A-Level Chemistry OCR Notes

5.3.1 Transition elements

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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
Across the periodic table from scandium to zinc, orbitals are added to 3d orbitals
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.
Transition metals and their compounds act as catalysts
Transition metals often form complexes. A complex is a 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.

Catalysts

Transition metals and their compounds can act as heterogeneous and homogeneous catalysts
Heterogeneous catalysts are in a different state to the reactants
Homogeneous catalysts are in the same state as the reactants
Examples of heterogeneous catalysts are
- in the Contact process V2O5 catalyses
  - 2SO2 (g) + O2 (g) ⇌ 2SO3 (g)
- In the Haber Process Fe catalyses
  - N2 (g) + 3H2 (g) ⇌ 2NH3 (g)
- Hydrogenation of vegetable fats to make margarine is catalysed by
  - Ni (-CH=CH- + H2 → -CH2-CH2- )
- The decomposition of hydrogen peroxide is catalysed by manganese (IV) oxide:
  - 2H2O2 → 2H2O + O2

Examples of homogeneous catalysts are
- The reaction between the persulphate ion (S2O82-) and the iodide ion is catalysed by Fe2+.
- S2O82- (aq) + 2I- (aq) → 2SO42- (aq) + I2 (aq)
- S2O82- (aq) + 2Fe2+ (aq) → 2SO42- (aq) + 2Fe3+ (aq)
- 2I- (aq) + 2Fe3+ (aq) → I2 (aq) + 2Fe2+ (aq)

Complex Ions

A complex ion consists of a metal centre surrounded by ligands (a molecule or ion that forms a coordinate bond by donating a lone pair of electrons)
The number of coordinate bonds in a complex gives the coordination number of the metal centre
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. [CuCl4]2-
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 colours of light are absorbed
The colour of a transition metal ion is the sum of the remaining colours are not absorbed

The colour of a complex depends on the identity of the metal, its oxidation state, and the ligands that are coordinately bonded to the metal

Substitution Reactions

All ligands contain at least one lone pair of electrons in their outer shell
Cu(II) ions undergo ligand substitution of H2O with NH3

[Cu(H2O)6]2+ (aq) + 4NH3 (aq) ⇌ [Cu(NH3)4(H2O)2]2+ (aq) + 4H2O (l)

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)

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

Precipitate Reactions of Transition Metals

A precipitate reaction is when two solutions containing soluble ions are mixed, forming an insoluble compound. They occur when sodium hydroxide or ammonia is added to transition metal solutions
Fe2+ eventually oxidised by air to Fe3+ making brown Fe(OH)3 (s)
If excess ammonia is added to Cr(OH)3, purple [Cr(NH3)6]3+ forms
If excess ammonia is added to Cu(OH)2, deep blue [Cu(NH3)4(H2O)]2+ forms

Metal-aqua ion	NH3 / OH-
[Fe(H2O)6]2+ (aq) Pale green solution	Fe(OH)2 Green ppt*
[Cu(H2O)6]2+ (aq) Pale blue solution	Cu(OH)2 Pale blue ppt
[Fe(H2O)6]3+ (aq) Purple/yellow/brown solution	[Fe(OH)3] Brown ppt
Mn2+ (aq) Pale pink solution	Mn(OH)2 Light brown ppt
Cr3+ (aq) Colourless solution	Cr(OH)3 Grey green ppt

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)

Manganate (VII) ions are readily reduced to Mn2+ ions under 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+

When transition metals are oxidised, the 4s electrons are lost before the 3d electrons

Tests for Ions

Qualitative tests give information on the identity of ions present in a sample
To test for the presence of carbonate ions in a sample:
- Add a dilute strong acid to the sample. If carbonate ions are present CO2 (g) will evolve
- Test the gas evolved by bubbling it through limewater, if it is CO2 the limewater turns cloudy due to the formation of CaCO3
To test for the presence of halide ions in solution, add dilute nitric acid and an aqueous solution of AgNO3

Silver Halide	Colour of Precipitate	Addition of Dilute Ammonia	Addition of Concentrated Ammonia
AgCl	White	Dissolves to give a colourless solution	Dissolves to give a colourless solution
AgBr	Cream	Remains	Dissolves to give a colourless solution
AgI	Yellow	Remains	Remains

To test for the presence of sulfate ions in a sample, add barium ions.
- Ba2+ (aq) + SO42- (aq) → BaSO4 (s).
- BaSO4 is a white precipitate
To test for ammonium ions add sodium hydroxide solution and warm gently
NH4+ (aq) + OH- (aq) → NH3 (g) + H2O (l)
- Test the gas evolved with damp red litmus paper. If the sample contains ammonium ions, the red litmus paper will turn blue
To test for transition metal ions, add aqueous ammonia or sodium hydroxide dropwise
- Cu(II) → Blue precipitate
- Fe(II) or Cr (III) → Green precipitate
- Mn(II) → Brown precipitate
To distinguish Fe(II) or Cr(III) add excess ammonia
- Fe(II) → No change
- Cr(III) → Purple solution

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