Electron Transfer

Rusting of Iron Nails

Oxidation and Reduction in Terms of Electron Transfer
2I^- (aq) –> I₂ (aq) + 2e
Oxidation: Iodide ion, I^- is a reducing agent because it donates/loses electrons to become I₂.
Br₂ + 2e –> 2Br^- (aq)
Reduction: Bromine water, Br₂ is an oxidising agent because it receives/accepts electrons from I^- to form bromide ions, Br^-.
–> Overall reaction: 2I^- (aq) + Br₂ –> I₂ (aq) + 2Br^- (aq)

Conversion of Fe²⁺ Ions to Fe³⁺ Ions and Vice Versa
A) Common oxidising agent (change Fe²⁺ ions to Fe³⁺ ions):

• bromine, Br₂
• chlorine, Cl₂
• hydrogen peroxide, H₂O₂
• concentrated nitric acid, HNO₃
• acidified potassium manganate(VII), KMnO₄ solution
• acidified potassium dichromate(VI), K₂Cr₂O₇ solution

Fe²⁺ (aq) –> Fe³⁺ (aq) + e
Oxidation: Iron(II) ion, Fe²⁺ is a reducing agent because it donates/loses one electron to become Fe³⁺.
Br₂ (aq) + 2e –> 2Br^- (aq)
Reduction: Bromine water, Br₂ is an oxidising agent because it receives/accepts electrons from Fe²⁺ to form bromide ions, Br^-.
–> Observation: iron(II) sulphate solution changes colour from pale green to yellowish-brown.
–> Overall reaction: 2Fe²⁺ (aq) + Br₂ (aq) –> 2Fe³⁺ (aq) +2Br^- (aq)
B) Common reducing agent (change Fe³⁺ ions to Fe²⁺ions):

• zinc powder, Zn
• aluminium, Al
• Magnesium, Mg
• Calcium, Ca
• Sulphur dioxide, SO₂
• Hydrogen sulphide, H₂S
• Sodium sulphide solution, Na₂SO₃
• Tin(II) chloride solution, SnCl₂

Zn (s) –> Zn²⁺ (aq) + 2e
Oxidation: Zinc powder, Zn is a reducing agent because it donates/loses two electrons to form zinc ions, Zn²⁺.
Fe³⁺ (aq) + e –> Fe²⁺ (aq)
Reduction: Iron(III) ion, Fe³⁺ is an oxidising agent because it receives/accepts one electron to become Fe²⁺.
–> Observation: iron(III) sulphate solution changes colour from yellowish-brown to pale green.
–> Overall reaction: 2Fe³⁺ (aq) + Zn (aq) –> 2Fe²⁺ (aq) + Zn²⁺ (aq)
C) Investigate the presence of iron(II) and iron(III) ions

Reagent	Ions	Observations
NaOH solution / NH3 solution	Fe2+	Green precipitate,insoluble in excess alkali
NaOH solution / NH3 solution	Fe3+	Brown precipitate,insoluble in excess alkali
Potassium hexacyanoferrate(II) solution	Fe2+	Light blue precipitate
Potassium hexacyanoferrate(II) solution	Fe3+	Dark blue precipitate
Potassium hexacyanoferrate(III) solution	Fe2+	Dark blue precipitate
Potassium hexacyanoferrate(III) solution	Fe3+	Greenish-brown solution
Potasium / Ammonium thiocyanate solution	Fe2+	Pale red colouration
Potasium / Ammonium thiocyanate solution	Fe3+	Blood-red colouration