AQA A Level Chemistry - Transition Metals

Transition Elements

Transition Elements

Incomplete d sub-level in one or more of their oxidation states.

Complex Ion

Made when a number of ligands co-ordinate bond to a central metal ion.

Ligand

A molecule/ion with a lone pair, that can co-ordinate bond to a metal

Co-ordinate Bond

One species donates both electrons in a covalent bond.

Co-ordination Number

Number of co-ordinate bonds to ligands that surround the d-block metal ion.

Monodentate Ligand

A species that only forms one coordinate bond to a metal ion.

e.g. water, ammonia, hydroxide ion, chloride ion, cyanide ion.

Bidentate Ligand

A molecule or ion that can form two co-ordinate bonds to a metal.

Multidentate Ligand

A molecule or ion that can form several co-ordinate bonds to a metal.

e.g. ethane-1,2-diamine (en), ethanedioate ion (oxalate), EDTA

Heterogenous Catalyst

Catalyst in a different phase than the reactants.

Homogenous Catalyst

Catalyst in the same phase as the reactants.

Autocatalysis

Occurs when one of the products of a reaction is the catalyst for that reaction.

Ligand Substitution

Water molecules in aqua ions can be replaced by other ligands because the other ligand forms stronger co-ordinate bonds OR because they are present in higher concentration.

Chelate Effect

Complexes formed are very stable, the equilibria are far to RHS and few aqua ions are present. Products are thermodynamically more stable than reactants.

Cr Electron Configuration

1s2 2s2 2p6 3s2 3p6 4s1 3d5

Cu Electron Configuration

1s2 2s2 2p6 3s2 3p6 4s1 3d10

Chemical Properties

Variable oxidation states, colour, catalysis, complex formation.

Linear Complexes

Co-ordination number 2
Ag+ complexes
e.g. [Ag(NH3)2]+

Tetrahedral Complexes

Co-ordination number 4
Large ligands (e.g. Cl)
e.g. [CuCl4]2-

Square Planar Complexes

Co-ordination number 4
Pt2+ complexes
e.g. [PtCl4]2-

Octahedral Complexes

Co-ordination number 6
Most common
e.g. [Cu(H2O)6]2+

Why are substances coloured?

When white light shines on/through a surface, some of the colours from the spectrum are absorbed, others are reflected/transmitted. The colour we see is due to the reflected/transmitted radiation.

Red absorbed = See cyan
Blue and green absorbed = See red
All 3 absorbed = black
All 3 reflected = white

Why are transition metal complexes coloured?

They have part filled d orbitals, so the electrons can move from one d orbital to another.

In a compound, the d orbitals have slightly different energies.

When electrons move from one d orbital to another of a higher energy level (excited state), they absorb energy in the visible region of the spectrum equal to the difference in energy between the levels.

You see the colours with are not absorbed.

Why are different transition metal complexes different colours?

Different ox state = different number of electrons in d orbital = orbitals repelled by the ligand to different extents = energy gap is different sizes.

Different ligands repel the d orbitals to different extents, so alter the size of the energy gap.

Factors affecting colour

Metal
Oxidation state
Ligand
Coordination number

Potassium Manganate (VII) Redox Equation

MnO4- + 8H+ + 5e- --> Mn2+ + 4H2O

Potassium Manganate oxidising Fe2+

5Fe2+ + MnO4- + 8H+ --> 5Fe3+ + Mn2+ + 4H2O

Autocatalysis example:
Potassium manganate with ethanedioate.

2MnO4- + 16H+ + 5C2O4 2- --> 2Mn2+ + 8H2O + 10CO2

Catalysed by Mn2+:
4Mn2+ + MnO4- + 8H+ --> 5Mn3+ + 4H2O
2Mn3+ + C2O4 2- --> 2CO2 + 2Mn2+

Describe how a catalyst provides an alternative route.

Reactant(s) adsorbed onto the [e.g. Pt] provides a surface.

Reaction on the surface/bond breaking (weakening) occurs on surface.

Desorption of the product.

ADSORB, REACT, DESORB

Contact Process (heterogenous catalysis)

Overall: SO2 + ½O2 --> SO3

Catalysed by V2O5:
V2O5 + SO2 --> V2O4 + SO3
V2O4 + ½O2 --> V2O5

Why can transition metals act as catalysts?

They have the ability to vary oxidation states.

Homegenous catalysis example:
Iodide ions with peroxodisulfate ions.

Overall: 2I- + S2O8 2- --> I2 + 2SO4 2-

Fe3+ provides alternative route:
2Fe2+ + S2O8 2- --> 2Fe3+ + 2SO4 2-
2Fe3+ + 2I- --> 2Fe2+ + !2