CATALYTIC ACTIVITY Some reactions catalysed by transition elements/their compounds. process reaction Catalyst Contact process 2SO 2 (g)+O 2 (g)2SO 3 V 2 O 5 Haber process N 2 (g)+3H 2 (g)2NH 3 (g) Fe(s) Hydrogenation CH 2 =CH 2 (g)+H 2 (g)CH 3 CH 3 Ni(s) Thermal decomposition 2KCLO 3 (s)2KCL(s)+3O 2 (g) MnO 2 (S) Redox 2I - (aq)+S 2 O 8 2- (aq)I 2 (aq) +2SO 4 2- (aq) Fe 3+ (aq) Homogenous Catalysis 1. In homogenous catalysis, the catalyst and the reactants are in the same physical states. 2. Homogenous catalysis is usually explained by the intermediate product theory. 3. Using the oxidation of iodide ions by peroxodisulphate ions as an example. S 2 O 8 2 (aq) + 2I - (aq) ---------------------> 2SO 4 2- (aq) + I 2 (aq) a) The activation energy for an catalysed reaction is high because it involves the direct collision of negative charge ions,which tends to repel each each other. b) However ,in the presence of Fe 3+ (aq) as catalyst, the activation energy is lower because it involves the collision of opposite charged particles. 4. Transition elements and their compounds are good homogenous catalysts because they exhibit variable oxidation states. Heterogeneous catalysis Fe 3 +
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CATALYTIC ACTIVITY
Some reactions catalysed by transition elements/their compounds.
4. The equilibrium constant of the complex ion in solution is called the stability constant. All complexes
dissociate partially in solution according to the equation:
M(aq) + nL(aq) MLn(aq) where M = Transition metal ion
L = Ligand
n = Number of ligands
The equilibrium constant, K, of the reaction at constant temperature is given as:
K is the stability constant used to measure the stability of the complex ion with respect to its constituent
species. The larger the value of K, the higher the concentration of MLn in solution and thus the more
stable the complex.
5. Stability constant have a wide range of values. As such, thet are better expressed as their logarithm,
log10 K or simply lg K.
6. The table below shows the stability constant of some complex ions.
Complex ion Stability constant (K) lg K
[Fe(Cn)6]3- 1 x 1042 42
[Fe(Cn)6]4- 1 x 1037 37
[Co(NH3)6]3+ 4.5 x 1033 33.7
[AlF6]3- 6.7 x 1019 19.8
[Cu(NH3)4]2+ 1.4 x 1013 13.0
[CuCl4]2- 3.98 x 105 5.6
7. The stability of a complex depends on the following factors.
a) the oxidation state of the central metal ion
The higher the charge density of the central ion, the more stable is the complex.
eg : Iron (II) in [Fe(Cn)6]4- lg K = 24
Iron (II) in [Fe(Cn)6]3- lg K = 31
b) the size of the halide ion
The strength of the halide ion ligands increase in the order Cl- < Br- < I-.
Eg : [CdCl4]2- < [CdBr4]2- < [CdI4]2-
lg K = 2.3 lg K = 3.7 lg K = 6.1
c) the presence of polydentate ligands
The stability of complex increase significantly with the presence of polydentate ligands. As such,
hexadentate ligand like EDTA will produce a far more stable complex than monodentate and
bidentate ligsnds.
eg : [Co(NH3)6]2+ < [Co(H2NCH2CH2NH2)]2+ < [Co(EDTA)]2- g
lg K = 4.9 lg K = 10.6 lg K = 19.0
Even though four of lone pairs in EDTA are from nitrogen atoms as in ammonia, the binding effect is
much stronger than ammonia ligands. EDTA liberates six water molecules during bonding, resulting in a
strong ‘clawing’ effect on the complex and thus it is stable entropically.
USES OF TRANSITION METAL AND THEIR COMPOUNDS
Titanium
1. Titanium has the same mechanical strength of steel but it is lighter and does not corrode.
2. The main titanium ores are rutile, TiO2, and ilmenite which is a combination of rutile and iron(II) oxide,
FeTio3.
3. Extraction of titanium can be tedious because it combines strongly with non-metals like nitrogen,
oxygen and carbon. Titanium can be extracted by Kroll process which consists of four stages:
(a) Purification of oxide
The first stage involves the removal of impurities like clay and granite that come with the ore. Pure
titanium(IV) oxide, being amphoteric is obtained from rutile by dissolving it in concentrated sulphuric
acid. Titanyl sulphate, [TiO][SO4], is first formed.
