A-Level Chemistry AQA Notes

3.2.5 Transition metals (A-Level)

General Properties of Transition Metals

A transition element is a d-block element which has at least one stable ion with an incomplete d-subshell
Transition metal ions form coloured compounds because of the movement of electrons in partially filled d-orbitals. Particular wavelengths of light are absorbed, so the remaining wavelengths are the colour perceived.
All the transition metals have more than one oxidation state.
The highest oxidation states will readily accept electrons (be reduced), which makes them powerful oxidising agents.
Transition metals and their compounds act as catalysts by:
- Providing a surface for the reaction to take place
- Binding to reactants to form intermediates
Transition metals often form complexes. A complex is a central metal atom or ion surrounded by ligands.
A ligand is a molecule or ion that forms a co-ordinate bond with a transition metal by donating a lone pair of electrons.
The co-ordination number is the number of co-ordinate bonds to the central metal atom or ion.

Substitution Reactions

All ligands contain at least one lone pair of electrons in their outer shell
Monodentate ligands can donate just one lone pair of electrons each and form one coordinate bond. E.g. H2O, NH3 and Cl^−
The ligands NH3 and H2O are similar in size and are uncharged. Therefore, exchange of a H2O ligand for a NH3 occurs without change of co-ordination number.

[Cu(H2O)6]^2+ (aq) + 4NH3 (aq) ⇌ [Cu(NH3)4(H2O)2]^2+ (aq) + 4H2O (l)
This ligand substitution is incomplete

[Co(H2O)6]^2+ (aq) + 6NH3 (aq) ⇌ [Co(NH3)6]^2+ (aq) + 6H2O (l)
This ligand substitution is complete

The Cl− ligand is larger than the uncharged ligands NH3 and H2O. Therefore, exchange of the ligand H2O by Cl– can involve a change of co-ordination number

[Cu(H2O)6]^2+ (aq) + 4Cl- (aq) ⇌ [Cu(Cl)4]^2- (aq) + 6H2O (l)
[Co(H2O)6]^2+ (aq) + 4Cl- (aq) ⇌ [CoCl4]^2- (aq) + 6H2O (l)
[Fe(H2O)6]^3+ (aq) + 4Cl- (aq) ⇌ [FeCl4]^- (aq) + 6H2O (l)

Bidentate ligands have 2 lone pairs it can donate e.g, H2NCH2CH2NH2 and C2O4^2–

Multidentate ligands can donate multiple lone pairs of electrons from different atoms within the ligands to form multiple coordinate bonds eg EDTA^4–

Chelation is when multidentate ligands replace monodentate ligands in complexes

[Cu(H2O)6]^2+ (aq) + EDTA^4- (aq) → [Cu(EDTA)]^2- (aq) + 6H2O (l)

For the reaction to be thermodynamically feasible, the Gibbs free energy change must be negative.
The haem group in haemoglobin is n Fe(II) complex with a multidentate ligand
Oxygen can reversibly form a coordinate bond to the Fe2+ ion and travel through the bloodstream, being released where it’s needed

Carbon monoxide is toxic because it binds irreversibly to haemoglobin, forming a stronger bond than oxygen does.

Complex Ions

Complex ions most commonly form octahedral complexes with small ligands (eg H2O and NH3). Octahedral complexes can display cist-trans isomerism with monodentate ligands and optical isomerism with bidentate ligands

Tetrahedral complexes are commonly formed with larger ligands e.g. Cl^-. [Ag(NH3)2]^+ has a linear shape- this is formed when Tollens’ reagent is used

Cis and trans isomers can also exist if the complex is square planar with two pairs of identical ligands.e.g. cisplatin, which acna bind to DNA to prevent replication causing death of cancer cells. But transplatin cannot.

Formation of Coloured Ions

Transition metal ions have distinctive colours which are used to identify them
d electrons move from the ground state to an excited state when light is absorbed

∆E difference in energy (J)
h Planck's constant (Js)
v frequency of light absorbed (s^-1 or Hz)
c speed of light in vacuum (3.00 x 108 ms^-1)
λ wavelength of light absorbed (m)

Changes in oxidation state, co-ordination number and ligand alter ∆E and this leads to a change in colour
A simple colorimeter can be used to calculate the concentration of transition metal ions in solution

The wavelength of visible light absorbed can also be used to spectroscopy for identification of ions.

Variable Oxidation States

Vanadium has 4 oxidation states II, III, IV & V
Vanadium species in oxidation states IV, III and II are formed by the reduction of vanadate(V) ions by zinc in acidic solution

Reduction from V(V) to V(IV)
2VO2+ (aq) + 4H+ (aq) + Zn (s) → 2VO2+ (aq) + Zn2+ (aq) + 2H2O (l)

Reduction from V(IV) to V(III)
2VO2+ (aq) + 4H+ (aq) + Zn (s) → 2V3+ (aq) + Zn2+ (aq) + 2H2O (l)

Reduction from V(III) to V(II)
2V3+ (aq) + Zn (s) → Zn2+ (aq) + 2V2+ (aq)

Electrode potentials for a transition metal ion changing from a higher to a lower oxidation state gives the thermodynamic feasibility of the reduction.
redox potentials are influenced by pH and by the ligand. Redox potentials are larger in more acidic solutions as it is easier to reduce the ion
redox titrations can be carried out to show how much oxidising agent is needed to react exactly with a reducing agent.
Manganate(VII) ions are readily reduced to Mn2+ ions underin acidic conditions (purple to colourless). This can be used to find the amount of Fe3+ in a solution:

Oxidation: Fe2+ → Fe3+ + e-
Reduction: 8H+ + MnO4- + 5e- →Mn2+ + 4H2O
Overall: 8H+ + MnO4- + 5Fe2+ → Mn2+ + 4H2O + 5Fe3+

Catalysts

Transition metals and their compounds can act as heterogeneous and homogeneous catalysts
Heterogeneous catalysts are spread onto a support medium to maximise their surface area and minimise the cost
Heterogeneous catalysts can become poisoned by impurities that block the active sites and consequently have reduced efficiency. They can be expensive to replace.
in the Contact process V2O5 acts as a heterogeneous catalyst 2SO2 (g) + O2 (g) ⇌ 2SO3 (g)
- Sulphur dioxide adsorbs onto vanadium(V) oxide
- V2O5 (s) + SO2 (g) → V2O4 (s) + SO3 (g)
- V2O4 (s) + 1/2O2 (g) → V2O5 (s)
Desorption
The reaction between the persulphate ion (S2O8^2-) and the iodide ion is catalysed by the homogenous catalyst Fe^2+.
S2O8^2- (aq) + 2I- (aq) → 2SO4^2- (aq) + I2 (aq)
- S2O8^2- (aq) + 2Fe^2+ (aq) → 2SO4^2- (aq) + 2Fe^3+ (aq)
- 2I^- (aq) + 2Fe^3+ (aq) → I2 (aq) + 2Fe^2+ (aq)

The reaction between ethanedioate ions and potassium manganate(VII) which is catalysed by Mn^2+ (autocatalysis).
2MnO4-(aq)+5C2O4^2-(aq)+16H+ (aq) → 2Mn^2+(aq) + 10CO2 (g) + 8H2O(I)
- 2MnO4- (aq) + 16H^+ (aq) + 8Mn^2+ (aq) → 8H2O (l) + 10Mn^3+ (aq)
- 5C2O4^2- (aq) + 10Mn^3+ (aq) → 10CO2 (g) + 10Mn^2+ (aq)