H2 Chem Summary of Transition Element

October 8, 2017 | Author: onnoez | Category: Ligand, Catalysis, Transition Metals, Coordination Complex, Chemical Reactions
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H2 Chem Summary of Transition Element...


Meridian Junior College Summary of Transition Elements 

A transition element is a d-block element which forms one or more stable ions with incomplete d-orbitals.








[Ar]3d24s2 [Ar]3d34s2 [Ar]3d54s1 [Ar]3d54s2 [Ar]3d64s2 [Ar]3d74s2 Electronic configuration Across the period, additional electron enters the 3d orbital. When forming ions, 4s electrons are removed first before removing 3d electrons.





Atomic radius Mn+ ionic radius (Graph 1) First I.E. (Graph 2)

Across the transition elements series,  the nuclear charge increases but electrons are added to the inner 3d orbital and thus provide shielding of the 4s electrons.  Hence, increase in shielding effect almost cancel off the increase in nuclear charge  Effective nuclear charge increases slightly.  Electrostatic force of attraction between outer electrons and nucleus increases slightly.  Thus, atomic and ionic radius decreases slightly. Note: T.E . has smaller atomic radius and ionic radius than s-block elements due to their greater nuclear charge. Greater 1st I.E. than s-block elements due to  in nuclear charge and relatively invariant atomic radius.

Second I.E. (Graph 2)

2nd I.E. for Cr and 2nd I.E. for Cu is slightly higher than expected. Cr+  Cr2+ + e Cu+  Cu2+ + e 5 4 10 [Ar] 3d [Ar] 3d [Ar] 3d [Ar] 3d9 Reason:  Removal of an outer electron disrupts the stable d5 or d10 configuration.  Hence, larger amount of energy is required to remove valence electron in Cr+ and Cu+.

3rd I.E. and higher (Graph 2)

  

3rd and 4th IE involves removal of electrons from inner 3d subshell. The remaining d electrons provide poor shielding effect. Therefore, there is a significant increase in effective nuclear charge that leads to a rapid increase in 3rd and 4th IE.

3rd I.E. for Fe is lower than expected. Fe2+  Fe3+ + e 6 [Ar] 3d [Ar] 3d5 Reason:  In Fe2+, the 3d electron to be removed is a paired electron.  Inter-electron repulsion result in less energy required to remove the valence electron from Fe2+.


Graph 1 Graph 2



2nd 1st


Properties Melting point

    








Electrical conductivity

  


 

T.E. has higher density than s-block elements. Gradual  in density across the period.

d-block elements have greater Ar and smaller atomic volume (i.e.

Electrical conductivity  across period. Electrical conductivity of d-block elements is better than s-block elements since both 3d and 4s electrons are contributed to the sea of delocalised electrons for metallic bond formation.

4 3  r ) than s-block elements. 3

Variable oxidation state

[Ar]3d24s2 [Ar]3d34s2 [Ar]3d54s1 [Ar]3d54s2 [Ar]3d64s2 [Ar]3d74s2 [Ar]3d84s2 +1 to +4 +1 to +5 +1 to +6 +1 to +7 +1 to +6 +1 to +5 +1 to +4  3d and 4s orbitals are close in energies.  Hence variable number of 4s and 3d electrons can be removed to form ions of similar stability.

Complex formation

   


d-block elements have higher m.p. than s-block elements. For s-block elements, only s electrons are delocalised. For example, Ca has only 2 valence electrons for formation of metallic bonds. For transition elements, the 3d and 4s orbitals are close in energies and hence both the 3d and 4s electrons can be delocalised to the sea of electrons. The increase in number of electrons added to the sea of electrons increases the strength of metallic bonding. A larger amount of energy is required to overcome the stronger metallic bonding in transition metals than that in s-block metals.

[Ar]3d104s1 +1 to +3

A complex ion is one which contains a central metal atom or ion closely surrounded by a cluster of other molecules or ions called ligands through dative bonds. A ligand is a molecule or ion that has at least one lone pair of electrons for dative bond formation with a transition metal atom or ion. Types of ligands : Monodentate ligand (1 dative bonds per ligand, e.g. H2O), Bidentate ligand (2 dative bonds per ligand, e.g. H2NCH2CH2NH2), Hexadentate ligand (6 dative bonds per ligand, EDTA), Shape of complexes: linear, tetrahedral, square planar and octahedral.

Note:  Transition metal ions have high charge density and are able to attract ligands.  Transition metal ions have energetically available and accessible vacant d orbitals to accommodate the lone pair of electrons from ligands to form dative bond.


Properties Colour of complexes


   








Transition metal ions have partially filled d-orbitals. In the presence of ligands, the 3d orbitals are split into 2 groups with different energy. This effect is known as d orbitals splitting. d-d transition takes place whereby d electrons from the lower energy level are promoted to the higher energy level by absorbing a certain wavelength of light from the visible region of the electromagnetic spectrum. The complex thus emits the remaining wavelength which appears as the colour of the complex observed.

