the chemistry of iron

Coloured ions and complex formation

Complex is formed when ligands form dative covalent bonds with the central atom/ion.  Transition metal ions like Fe2+ and Fe3+ can form complexes with ligands readily because they have large charge/radius ratio and low lying vacant d-orbitals available for accepting the lone-pair electrons of the ligands.  After forming complexes, the d-orbitals of the transition metal ions split into two groups with slightly different energy levels.  Thus, the 3d electrons can absorb light to transit from a lower energy level to a higher energy level.  This results in coloured ions.

When Fe(II) salt is dissolved in water, light green  is formed, resulting in a pale green solution.

 

When Fe(III) salt is dissolved in strong alkali, pale violet is formed.  However, Fe(III) solution in acidic or neutral medium is yellow due to the formation of yellow

When another stronger ligand is present, Fe ions would form another complex, resulting in a different solution of a different colour.  For example, Fe(III) ions react with cyanide CN(aq) to give a deep blue complex .

 

Reactions of Fe(II) compounds

1.Oxidation

  Fe2+ is a reducing agent and it can be oxidized by common oxidizing agents like KMnO4, conc. HNO3, etc.

   

  As mentioned before, Fe(III) is more stable than Fe(II).  Thus, nearly all compounds of Fe(II) can be oxidized readily to Fe(III) in air.

2. Precipitation with hydroxide

  Fe2+ reacts with OHto give green gelatinous Fe(OH)2(s), which quickly undergoes further oxidation to give brown Fe(OH)3(s).

             

     4Fe(OH)2(s) + O2(g) +2H2O(1)4Fe(OH)3(s)

3. Hydrolysis

  When dissolved in water, Fe2+ undergoes hydrolysis to give a slightly acidic solution.

   

4. Complex formation

  Other than forming complex with water, Fe2+ can also form complex with CN(aq) to give pale yellow .

 

Reactions of Fe(III) compounds

1. Reduction

  Fe3+ can be reduced by strong reducing agents like Zn, I

   

 

2. Precipitation with hydroxide

  Fe3+ reacts with OH to give brown gelatinous Fe(OH)3(s) which does not dissolve in excess NH3(aq) or NaOH(aq).

     

3. Hydrolysis

  Fe3+ undergoes extensive hydrolysis to give a very acidic solution.

 

  The acidity of Fe3+ is higher than that of Fe2+ because Fe3+  has a higher polarizing power.  Therefore, Fe3+ attracts more strongly to the electrons on the oxygen atom in the water molecule.  This weakens the O-H bond and thus, the hydrogen in water would be more easily ionized.

4. Complex formation

  Fe3+(aq) forms a lot of complexes.

  e.g. Fe3+(aq) reacts with NCS(aq) to give a red complex.

     

Ways to distinguish Fe(II) and Fe(III)

1. Fe(II) reacts with potassium hexacyanoferrrate(III) to give deep blue precipitates, while Fe(III) reacts to give a brown solution instead.

2. Fe(III) reacts with thiocyanate NCS(aq) to give a blood red solution due to the formation of the red complex .  Fe(II) does not form such complex.

Iron used as a catalyst

Iron is used as a catalyst in Haber process.

The reason for catalytic activity is due to variable oxidation state of iron.

Rusting of iron

Rusting is in fact an electrochemical process.  It requires the presence of oxygen and water.  When iron is exposed to air, a water film would form on its surface.  Some air would then dissolve into the water layer.  Due to the presence of impurities, non-uniform packing of atoms and non-uniform thickness of water film, some regions of the piece of iron would become more readily oxidized (the anodic region), while at other regions (the cathodic region), reduction of oxygen is more likely to occur.  With the presence of anodic and cathodic regions, redox reaction would occur.

At anode: Fe(s)  Fe2+(aq) + 2e

At cathode: O2(g) + 2H2O(1) + 4e4OH(aq)

The iron(II) ions formed react with hydroxide ions to give Fe(OH)2, which is immediately oxidized by air to give Fe(OH)3(s).

                   

         

Overall:    

Rate of rusting is increased by the presence of acids or other electrolytes, high temperature, contact with metals less reactive than iron and the presence of bent or sharp region in the piece of iron.

Reactions of iron

Iron shows all typical properties of metals.  Iron reacts with dilute acids to give hydrogen.

e.g. Fe(s) + 2HCl(aq)Fe2+(aq) + 2Cl(aq) + H2(g)

Iron is a strong reducing agent.

e.g. iron can reduce copper (II) ion to copper.

    Fe(s) + Cu2+(aq)Cu(s) + Fe2+(aq)