when the compound forms. This will either be
________ enthalpy if you are thinking about solids, or the ____________
enthalpies of the ions if you are thinking about solutions.
The more highly charged the ion, the more ____________ you have to remove and the
more __________ energy you will have to provide. But off-setting this, the more highly
charged the ion, the more energy is _________ either as ________ enthalpy or the
___________ enthalpy of the metal ion.
Transition metals have variable oxidation states as the 4s and 3d
electrons have _________ energies.
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A2 CHEMISTRY NOTES – Transition Metals
Redox Titrations
Manganate (VII)
Potassium permanganate is an oxidising agent and has a rich
_________ colour. If it is reduced in the presence of dilute
____, usually ________ acid it becomes Mn2+, a ____ solution
and so when it is used in a titration it is self indicating.
In the burette: ___________________________
In the conical flask: _______________________________
End point: Purple colour of permanganate ______ persists.
Oxidation half equation:
Reduction half equation:
Full Equation:
Mole Ratio: _______
Dichromate (VI)
Acidified potassium dichromate is an oxidizing agent and it is
_______ in colour. If it is reduced in the presence of dilute
________, usually _________ acid it becomes a ______ solution of
____. This end point would be difficult to see so an indicator is used.
In the burette: _________________________
In the conical flask: ________________________
End point: Depends on the _________________.
Oxidation half equation:
Reduction half equation:
Full Equation:
Mole Ratio: _______
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A2 CHEMISTRY NOTES – Transition Metals
Thiosulphate
A solution of thiosulphate ions (S2O32-) acts as a reducing
agent. It is a ______ solution and is often used to find out the
concentration of _________ solutions. The iodine is reduced
to _______ and the thiosulphate oxidized to tetrathionate
When nearing the end point of these titrations is reached, and
the brown colour of the iodine fades to _______, a ______
indicator is added. This forms a ____-_______ colour with
iodine. The ____-_______ colour just disappears as the end
point of the titration is reached
In the burette: _____________________
In the conical flask: ___________ (starch indicator added when colour fades to yellow)
End point: __________________________________________________
Oxidation half equation:
Reduction half equation:
Full Equation:
Mole Ratio: ______
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A2 CHEMISTRY NOTES – Transition Metals
FORMATION OF COMPLEX IONS
A complex ion has a metal ion at its centre with a number of other molecules or ions
called ligands surrounding it. Ligands use their _____ pairs of electrons to form
__-___________ bonds with the metal ion. All ligands are lone pair _______.
 The ligand is acting as a Lewis _______ – electron pair ______
 The metal ion is acting as a Lewis ______ - electron pair ________
A ligand is a molecule or ions that donate an ________ pair to a central metal ion
thus forming a __-__________ bond
Co-ordination Number
Co-ordination number is the number of pairs of ________ donated to the central metal
ion and is usually 2, 4 or 6.
Unidentate ligands donate one electron pair water, ammonia
and halide ions.
H2O and NH3 are uncharged and similar in ____
Bidentate ligands donate ____ electron pairs
H2NCH2CH2NH2, C2O42-
Multidentate ligands
[Ni (NH2CH2CH2NH2)3]
2+
[Cr (C2O4)3] 3-
[Cu(EDTA)]2-
Co-ordination number of
these complexes is __
Complexes containing bidentate and multidentate ligands are called ____________.
___________ are more stable than complexes and the chellation process has positive
__________.
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A2 CHEMISTRY NOTES – Transition Metals
THE SHAPES OF COMPLEX METAL IONS
linear [Ag(NH3)2]+
2-co-ordinated complex ions
tetrahedral [CrCl4]4-co-ordinated complex ions
square planar [Pt(NH3)2Cl2]
4-co-ordinated complex ions
octahedral [Co(NH3)6]3+, [Cr(H2O)6]2+ and other aqua complexes
6-co-ordinated complex ions
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A2 CHEMISTRY NOTES – Transition Metals
NAMING COMPLEX METAL IONS
The names of complex ions are long, but formulae are simply
systematic in much the same way that organic names are
systematic. The table shows the code for some ligands,
multipliers and for metal anions.
