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Sunday, August 21, 2011

Chemistry Form 5: Chapter 3 - Conversion of Iron (II) to Iron (III) and Iron (III) to Iron (II)


Conversion of Fe2+ to Fe3+

Oxidising agent : bromine water
Reducing agent :  Fe2+ ions

  • Fe2+ ions lose electrons and are oxidized to Fe3+.
  • The presence of Fe3+ ions is confirmed by the formation of brown precipitate with excess of NaOH solution.
                     Fe2+ -------->    Fe3+    +     e

Reduction half equation:
  • Bromine molecules which give bromine water its brown colour gain electrons and are reduced to colourless bromide ions.
                     Br2    +    2 e   ------->  2 Br-

Overall ionic equation:

2 Fe2+   +    Br2    ---------->  2 Fe3+    +    2 Br-

Observation:
  • Brown bromine water decolourises. The solution changes colour from pale green (Fe2+)  to yellow (Fe3+).



Conversion of Fe3+ to Fe2+

Oxidising agent : Fe2+ ions
Reducing agent :  zinc

Oxidation half equation:
  • Zinc atoms lose their electrons and are oxidized to zinc ions, Zn2+.
  • Zinc powder dissolves in iron (III) chloride.
Zn    ------->   Zn2+    +    2 e

Reduction half equation:
  • Fe3+ ions accept electrons and are reduced to Fe2+.
  • The presence of Fe2+ ions is confirmed by the formation of green precipitate with excess of NaOH solution.
Fe3+    +     e    -------->    Fe2+   

Overall ionic equation:

2 Fe3+   +    Zn    --------->  2 Fe2+    +    Zn2+

Observation:
  • Zinc powder dissolves into solution. The solution changes colour from brown (Fe3+) to pale green (Fe2+).

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