However, upon dilution Ti(OH)4 is precipitated. It is then filtered off, washed and heated strongly to
obtain the oxide.
Ti(OH)4 (s) TiO2 (s) + 2H2O (l)
(b) Formation of titanium(IV) chloride
The second stage is to produce titanium(IV) chloride by heating the oxide with chlorine and carbon to
about 900°C.
TiO2 (s) + 2C (s) + 2Cl2 (g) TiCl4 (l) + 2CO2 (g)
Titanium(IV) chloride produced is a colourless, covalent liquid (boiling point 136°C) which fumes strongly
in moist air. It is then separated and purified through fractional distillation. It is kept in a bronze
container containing argon to prevent hydrolysis by moist air.
TiCl4 (l) + 2H2O (g) TiO2 (s) + 4HCl (aq)
(c) Reduction of titanium(IV) chloride
The titanium(IV) chloride stored under inert condition is then reduced by molten magnesium at 1000°C
to produce the titanium metal.
TiCl4 (g) + 2Mg (l) Ti (s) + 2MgCl2 (s)
(d) Purification
Titanium produced by the Kroll process needs to be processed further to remove impurities like
unreacted magnesium and side-product magnesium chloride. Magnesium chloride cam be recycled
through electrolysis, producing chlorine gas which can in turn be used to produce titanium(IV) chloride.
4.Titanium is used in the making of aircraft body, space capsules and nuclear reactors. It is added to
steel in the form of alloy with iron to remove combined oxygen and nitrogen. Titanium(IV) oxide is used
as white pigments in paints. It is also used as ‘filters’ for plastic and rubber.
Chromium
1. Chromium is a bright, shiny metal which forms a transparent yet stable oxide layer on its surface.
2. The only commercial important chromium ore is chromite, FeCr2O4.
3. If pure chromium is needed, the ore is first concentrated and converted to chromium(III) oxide. The oxide is then reduced by Thermite process, in which a powdered mixture of chromium(III) oxide and aluminium is ignited.
Cr2O3 (s) + 2Al (s) 2Cr (l) + Al2O3 (s)
The reaction is highly exothermic and the molten chromium metal produced settles at the bottom of the refractory container and collected through a pipe.
4. If it is used to produce ferrochrome alloy, then the concentrated chromite ore is reduced by carbon in an electric arc furnace.
FeCr2O4 (s) + 4C (s) Fe (l) + 2Cr (l) + 4CO (g)
Molten chromium is then mixed with nickel to produce heat-resistant alloys or with iron and nickel to produce stainless steels.
5. Chromium is used to harden steel and increase its resistance to corrosion. It is used in electroplating. Alloy of chromium with vanadium and tungsten is used in high speed cutting tools.
6. Dichromate such as K2Cr2O7 are oxidizing agents and are used in quantitative analysis and also in tanning leather.
7. Chromium is used by the aircraft and other industries for anodizing aluminium. The refractory industry uses chromite for forming bricks and shapes, as it has a high melting point, moderate thermal expansion, and stable crystalline structure.
Cobalt
1. Cobalt is a hard silvery-white metal. Cobalt is harder than iron, but melts at a lower temperature and is somewhat heavier.
2. Most cobalt is obtained as a by-product in smelting nickel or copper ores.
3. Cobalt is used in magnet steels and stainless steels. It is used in alloys used in jet turbines and gas turbine generators and electroplating because of its appearance, hardness, and resistance to oxidation.
4. Cobalt-60, an artificial isotope, is an important γ ray source, and is extensively used as a tracer and a radio therapeutic agent. Single compact sources of 60Co are readily available.
5. Different kinds of compounds cobalt are used as pigment in pottery, glass enamels and paints. Some compounds of cobalt are used as driers, substances that promote drying in paints, varnishes and printing inks.
Manganese
1. Manganese is a pinkish-gray, chemically active element. It is a hard metal and is very brittle. It is hard to melt, but easily oxidized. Manganese is reactive when pure, and as a powder it will burn in oxygen, it reacts with water (it rusts like iron) and dissolves in dilute acids.
2. Manganese shows a variety of oxidation states (+2 to +7). The most stable oxidation state is +2 which is always ionic. While +4 may be ionic or covalent, +6 and +7 are always covalent.
3. Steel becomes harder when it is alloyed with manganese. Manganese dioxide is used to manufacture ferroalloys and dry cell batteries. It is used to "decolorize" glass and to dry black paints. Manganese sulfate (MnSO4) is used as a chemical intermediate and as a micronutrient in animal feeds and plant fertilizers.
