F325 Transition Elements (A2 Chemistry)

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  • Created on: 08-03-14 21:44
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Transition Elements
Typical Properties of transition elements
Variable oxidation states
The transition elements all exhibit two or more oxidation states in their compounds,
e.g. Iron: Fe2+ and Fe3+, chromium: Cr3+ and Cr7+
The ionisation energies of the transition elements are similar, with reasonably low
values. All the transition metals can form an ion of oxidation state 2, which
represents the loss of the two 4s-electrons.
Formation of coloured ions
In aqueous solutions:
Cu2+ Copper(II) ions ­ blue Cr3+ Chromium(III) ions ­ violet
Cr2O72- Dichromate (VI) ions ­ orange MnO42- Manganate ions ­ purple
2+
Fe Iron(II) ions ­ pale green Fe3+ Iron(III) ions - yellow
The ions are coloured is a consequence of the presence of an ion containing
incompletely filled d-orbitals that has an ion or molecule attached to it. Water is a
common attachment and it is this that makes aqueous copper(II) salts appear blue.
Use as catalysts
Transition metals are often used as heterogeneous catalysts. This is because a
transition metal can use its d-orbitals to bind other molecules or ions to its surface.
Production of ammonia in the Haber process ­ iron is used as catalyst.
Catalytic converter in cars ­ platinum, palladium and rhodium is used as catalyst.
Hydrogenation of alkenes ­ nickel is the catalyst.
Precipitation reactions
When NaOH(aq) is added to metal ions in aqueous solutions:
Cu2+(aq) + 2OH-(aq) -> Cu(OH)2 (s)
Light blue precipitate is formed. The blue solution of the copper salt
solution fades in colour.
Co2+(aq) + 2OH-(aq) -> Co(OH)2 (s)
Blue-green precipitate is formed. The pink colour of the Co2+(aq) fades.
Fe2+(aq) + 2OH-(aq) -> Fe(OH)2 (s)
DARK-GREEN precipitate is formed in the pale green aqueous solution
containing Fe2+. The solutions turns brown at the surface due to the
oxidation to Fe(OH)3 by air.
Fe3+(aq) + 3OH-(aq) -> Fe(OH)3 (s)
Rusty-brown precipitate is formed. Fe3+(aq) are yellow in dilute solution, and
appear orange in more concentrated solutions. Fe(OH)3 forms readily when
iron metal is exposed to wet conditions. It forms because the air promotes
oxidation on the surface of the iron to form iron(III) ions and there are
enough hydroxide ions in water to start the precipitation.
Ying G.

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Ligands and the formation of complex ions
The lone pairs of electrons on the oxygen atom of a water molecule are attracted
strongly to the positive ions in the ionic lattice and bind with them firmly enough to
allow the breakdown of the lattice to occur. This can happen with any ion that
possesses spare pairs of electrons, although the effectiveness of the attraction may
not be as strong as it is for water.…read more

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An octahedral complex has coordination number 6, square planar and tetrahedral
complexes have coordination number 4.
Monodentate and bidentate ligands
All the examples so far have involved monodentate ligands. These ligands have a
single point of attachment to the central cation. It is possible to have ligands that
attach at more than one point. For example, the bidentate ligand 1,2-diaminoethane
(often abbreviated to `en') has the structure:
The nitrogen atoms at the end of the molecule each have a lone pair of electrons.…read more

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Isomerism of complex ions
The square planar structure of Ni(NH3)2Cl2 shows cis-trans isomerism.
Trans form ­ ammonia molecules and chloride ions are on opposite sides of the
complex ion
Cis form ­ ammonia and chloride ions are alongside each other
Optical isomerism is also possible in complexes coordinated with polydentate ligands.
Ni(en)2+ is an example:
It is the asymmetry of the
structure that leads to optical
isomerism as the two
molecules cannot be
superimposed.…read more

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CuCl42- with a little [Cu(H2O)6]2+, the mixture becomes progressives
more blue as the chloride ions of the [CuCl4]2- are substituted by H2O until blue
[Cu(H2O)6]2+ ions predominate.
Cobalt chloride exists as Co2+[CoCl4]2- in the anhydrous state. [CoCl4]2- is blue, but the
addition of water results in the formation of a pink solution containing [Co(H2O)6]2+
ions. This is the basis of a test for water using cobalt chloride paper, which is blue but
turns pink when it comes into contact with water.…read more

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Silver ions form a soluble complex ion with two ammonia ligands, [Ag(NH3)2]+. This
complex ion is formed when excess aqueous ammonia is added to a precipitate of
silver chloride. It does not form if excess aqueous ammonia is added to a precipitate
of silver iodide. It is a useful means of distinguishing between the two precipitates.
The complex ion has the stability constant 1.7 x 107 mol-2dm6.…read more

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