ligand
H2O
NH3
OHCl-
coded by
AQUA
AMMINE
HYDROXO
CHLORO
no of
ligands
coded
by
2
3
4
5
6
DI
TRI
TETRA
PENTA
HEXA
Metals in
anionic
changed to
complexes
cobalt
COBALTATE
aluminium ALUMINATE
chromium CHROMATE
vanadium
VANADATE
copper
CUPRATE
iron
FERRATE
Only one type of ligand:
E.g [Cu(H2O)6]2+ is called the __________copper(II) ion.
two "a"s next to each
other in the name is OK)
More than one type of ligand in an ion:
The ligands are named in ___________ order - ignoring the prefixes. [Cu(NH3)4(H2O)2]2+
is called the _____________________(II) ion. The "ammine" is named before the
"aqua" because "am" comes before "aq" in the alphabet. The "tetra" and "di" are ignored.
Naming the metal
For positively charged complex ions
A positively charged complex ion is called a _________ complex. The metal in this is
named exactly as you would expect, with the addition of its oxidation state. E.g.
[Cu(H2O)6]2+ is called the _______________(II) ion because the copper's oxidation
state is +2.
For negatively charged complex ions
A negatively charged complex ion is called an ___________ complex.. In this case the
name of the metal is modified to show that it has ended up in a negative ion. This is
shown by the ending -ate. With many metals, the basic name of the metal is changed as
well - sometimes drastically! Common examples include:
[CuCl4]2-. _____________________________ ion.
[Al(H2O)2(OH)4]- ___________________________ ion.
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A2 CHEMISTRY NOTES – Transition Metals
Some Important Complexes
Cisplatin
Cis-platin
Use:
Side effects:
trans[PtCl2(NH3)2]
As an __________ drug
nausea, vomiting, allergic reactions, hearing loss, kidney damage
Hemoglobin
The Role of Fe2+
The haem part of haemoglobin is a ___ (II) complex with a
multidentate ligand. The Fe2+ is central to the action of ____
blood cells in carrying ___________.
Why is carbon monoxide toxic?
Oxygen binds ___________ to the Fe2+ ion at the
centre of the hemoglobin complex. Carbon monoxide,
however binds strongly to the complex and thus prevents
________ being carried round the body.
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A2 CHEMISTRY NOTES – Transition Metals
THE COLOURS OF COMPLEX METAL IONS
Why is copper(II) sulphate solution blue?
If white light passes through copper(II) sulphate solution,
some wavelengths are absorbed by the solution. Copper(II)
ions in solution absorb light in the ___ region of the spectrum.
The light which passes through the solution and out the other side
will have all the colours in it except for the ___. We see this
mixture as _____, the complementary colour of ___. This is a
mixture of the remaining wavelengths
The formation of coloured compounds
When the ligands bond with the transition metal ion
the d electron energy levels _____ into two groups.
When white light is passed through a solution of this
ion, some of the energy in the light is used to
promote an _______ from the lower set of orbitals
into a space in the upper set. The ______ light that
is absorbed leaves other colours to pass through.
ΔE = h
Where
ΔE is the enery ___ between the split d orbitals
h is Planks constant (6.626 x 10-34Js)
 is the frequency of light absorbed in Hz
Attaching ligands to a metal ion splits the energies of the d orbitals. Light is
absorbed as electrons move up between one __ orbital and another.
Some common examples
The size of the energy gap between them varies
with the nature of the transition ______ ion, its
__________ state, the __-_________ number and
the nature of the ____________.
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A2 CHEMISTRY NOTES – Transition Metals
Colorimetry
A spectrometer measures the degree of
absorbance of a solution and can be used to find
the ______________ of a solution.


a filter is used which is usually a _____________ colour to the solution to make
the readings more sensitive.
a ____________ curve plotted with samples of ________ concentrations.
