Chemqueries: Evidence in favour of WERNER'S THEORY

Wednesday, 14 November 2018

Evidence in favour of WERNER'S THEORY

1. Electrical Conductance Measurement:

  • Molar conductance of a substance depends upon the number of charges on the particle produced by the substance.
  • As the number of the particle (i.e. ions) and charges on the coordination sphere increases, molar conductance of the compound increases.
  • For example molar conductance for aqueous solution of cobalt(III)chloride complexes with ammonia decreases in the order,
                                      [Co(NH3)6]Cl3 > [Co(NH3)5Cl]Cl2  > [Co(NH3)4Cl2]Cl  > [Co(NH3)3Cl3]
number of particle/ions             4                            3                             2                         1
                          charge             -3                           -2                           -1                         0
           1.[Co(NH3)6]Cl3 →  [Co(NH3)6]3+ + 3Cl-
           2.[Co(NH3)5Cl]Cl2  → [Co(NH3)5Cl]2+ + 2Cl-
           3.[Co(NH3)4Cl2]Cl→ [Co(NH3)5Cl]++ Cl-
            4.[Co(NH3)3Cl3]
      If we notice 1st equation, it is clear that  [Co(NH3)6]Cldissociates to give 4 particles i.e.one  [Co(NH3)6]3+ ion and three Cl-  ion And charge on the coordination sphere is -3.
So if we consider all the above equation, we will notice that there is a decreasing order in the number of particle and charges, on the coordination sphere from top to bottom. Therefore molar conductance decreases from top to bottom.

2.Cryoscopic Measurement:

  • The cryoscopic measurement means, measurement of depression in freezing point.
  • The depression in freezing point is a colligative property and depends upon the number of particles in the solution. Greater the number of particle more will be the depression in freezing point.
  • If a species dissociates into two, three, four.......ions, then depression in freezing point will be two, three, four ....... times respectively.
  • Thus the cryoscopic measurement gives the number of ions produced by the dissociation of an ionic compound.

3.Precipitation Reaction:

  • The number of ions furnished by a coordination compound can also be determined by precipitation reactions.
  • When the complexes CoCl3.6NH3,CoCl3.5NH3 ,CoCl3.4NH3 are treated with excess of AgNO3 solution, the number of chloride ion precipitated as AgCl is 3, 2, 1 respectively. This indicate that number of chloride ion in ionization sphere is 3, 2 and 1 respectively. The reactions are given below ,

           CoCl3.6NH3 +3 AgNO3 → 3AgCl + [Co(NH3)6](NO3)3
          CoCl3.5NH3 + 2AgNO3 → 2AgCl + [Co(NH3)5Cl](NO3)2

          CoCl3.4NH3 + AgNO3 → AgCl + [Co(NH3)4Cl2](NO3)

  • The structure provided by precipitation reactions are similar to those given by Werner.
  • The complex gives no precipitate with the solution. This indicates that all the three chloride ion are present in the coordination sphere in this compound.

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