CHEM5 - Transition Metals

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TM - The Basics

Transition Metal - A metal that can form one or more stable ions with a partially filled d-subshell. (Ti - Cu)
      Sc and Zn aren't TM's because they only form ions without partially filled d-subshells. The 4s orbital fills and empties first.
Ti - [Ar] 3d(2)4s(2);                              V - [Ar] 3d(3)4s(2)
Cr - [Ar] 3d(5)4s(1)      [1 electron in each orbital of 3d-subshell gives more stability]
Mn - [Ar] 3d(5)4s(2)                             Fe - [Ar] 3d(6)4s(2)
Co - [Ar] 3d(7)4s(2)                             Ni - [Ar] 3d(8)4s(2)
Cu - [Ar] 3d(10)4s(2)  [full 3d-subshell gives more stability]

TM's have similar physical properties (i.e. high density; high mp+bp; similar ionic radii; form complex ions; form coloured ions; are good catalysts; variable oxidation states)

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TM - Complex Ions

Complex ion - Metal ions surrounded by coordinately bonded ligands.
Coordinate bond - Covalent bond in which both electrons in the shared pair come from the same atom
Ligand - Atom/ion/molecule that donates a pair of electrons to a central metal ion.
Coordination number - Number of coordinate bonds formed with the cental metal ion.
Multidentate Ligand - A ligand with more than one lone pair of electrons to form a coordinate bond (e.g. NH2CH2NH2 or EDTA)

Octahedral shape: H2O and NH3 form 6 bonds.
Tetrahedral shape: Cl- forms 4 bonds.
Square planar: cisplatin [Pt(NH3)2Cl2]
Linear: Ag+ complexes forming 2 bonds. [Ag(NH3)2]3+

The oxidation state of a metal ion = Total Oxidation State - Sum of Oxidation States of Ligands. e.g. [CoCl4]2- (Total -2), Each Cl- is -1. -2-(4x-1) = +2.  

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TM - Complex Ions

Haem contains a multidentate ligand. N-H2O-N-N-N(globin)-N bonded to Fe.

Haemoglobin is a protein found in blood that transports oxygen around the body. Hb contains Fe2+, hexa-coordinated: 6 lone pairs donated to form 6 coordinate bonds.

4 of the lone pairs come from N atoms which form a circle around the Fe2+ (haem)

Protein called globin and either an O or H2O molecule also binds to Fe2+ to make an octahedral structure.

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TM - Coloured Ions

- Presence of the ligands splits the 3d-subshell into 2 energy levels: Some orbitals given more energy than others.
- Electrons occupy the lower orbitals (ground state). To jump up to the higher orbitals (excited state) they need energy equal to the energy gap (^E). This energy is gained from visible light.
- ^E = hv       h = 6.63x10-34 (Constant)           v = Frequency of light absorbed.

- Amount of energy needed depends upon central metal ion and its oxidation
state/ligands/coordination number as these effect the size of the energy gap.

- The colour of the compounds are complements of those that are absorbed: when visible light hits a TM ion, some frequencies are absorbed when electrons jump up to the higher orbitals. The frequencies depend on the energy gap.
The rest of the frequencies are reflected and these combine to make the complement of the colour that is absorbed. (e.g. [Cu(H2O)6]2+ absorbs yellow light and remaining frequencies combine to produce blue colour). 

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TM - Coloured Ions

The colour of the complex can be altered by:  
   - Oxidation state. [Fe(H2O)6]2+    >      [Fe(H2O)6]3+ (pale green > yellow)
[V(H2O)6]2+       >         [V(H2O)6]3+ (violet > green
   - Coordination number [Cu(H2O)6]2+(aq) + 4Cl- > [CuCl4]2-(aq) + 6H2O (blue > yellow)
  - Ligand [Co(H2O)6]2+   +    6NH3    >    [Co(NH3)6]2+     +     6H2O   (pink > straw-yellow)

Spectrometry can be used to determine the concn of a solution by measuring how much light it absorbs: 1) White light is shone through a filter, chosen to only let colour of light through thats absorbed. 2) Light then passes through sample to colorimeter which calculates how much light was absorbed. 3) The more concentrated a solution, the more light it will absorb. Use this measurement to work out the concn of solution of TM ions. 5) Produce a calibration graph by measuring absorbances of known concn of solutions and plot results. 6) Measure absorbance of sample and read concn off graph.

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TM - Variable Oxidation States

Chromium can exist as +2, +3 or +6.
    +6  -   Cr2O7(2-)    -    Orange
    +6  -   CrO4(2-)       -    Yellow
    +3  -   Cr(3+)         -     Green (violet) [When Cr3+ are surrounded by 6H2O ligands they're violet but H2O is often substituted] 
    +2  -   Cr(2+)         -      Blue

- Cr2O7 and CrO4 exist in equilibrium. [Cr2O7(2-) + OH- > 2CrO4(2-) + H+] 
 [Orange > Yellow because CrO4(2-) formed.]
- 2CrO4(2-) + H+ > Cr2O7(2-) + OH-           [Yellow > Orange
- Cr2O7(2-) + H2O >< 2CrO4(2-) + 2H+            
The position of equilibrium depends on the pH. If H+ ions are added, the eqm shifts to the left so it will be orange. If OH- ions added, H+ ions removed so eqm shifts right forming yellow. 