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A2 CHEMISTRY NOTES – Transition Metals
CATALYTIC ACTIVITY
Transition metals and their compounds function as catalysts either because of their ability
to change ____________ state or, in the case of the metals, to adsorb other substances
on to their __________ and activate them in the process.
Transition metals as heterogeneous catalysts
Heterogeneous catalysts are in a different _______ to
the reactants. The reaction occurs on the ________ of
the catalyst and provides an _____________ route with
a _________ activation energy
Use of Catalyst Supports
Transition metal catalysts can be very costly and since it
only the ________ that is the active part of the catalyst
a support can be used on which the transition metal is
coated. The support provides a large ________ ______.
Poisoning of Catalysts
There are some substrates that bind strongly to the catalyst surface and blocks the
________ sites. This is called poisoning of the catalyst. It happens with sulphur
particles in the Haber Process and lead particles can do the same with the palladium
catalyst in catalytic converters. This means that the catalyst is not working as
efficiently as it should and will have implications in terms of the overall _____ of the
process as it may take ________ and produce _____ product.
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A2 CHEMISTRY NOTES – Transition Metals
Iron in the Haber Process
2N2(g) + 3H2(g)
The Haber Process combines hydrogen and
nitrogen to make ___________ using an
_____ catalyst.
2NH3(l)
Iron
Chromium oxide in the production of methanol
Methanol is used a _____ and a ________ and it can
be produced from reaction of carbon monoxide and
hydrogen.
Vanadium(V) oxide in the Contact Process
At the heart of the _________ Process is a
reaction which converts sulphur dioxide into
sulphur trioxide. Sulphur dioxide gas is passed
together with air (as a source of oxygen) over a
solid vanadium(V) oxide catalyst.
The ability of vanadium to change it’s
_________ state allows it to catalyse this
reaction
CO(g) + 2H2(g)

CH3OH(l)
Cr2O3
2SO2(g) + ½ O2(g)
SO2(g) + V2O5(s)
V2O5
2SO3(g)
 SO (l) + V O (s)
3
2
4
The sulphur dioxide is oxidised to sulphur _________ by the vanadium(V) oxide. In the
process, the vanadium(V) oxide is _________ to vanadium(IV) oxide.
The vanadium(IV) oxide is then re-oxidised by _______.
Although the catalyst has been temporarily changed
during the reaction, at the end it is chemically the same
as it started.
13
V2O4(s)+ ½ O2(g)
 V O (s)
2
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A2 CHEMISTRY NOTES – Transition Metals
Transition metalcompounds as homogeneous catalysts
Homogeneous catalysts are in the same ______ as
the reactants and work by forming an intermediate
state with a different oxidation number. This
route has a _______ activation energy than the
uncatalysed route
Fe3+ catalyst
Iron(II) ions act as a homogenous catalyst for the reaction between peroxodisulphate
and iodide ions
S2O82-(aq) + 2eFe3+(aq) + eI2(aq) + 2e2-
2SO42-(aq) +2.01
Fe2+ (aq)
+0.77
2I (aq)
+0.54
The oxidation of Fe2+ -by the peroxodisulphate will be the first stage of the reaction
S2O82-(aq) + 2Fe2+(aq)  2Fe3+ (aq) + 2SO42-(aq)
E = _____________
The Fe3+ ions are then reduced by the iodide ions
2I-(aq) + 2Fe3+(aq)  2Fe2+ (aq) + I2(aq)
E = _____________
Autocatalysis
The manganate (VII) ions produce Mn2+, carbon dioxide gas and water when it reacts with
acidified aqueous ethandiote ions.
(notice that oxidizing and reducing agents are both negatively charged which makes a
straightforward route kinetically unfavoured.)
The reaction is initially slow but gets faster as the Mn2+ produced catalyses the reaction
as follows:
The ethanedioate ion then reduces the Mn3+ ion
5Mn3+(aq) + 5C2O42-(aq)  5Mn2+(aq) + 10CO2(g)
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A2 CHEMISTRY NOTES – Transition Metals
Hexaqua ions (6 water ligands around a central ion)
The positive charge on the transition metal ions attract _____ water molecules. The
ions have vacant _______ which allow a __-_________ bond to form. The small size of
the water molecules means there is room for __ waters around the ion.
