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 Oxidation half equation: 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+).