Examples of Redox Reaction - Change of Iron(II) Ion to Iron(III) Ion

  1. Iron shows two oxidation numbers, that is +2 and +3.
  2. The aqueous solution of iron(II) ion Fe2+ is light green in colour. The aqueous solution of iron(III) ion Fe3+ is brown in colour.
  3. The change of iron(II) ion for iron(III) ion is an oxidation process. This can be done by mixing an oxidation agent.


Example
Procedure:

  1. 2 cm³ of iron(II) sulphate solution is poured into a test tube.
  2. Bromine water is added drop by drop into the solution until no further changes are observed.
  3. The mixture is then shaken and warmed gently.
  4. The observation is recorded.

Observation:

  1. The brown colour of bromine water turns colourless. 
  2. The colour of the solution changes from light green to yellowish brown.

Half Equations:

Fe2+ → Fe3+ + e

Br2 + 2e → 2Br-

Ionic Equation

2Fe2+ + Br2 → 2Fe3+ + 2Br-

Explanation:

  1. The light green colour of iron(II) sulphate solution turns brown because iron(II) ions Fe2+ are oxidise
  2. to become iron(III) ion, Fe3+.
  3. Iron(II) ion, Fe2+ undergoes oxidation by releasing electron to form iron(III) ion, Fe3+.
  4. The brown colour of bromine water turn colourless because bromine molecules are reduced to become bromide ions. 
  5. Bromine molecules receive electrons and undergoesreduction to form bromide ion, Br–.


Oxidising agent: Bromine water
Reducing agent:  Iron(II) ions Fe2+


Confirmation Test

2 cm³ of solution of the product is filled into a test tube.

Test 1 : Dilute sodium hydroxide solution (NaOH) is then added into the test tube until excess.
Result : Brown precipitate formed. The precipitate does not dissolve in excess sodium hydroxide solution.

Test 2 : Dilute ammonium hydroxide solution (NH4OH)/ammonia aqueous (NH3) is then added into the test tube until excess
Result :Brown precipitate formed. The precipitate does not dissolve in excess ammonium hydroxide solution /ammonia aqueous.

Test 3 : 2cm³ of potassium thiocyanate is added into the test tube.
Result : Red blood solution formed.



Other oxidation agents that get to replace bromine water
  1. Chlorine water
    Half Equations
    Cl2 + 2e → 2Cl-
  2. Acidic potassium manganate (VII)
    Half Equations
    MnO4- + 8H+  +  5e   →      Mn2+ +      4H2O
  3. Potassium dichromate (VI)
    Half Equations
    Cr2O72- +   14H+ +  6e   →      2Cr3+ +     7H2O
  4. Hydrogen peroxide
    Half Equations
    H2O2 +     2H+ +   2e  →     2H2O
  5. Concentrated nitric acid
    Half Equations
    NO3- +    4H+ +   3e  →      NO +   2H2O



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