M2+
[Cu(H2O)6]2+
[Co(H2O)6]2+
[Fe(H2O)6]2+
Colour
Colour
3+
[Al(H2O)6]
[Cr(H2O)6]3+
[Fe (H2O)6]3+
The acidity of the hexaaqua (III) ions: Hydrolysis Reactions
Cr3+
3+ ions formed in solution are ________ and have a
pH in the ___- ___ range.
The small ______ and the large _______ make the M3+ ion
very _________. A _________ ion is lost from one of the
ligand water molecules as the M3+ ion pulls electron density
away from the O-H bond in the water ligands.
The complex ion is acting as an ______ by donating a hydrogen ion to water molecules in the
solution. The water is acting as a ________ by accepting the hydrogen ion.
For chromium
For Iron
For Aluminium
The acidity of the hexaaqua (II) ions
The +2 ions are not as polarizing as they have larger radii and smaller charges and so they
are not able to pull electron density from the O-H bond as strongly as the +3 ions. These
complexes are less acidic.
For Iron
For Cobalt
For Copper
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A2 CHEMISTRY NOTES – Transition Metals
Hydrolysis Reactions Of Hexaaqua Metal Ions With Hydroxide Ions
The hydroxide ion removes a hydrogen ion from one of the ligand water molecules. This is
an hydrolysis reaction.
Adding OH-(aq) ions to 2+ hexaaqua ions
16
Adding OH-(aq) to 3+ hexaaqua ions
A2 CHEMISTRY NOTES – Transition Metals
Reaction of complex ions with NaOH(aq)
hexaaquacobalt(II)
hexaaquacopper(II)
hexaaquairon(II)
Hexaaquaaluminium
hexaaquairon(III)
hexaaquachromium(III)
Reversing the process
If adding hydroxide ions removes hydrogen ions from the hexaaqua complex one at a
time, it doesn't seem unreasonable that you could put them back again by adding an
______. That's just what happens!
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A2 CHEMISTRY NOTES – Transition Metals
Reactions Of Hexaaqua Metal Ions With Carbonate Ions
The general case for 2+ ions
These have the form [M(H2O)6]2+. They are only very weak _______ and so when the
carbonate ions are added they form the metal(II) carbonate which is _______ in water:
[M(H2O)6]2+(aq) + CO32-(aq)  MCO3(s) + 6H2O(l)
Copper (II)
[Cu(H2O)6]2+(aq) + CO32-(aq)  CuCO3(s) + __H2O(l)
_______
__________
Cobalt (II)
[Co(H2O)6]2+(aq) + CO32-(aq)  CoCO3(s) + __H2O(l)
_______
__________
Iron (II)
[Fe(H2O)6]2+(aq) + CO32-(aq)  FeCO3(s) + __H2O(l)
_______
__________
The general case for 3+ ions
These have the form [M(H2O)6]3+. They are able to act as ________.
[M(H2O)6]3+(aq) +3H2O(l)
[M(H2O)3(OH)3](s) + 3H3O+ (aq)
In this case the carbonate react as a base with two H3O+(aq) to form ____ and water
CO32-(aq) + 2H3O+(aq)  3H2O(l) + CO2(g)
Overall the reaction is
2[M(H2O)6]3+(aq) + 3CO32-(aq)  [M(H2O)3(OH)3](s) + 3H2O(l) +3CO2
Aluminium
2[Al(H2O)6]3+(aq) + 3CO32-(aq)  2[Al(H2O)3(OH)3](s)+ 3H2O(l) +3CO2(g)
_______
__________
Chromium
2[Cr(H2O)6]3+(aq) + 3CO32-(aq)  2[Cr(H2O)3(OH)3](s)+ 3H2O(l) +3CO2(g)
_______
__________
Iron(III)
2[Fe(H2O)6]3+(aq) + 3CO32-(aq)  2[Fe(H2O)3(OH)3](s)+ 3H2O(l) +3CO2(g)
_______
__________
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A2 CHEMISTRY NOTES – Transition Metals
Amphoteric hydroxides
An amphoteric substance has both _______ and _____ properties. In other words, it
will react with both bases and acids. Some of the metal hydroxides we've been looking at
are doing exactly that.
