Redox Reactions by the Transfer of Electrons at a Distance

Redox Reactions by the Transfer of Electrons at a Distance
Set I

Reducing agent	Oxidising agent	Test on the solution in the reducing agent arm of U-tube
Iron(II) sulphate, FeSO4 solution	Acidified potassium dichromate(VI), K2Cr2O7 solution	Add a few drops of potassium thiocyanate, KSCN solution
Observation		Inference
The electrode in the iron(II) sulphate, FeSO4 solution acts as the negative terminal, whereas the electrode in the acidified potassium dichromate(VI), K2Cr2O7 solution acts as the positive terminal.		Electrons flow from iron(II) sulphate, FeSO4 solution to acidified potassium dichromate(VI), K2Cr2O7 solution
Iron(II) sulphate solution changes from pale green to yellow/brown. It gives blood-red colouration with potassium thiocyanate solution (KSCN)		Iron(III) ions are present. Iron(II) ions are oxidised to  iron(III) ions.
Acidified potassium dichromate(VI), K2Cr2O7 solution changes colour from orange to green.		Dichromate(VI) ions are reduced to chromium(III) ions.

• Oxidation half-equation: Fe²⁺(aq) –> Fe³⁺(aq) + e
• Reduction half-equation: Cr₂O₇^2-(aq) + 14H⁺(aq) + 6e –> 2Cr³⁺(aq) + 7H₂O(l)
• Overall reaction: Cr₂O₇^2-(aq) + 6Fe²⁺(aq) 14H⁺(aq) –> 2Cr³⁺(aq) + 6Fe³⁺(aq) + 7H₂O(l)

Set II

Reducing agent	Oxidising agent	Test on the solution in the reducing agent arm of U-tube
Iron(II) sulphate, FeSO4 solution	Acidified manganate(VII), KMnO4 solution	Add sodium hydroxide, NaOH solution
Observation		Inference
The electrode in the iron(II) sulphate, FeSO4 solution acts as the negative terminal, whereas the electrode in the acidified potassium manganate(VII), KMnO4 solution acts as the positive terminal.		Electrons flow from iron(II) sulphate, FeSO4 solution to acidified potassium manganate(VII), KMnO4 solution
Iron(II) sulphate solution changes from pale green to yellow/brown. It formed a brown precipitate when the brown solution is tested with sodium hydroxide solution (NaOH)		Iron(III) ions are present. Iron(II) ions are oxidised to  iron(III) ions.
Purple acidified manganate(VII), KMnO4 solution turns colourless.		Manganate(VII) ions are reduced to manganese(II) ions.

• Oxidation half-equation: Fe²⁺(aq) –> Fe³⁺(aq) + e
• Reduction half-equation: MnO₄^-(aq) + 8H⁺(aq) + 5e –> Mn²⁺(aq) + 4H₂O(l)
• Overall reaction: MnO₄^-(aq) + 5Fe²⁺(aq) + 8H⁺(aq) –> Mn²⁺(aq) + 5Fe³⁺(aq) + 4H₂O(l)

Set III

Reducing agent	Oxidising agent	Test on the solution in the reducing agent arm of U-tube
Potassium iodide, KI solution	Bromine water, Br2	Add a few drops of starch solution
Observation		Inference
The electrode in the potassium iodide, KI solution acts as the negative terminal, whereas the electrode in the bromine water acts as the positive terminal.		Electrons flow from potassium iodide, KI solution to bromine water, Br2 (aq).
Colourless potassium iodide solution turns brown. It formed a dark blue colouration when the brown solution is tested with starch solution.		Iodine is present.Iodide ions have oxidised to iodine.
Brown bromine water turns colourless.		Bromines are reduced to bromide ion.

• Oxidation half-equation: 2I^-(aq) –> I₂(aq) + 2e
• Reduction half-equation: Br₂(aq) + 2e –> 2Br^-(aq)
• Overall reaction: Br₂(aq) + 2I^-(aq) –> 2Br^-(aq) + I₂(aq)

Other pairs of reducing agent and oxidising agent

Reducing agent	Oxidising agent
Potassium iodide,KI solution	Iron(III) sulphate,Fe2(SO4)3 solution
Potassium iodide,KI solution	Acidified potassium dichromate(VI),K2Cr2O7 solution
Potassium bromide,KBr solution	Chlorine, Cl2 water