Factors affecting colour of complex/compound (a) (b) (c) (d)

Nature of metal and its oxidation state No. of d electrons (d1 to d9 but not d0 or d10) Shape of complex ion. Nature of the ligands

 Different ligands split the energy level of d orbitals to different extent.  Amount of energy, E, absorbed by d electron in d-d transition differ 

Relative d orbital splitting capacity: I- < Br- < Cl- < F- < H2O < NH3 < H2NCH2CH2NH2 < CN-

Weak field ligand results in in small E and long  absorbed Catalyst

Strong field ligand results in in large E and short  absorbed reference

Heterogeneous catalyst  Heterogeneous catalyst operates in a different physical phase to the reactants.  Transition metals and their compounds are good heterogeneous catalyst because of the availability of 3d and 4s electrons for temporary bond formation. Example  Production of ammonia via Haber Process using iron catalyst.  Hydrogenation of alkenes (eg ethene) on nickel surface. Mechanism:  Temporary bonds are formed with reactants molecules when they are adsorbed on the catalyst surface.  This adsorption weakens the bonds in reactant molecules, thereby lowering the activation energy and increasing the surface concentration of the reactants.  Reactant molecules are brought closer together and reaction can take place between the reactants molecules more easily.


Properties Catalyst

22Ti 23V 24Cr 25Mn 26Fe 27Co 28Ni 29Cu Hence the rate of reaction is increased. Homogeneous catalyst  Homogeneous catalyst operates in the same physical phase as the reactants.  Transition metals and their compounds are good homogeneous catalyst because of their ability to exist in various oxidation states, thus facilitating the formation of reaction intermediate via alternative pathways of lower Ea.

The catalysis of S2O82-(aq) (peroxodisulphate) / I-(aq) reaction: Without a catalyst: The redox reaction: S2O82- + 2I-  2SO42- + I2  

Ea of the above reaction is very high and thus reaction is slow. Reaction is kinetically not favourable since both negatively charged ions are involved and they repel each other.

The reaction is accelerated in the presence of transition metal ions which can act as homogeneous catalysts such as Fe2+(aq) or Fe3+(aq). With a catalyst: Fe3+(aq) Step 1:

Fe3+ reacts with I2Fe3+ + 2I-  2 Fe2+ + I2 Ecell = 0.77 – 0.54 = +0.23V > 0

Step 2:

Fe2+ intermediate reacts with S2O822Fe2+ + S2O82-  2SO42- + 2Fe3+ Ecell = 2.01 – 0.77 = +1.24V > 0


S2O82- + 2I-  2SO42- + I2

Both steps are feasible and the catalyst, Fe3+ is also regenerated. Both steps are favourable since oppositely charged ions are involved and attract each other.  Ea is lower and thus reaction is faster. 5

Ligand exchange and relative stability of complex ions Formation of soluble complexes When dilute ammonia is gradually added to a blue solution of Cu2+(aq), a pale blue precipitate of Cu(OH)2 is formed, which dissolves on adding more dilute ammonia to give deep blue solution of [Cu(NH3)4(H2O)2]2+ . Reason: When dilute ammonia is added initially, a pale blue precipitate of Cu(OH)2 is formed. NH4+ + OH-

NH3 + H2O

[Cu(H2O)6]2+ + 2OH-..

Cu(OH)2 + 6H2O


(from ammonia) In excess ammonia, 

both NH3 and OH- compete to combine with [Cu(H2O)6]2+

NH3 ligands replace H2O ligands to form a deep blue complex [Cu(NH3)4(H2O)2]2+ in eqm (2) [Cu(H2O)6]2+ + 4NH3

Since concentration of [Cu(H2O)6] 

[Cu(NH3)4(H2O)2]2+ + 4H2O 2+


is decreased, equilibrium position (1) shifts to the left to increase concentration of [Cu(H2O)6]2+.

Pale-blue precipitate of Cu(OH)2 dissolves.



oxygen-containing constituent of blood / contains Fe(II) ion / complex giant molecule

In haemoglobin molecule, Fe(II) ion is octahedrally bonded to five nitrogen atom and to an oxygen atom from a water molecule.

 

H2O ligand may be replaced by an O2 ligand to form oxy-haemoglobin in a reversible reaction. O2 is taken up by blood and distributed to cells.

 

CN- and CO are toxic because they can form strong bonds with Fe in haemoglobin in an irreversible reaction. Hence, reducing the amount of haemoglobin available for carrying O2.

Since O2 ligand cannot replace the stronger CO or CN- ligand, patient will die due to O2 starvation.


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