SPM Chemistry

Change of Iron(II) Ion to Iron(III) Ion and Vice Versa

spmchemistry

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 an electron to form iron(III) ion, Fe3+.
  4. The brown colour of bromine water turns colourless because bromine molecules are reduced to become bromide ions. 
  5. Bromine molecules receive electrons and undergo reduction to form bromide ion, Br–.

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

Confirmation Test

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

Test 1: Dilute sodium hydroxide solution (NaOH) is then added to 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 to 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

Change of Iron(III) Ion to Iron(II) Ions

  1. The change of iron(III) ion for iron(II) ion is a reduction process. This can be done by mixing a reducing agent.

Example

Procedure:

  1. 2 cm³ of iron(III) sulphate solution is poured into a test tube.
  2. Half spatula of zinc powder is added to the solution.
  3. The mixture is then shaken and warmed gently.
  4. The observation is recorded.

Observation:

  1. Zinc powder dissolves. 
  2. The brown coloured of iron(III) sulphate solution turn light green.

Half Equations:

Fe3+ → Fe2+ + e
Zn  → Zn2+ + 2e

Ionic Equation

2Fe3+ + Zn → 2Fe2+ + Zn2+

Explanation:

  1. The brown colour of iron(III) sulphate solution turns light green shows that iron(III) ions, Fe3+ are
  2. reduced to iron(II) ion, Fe2+.
  3. Iron(III) ion, Fe3+ undergoes reduction by receiving electron to form iron(II) ion, Fe2+
  4. Zinc powder added is oxidised to form zinc ion, Zn2+. 

Oxidising agent: Iron(III) ions Fe3+
Reducing agent:  Zinc powder

Confirmation Test

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

Test 1: Dilute sodium hydroxide solution (NaOH) is then added to the test tube until excess.
Result: Dirty green 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: Green precipitate formed. The precipitate does not dissolve in excess ammonium hydroxide solution /ammonia aqueous.

Test 3: 2cm³ of potassium thiocyanate is added to the test tube.
Result: No change observed

Other oxidation agents that get to replace bromine water

  1. Magnesium
    Half Equations
    Mg  → Mg2+ + 2e
  2. Sulphur dioxide gas
    Half Equations
    SO2 +     2H2O →      SO42- +   4H+ +  2e
  3. Hydrogen sulphide gas
    Half Equations
    H2S →     2H+ +  S +  2e
  4. Solution of sodium sulphide, Na2SO3
    Half Equations
    SO32- +  H2O →     SO42- + 2H+ +  2e
  5. Solution of tin(II) Chloride
    Half Equations
    Sn2+ →     Sn4+ +  2e