Chromium(III) hydroxide as an amphoteric hydroxide
With bases
[Cr(H2O)3(OH)3] (s) + 3OH-(aq)
[Cr(OH)6]3-(aq) + 3H2O(l)
With acids
[Cr(H2O)3(OH)3] (s) + 3H3O+-(aq)
[Cr(H2O)6]3+(aq) + 3H2O(l)
Aluminium hydroxide as an amphoteric hydroxide
With bases
[Al(H2O)3(OH)3] (s) + OH-(aq)
[Al(OH)4]-(aq) + 3H2O(l)
With acids
[Al(H2O)3(OH)3] (s) + 3H3O+-(aq)
[Al(H2O)6]3+(aq) + 3H2O(l)
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A2 CHEMISTRY NOTES – Transition Metals
Ligand Exchange Reactions with Ammonia
This type of reaction involves breaking the co-ordinate bond and replacing it with a
co-ordinate bond from a different ligand. Ammonia is similar in size to water and so the
final product is a hexammine complex. The initial reaction of the however is a hydrolysis
reaction to form the hydroxide precipitate. Some examples suggested by the syllabus:
Cobalt (+2 ion)
[Co(H2O)6]2+ + 2NH3(aq)
[Co(H2O)4(OH)2 ](s) + 2NH4+(aq)
When excess ammonia is added the hydroxide precipitate
dissolves and the H2O ligands are replaced by the NH3 ligands
[Co(H2O)6]2+ + 6NH3(aq)  [Co(NH3)6]2+(aq) + 6H2O(l)
However this substitution process can be incomplete and any of the following complexes
could be in existence in solution:
[Co(H2O)5(NH3) ], [Co(H2O)4(NH3)2 ], [Co(H2O)3(NH3)3 ], [Co(H2O)2(NH3)4], [Co(H2O) (NH3)4]
Chromium (+3 ion)
[Cr(H2O)6]3+ + 3NH3(aq)
[Cr(H2O)3(OH)3 ](s) + 3NH4+(aq)
With excess ammonia
[Cr(H2O)6]3+ + 6NH3(aq)  [Cr(NH3)6]3+ (aq) + 6H2O(l)
Copper (+2 ion that does not get fully substituted)
[Cu(H2O)6]2+ + 2NH3(aq)
[Cu(H2O)4(OH)2 ](s) + 2NH4+(aq)
Diagram of the
tetraamminediaquacopper (II)
complex
With excess ammonia just 4 of the water ligands are replaced
[Cu(H2O)6]2+ + 4NH3(aq)  [Cu(H2O)2 (NH3)4]2+(aq) + 4H2O(l)
Complex
Colour
2+
[Co(NH3)6] (aq)
[Cr(NH3)6]3+ (aq)
[Cu(H2O)2 (NH3)4]2+(aq)
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A2 CHEMISTRY NOTES – Transition Metals
Ligand Exchange Reactions with Chloride ions




Chlorine ions are larger than water and so the final complex has a co-ordination
number of 4.
The complex will also change in charge as the chloride ion is charged.
Concentrated hydrochloric acid is used as a source of chloride ions.
These reactions are reversible and adding excess water moves the equilibrium back
to the hexaqua ion.
Cobalt
[Co(H2O)6]2+ + 4Cl-(aq)
________
[CoCl4]2-(aq) + 6H2O(l)
_______
Copper
[Cu(H2O)6]2+ + 4Cl-(aq)
________
[CuCl4]2-(aq) + 6H2O(l)
_______
Some more complex reactions
Chromium
Common Ions: Cr2+ [Ar]
, Cr3+[Ar]
, Cr6+[Ar]
Aqueous potassium chromate is commonly used as an oxidizing agent in organic chemistry.