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TM - Variable Oxidation States

Chromium ions can be oxidised and reduced.
- Cr2O7(2-)(aq) + 14H+(aq) + 3Zn(s) > 3Zn(2+)(aq) + 2Cr(3+)(aq) + 7H2O(l)

- 2Cr(3+)(aq) + Zn(s) > Zn(2+)(aq) + 2Cr(2+)(aq)

- 2Cr(3+)(aq) + 10OH-(aq) + 3H2O2 > 2CrO4(2-)(aq) + 8H2O(l)

Cobalt can exist as Co2+ or Co3+.
- 2Co(2+)(aq) + H2O2 > 2Co(3+)(aq) + 2OH-
Place Co(2+) ions in excess of NH3(aq) to form [Co(NH3)6]2+
- [Co(H2O)6]2+(aq) + 2NH3(aq) > [Co(H2O)4(OH)2](s)

- [Co(H2O)4(OH)2](s) + 6NH3(aq) > [Co(NH3)6]2+(aq)

- Standing in air = [Co(NH3)6]3+(aq)        (dark brown)

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TM - Titrations

- Allows you to find out how much oxidising agent is needed to exactly react with a quantity of reducing agent.
1) Measure a quantity of reducing agent (e.g. Fe2+(aq)) using a pipette and put into a conical flask.
2) Using measuring cylinder, add 20cm3 of dilute H2SO4 to the flask.
3) Add the oxidising agent (KMnO4(aq)) to the reducing agent using a burette.
4) Stop when mixture in the flask just becomes tainted with the colour of the oxidising agent and record the volume of the oxidising agent added.
5) Do some accurate titrations until 2 or more readings gained within 0.10cm3.

- MnO4- (purple) + 8H+(aq) + 5e- > Mn(2+) + 4H2O
- Cr2O7(2-) (orange) + 14H+(aq) + 6e- > 2Cr(3+) + 7H2O

Eg/ Calculate concn of Fe2+ in solution.
n(MnO4-) added = c x (v/1000) = Z
n(Fe2+) =  Z x 5 = Y
Concn = Y/volume 

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TM - Uses

- TM catalysts work by changing oxidation states by gaining/losing electrons within their d-orbitals. E.g. V2O5 oxidises SO2 to SO3 because it can be reduced to V2O4.        (V2O5 + SO2 > V2O4 + SO3;    2V2O4 + O2 > V2O5)
- Heterogeneous catalysts are in a different phase from the reactants (i.e. gases passing over a solid catalysts)
1) One or more reactants are adsorbed on surface of the catalyst at active sites. 
2) When reactant molecules are on surface, they become more reactive.
3) The reaction occurs (when reactants are correctly orientated)
4) Product molecules are desorbed.

A good catalyst: - Must not adsorb too strongly as it is difficult for desorption;           - Mustnt adsorb too weakly as attachments not strong enough; - Should be prepared so surface area is maximised.

Catalytic poisoning: Occurs when waste products become too strongly adsorbed and block catalyst active sites.

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TM - Uses

Examples of catalysts: - Rh (on ceramic support); 2CO + 2NO > 2CO2 + N2
- [Contact Process] 2SO2 + O2 > 2SO3

Homogeneous catalysts are in the same phase as the reactants.
Example: S2O8(2-) + 2I- > 2SO4(2-) + I2
1) S2O8(2-) + 2Fe(2+) > 2SO4(2-) + 2Fe(3+)
2) 2Fe(3+) + 2I- > 2Fe(2+) + I

The collision between +ve and -ve ions is more likely to be successful than the collision between 2 -ve ions.

Autocatalysis - The reaction is catalysed by one of its reactants. E.g/
2MnO4- + 16H+(aq) + 5C2O4(2-) > 2Mn(2+) + 10CO2 + 8H2O
MnO4- + 8H+(aq) + 4Mn(2+) > 5Mn(3+) + 4H2O
2Mn(3+) + C2O4(2-) > 2CO2 + 2Mn(3+

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REDOX Equations

Oxidation - A loss of electrons.
Reduction - A gain of electrons.
An oxidising agent accepts electrons and becomes reduced. A reducing agent donates electrons and becomes oxidised. 

Uncombined elements have an oxidation state of O.
The oxidation state of a simple monatomic ion (na+) is the same as its charge.
In compound ions, the overall oxidation state is the ion charge (SO4(2-)) and for a neutral compound it is 0.
Combined oxygen is nearly always -2, except in peroxides (H2O2) where its -1.
Combined hydrogen is +1, except in metal hydrides (NaH) where its -1.

Zn(s) > Zn(2+)(aq) + 2e-               OXIDATION
Ag+(aq) + e- > Ag(s)                     REDUCTION 

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