4. Manganese metal is used as a brick and ceramic colorant, in copper and aluminum alloys, and as a chemical oxidizer and catalyst. Potassium permanganate (KMnO4) is used as a bactericide and algicide in water and wastewater treatment, and as an oxidant in organic chemical synthesis.
GEOMETRY OF COMPLEX IONS
These shapes are for complex ions formed using monodentate ligands - ligands which only form one
bond to the central metal ion.
You will probably be familiar with working out the shapes of simple compounds using the electron pair
repulsion theory. Unfortunately that doesn't work for most complex metal ions involving transition
metals. The answer is just to learn the shapes you need to know about. As you will see, it isn't difficult.
6-co-ordinated complex ions
These are complex ions in which the central metal ion is forming six bonds. In the simple cases we are
talking about, that means that it will be attached to six ligands.
These ions have an octahedral shape. Four of the ligands are in one plane, with the fifth one above the
plane, and the sixth one below the plane.
The diagram shows four fairly random examples of octahedral ions.
4-co-ordinated complex ions
These are far less common, and they can take up one of two different shapes.
Tetrahedral ions
These are the ones you are most likely to need for A' level purposes in the UK. There are two very similar
ions which crop up commonly at this level: [CuCl4]2- and [CoCl4]2-.
The copper(II) and cobalt(II) ions have four chloride ions bonded to them rather than six, because the
chloride ions are too big to fit any more around the central metal ion.
A square planar complex
Occasionally a 4-co-ordinated complex turns out to be square planar. There's no easy way of predicting
that this is going to happen. The only one you might possibly come across at this level is cisplatin which
is used as an anti-cancer drug.
Cisplatin is a neutral complex, Pt(NH3)2Cl2. It is neutral because the 2+ charge of the original platinum(II)
ion is exactly cancelled by the two negative charges supplied by the chloride ions. The platinum, the two
chlorines, and the two nitrogens are all in the same plane.
Bonding In Complex Ions
1.In complex compounds, transition metals ions use empty orbitals (3s, 4s and 4p) to
form coordinate bonds with ligands. Ligands like chloride ion donates their lone pair of electrons and
occupy the available empty orbitals. For example, in complex ion [CuCl4]2- the copper(II) ion has four
empty 4s and 4p orbitals to accommodate the four lone pair of electrons from the chloride ions.
With four equivalent bonds, [CuCl4]2- ion has a tetrahedron shape.
Bonding in tetrahedral complex,[CuCl4]2-
2. For complex ions to form an octahedral, the single electron in the 3d orbitals paired up leaving six
empty orbitals. Initially, the iron(II) ion is paramagnetic (can be attracted by a magnetic field) caused by
unpaired electrons in the d suborbitals. After all the electrons in the d suborbitals are paired up, the six
empty orbitals are filled up by the lone electrons from the water or cyanide ligands. The complex ion is
no longer influenced by magnetic fields and is now called diamagnetic. For example, in
hexacyanoferrate(II) ion, [Fe(CN)6]4- ,
Types Of Ligands
Cl -
Cl -
Cl -
Cl Cu 2+
Cl
Cl
Cl Cu
2-
1. The molecules or ions that surround the centre metal ions through dative bonds are called ligands.
Ligands play the role of Lewis bases as they act as electron donors. On the other hand, the centre metal
ions are Lewis acids, acting as electron recipients.
2. Depending on the number of electron pairs donated, lagands are classified as monodentate,
bidentate or polydentate.
3. H2O, CN-, Cl- and NH3 are monodentate ligands donating only one donor pair of electron for
coordinate bonds.
4. Two known bidentate ligands are ethylenediamine (in short, en) and oxalate ion (in short, ox),
whereby there are two pairs of donor electrons in each ligand.
Figure below shows complexes with these two bidentate ligands.
5. One popular example of polydentate ligand is ethyenediamminetetraacetate ion(EDTA), a
hexadentate. There are six donor pairs forming a very stable complex ion.
The stability of the complex formed with polydentate ligands is due to chelating effect. Unlike cyanide
ion, EDTA is non-toxic. As such, it is useful madically as it can be used to treat metal poisoning such as
lead and cadmium. EDTA acts as sequestering agent, forming complex with the toxic lead ions and thus
dissolving it. It can be then removed from the blood and tissues by excreting from the body.