The dichromate ion is reduced to chromium(III)
Cr2O72-(aq) + 14H+(aq) +6e-  2Cr3+(aq) + 7H2O(l)
________
_______
The chromate(VI)-dichromate(VI) equilibrium
Changing between the orange dichromate(VI) ion, Cr2O72- and the
yellow chromate(VI) ion, CrO42- is easy:
If you add dilute sulphuric acid to the yellow solution it turns
orange. If you add sodium hydroxide solution to the orange
solution it turns yellow.
The equilibrium reaction at the heart of the interconversion is:
.
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A2 CHEMISTRY NOTES – Transition Metals
Reduction of Dichromate ions
Aqueous potassium chromate is reduced with ____ and
_____________ acid first to the __ oxidation state.
+6 to +3
Cr2O72-(aq) + 14H+(aq) +6e-  2Cr3+(aq) + 7H2O(l)
Zn (s)  Zn2+ + 2e
3Zn (s) + Cr2O72-(aq) + 14H+(aq)  2Cr3+(aq) + 7H2O(l) + 3Zn2+
BUT 2Cr3+(aq) is [Cr(H2O)6]3+ and this undergoes ligand exchange with Cl- ions
[Cr(H2O)6]3+(aq)
+ 2Cl- (aq)
[Cr Cl2 (H2O)4]+1(aq)
+3 to +2
[Cr Cl2 (H2O)4]+1(aq)
________
[Cr(H2O)6]2+(aq)
______
But if any air is present
+2 to +3
[Cr(H2O)6]2+(aq)
_____
[Cr Cl2 (H2O)4]+1(aq)
_______
Oxidation in alkaline solution
+3 to +6
There are 3 stages in oxidizing the Cr(III) to a yellow
solution of CrO42-with H2O2. A _____ precipitate forms
in the first stage
[Cr(H2O)6]3+(aq) + 3OH-(aq)  Cr(OH)3(s) + 3H2O(l)
The ______ precipitate re-dissolves in excess
hydroxide forming a dark green solution
Cr(OH)3(aq) + 3OH-(aq)  Cr(OH)6(aq) + 3H2O(l)
H2O2 is added to the green solution and the mixture is
warmed giving the ________ solution.
Cr(OH)6(aq) + 3H2O2 (aq)  CrO42-(aq) + 8H2O(l) + 2OH-
22
+ 2H2O(l)
A2 CHEMISTRY NOTES – Transition Metals
Cobalt
Oxidation in alkaline solution
+2 to +3
A blue-green precipitate forms when hydroxide is added to cobalt(II) solution
[Co(H2O)6]3+(aq) + 2OH-(aq)  [Cr(H2O)4(OH)2](s) + 2H2O(l)
Hydrogen peroxide is added to the blue-green precipitate and the mixture is warmed
forming a dark brown precipitate.
2[Co(H2O)4(OH)2](s) + H2O2 (aq)  2[Co(H2O)3(OH)3](s) + 2H2O(l)
Oxidation by air with ammonia
+2 to +3
There are 3 stages in oxidizing the pink Co(II) solution to a dark brown mixture
containing hexaamiminebobalt(III).
A blue-green hydroxide precipitate forms in the first stage
[Cr(H2O)6]3+(aq) + 2NH3(aq)  [Co(H2O)4(OH)2](s) + 2NH4+(l)
The blue-green precipitate re-dissolves in excess ammonia forming a straw coloured
solution of hexamminecobalt(II) solution
[Co(H2O)4(OH)2](s) + 6NH3(aq)  [Co(NH3)6]2+(aq) + 4H2O(l) +2OH-(aq)
The air oxidizes the straw coloured hexamminecobalt(II) solution to yellow
hexamminecobalt(III) – the solution actually appears to be dark brown due to the
presence of other ions
[Co(NH3)6]2+(aq)  [Co(NH3)6]3+(aq) + e-
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