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Group 15: Chemical Reactivity

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Group 15: Chemical Reactivity - Lesson Summary

Reactivity towards hydrogen:
All the elements of group 15 form hydrides of the type EH 3.Here E can be nitrogen, phosphorus, arsenic, antimony or bismuth.
      
            2E    +     3H 2           →        2EH 3
                         Hydrogen              Hydride
             N2    +    3H 2           →        2NH 3
         Nitrogen                                Ammonia
             P 4       +     6H 2            →             4PH 3
       Phosphorus                             Phosphine
            AS 4         +    6H 2            →             4ASH 3
         Arsenic                                    Arsine
            4Sb       +    6H 2            →             4SbH 3
           
Antimony                                 Stibine
            4Sb       +    6H 2            →             4SbH3
         Bismuth                                  Bismuthine          

Stability:
The stability of hydrides decreases from ammonia to bismuthine. This is because the central atom E increases in size down the group. With the increase in the size of the central atom,   the   E – H bond becomes weaker.

With the increase in size, the overlapping between the orbitals of E and hydrogen will not be effective. This accounts for the decrease in the stability of hydrides as we move down the group.



Reducing character:
Hydrides of group 15 elements are strong reducing agents. The reducing character of hydrides increases from ammonia to bismuthine due to a decrease in the strength of the E – H bond down the group. Bismuthine is the strongest reducing agent among all the hydrides of group 15 elements.

Basic nature:
These hydrides are basic in nature. They act as Lewis bases due to the availability of a lone pair of electrons on the central atom. The basic character decreases down the group with an increase in the size of the central atom.

Boiling point:
The boiling point of hydrides decreases from ammonia to phosphine, and then increases from   phosphine to bismuthine. The same trend is observed for their melting points.

Reactivity towards oxygen:
Group 15 elements form 2 types of oxides; They are E 2O 3 and E 2O 5.
Nitrogen shows a strong tendency to form pπ-pπ bonds with oxygen. Hence nitrogen forms a wide range of oxides. Nitrogen forms 5 stable oxides,  

They are    Oxidation State                                           Oxides                  N      P     As      Sb     Bi             +1

            +2

            +3

            +4

            +5              N 2O
       Nitrous oxide
              NO
       Nitric oxide
             N 2O 3
    Dinitrogen trioxide
             N 2O 4
   Dinitrogen tetroxide
             N 2O 5
    Dinitrogen pentoxide     -

    -

 P 4O 6

    -

 P 4O 10      -

    -

 As 4O 6

     -

  As 4O 10 -

-

 Sb 4O 6

      -


 Sb 4O 10 -

-

 Bi 2O 3

    -


   ⊗
 Electronic Configuration of Bi: [Xe]4f 145d 106S 26P 3
                                                                               ↓
                                                                         Inert Pair

Bismuth can’t form oxides in +5 oxidation state because of the inert pair effect. Oxides in the higher oxidation state of an element are more acidic than that of the lower oxidation state. Hence the acidic strength of oxides of nitrogen increases from N 2O 3 to N 2O 5.

As we move down the group, the acidic character decreases. In other words, the basic character of oxides increases on moving down the group.

Reactivity towards halogens:
All the elements of group 15 form trihalides and pentahalides with the general formula EX 3 and EX 5.

    2E      +       2X 2          →          2EX 3
                    Halogen                  Trihalide

    2E      +       5X 2          →          2EX 5
                    Halogen                  Pentahalide

EX:  NF 3, PF 3, AsF 3, SbF 3 and BiF 3 are trihalides.

All the trihalides of these elements, except those of nitrogen, are stable. Phosphorus, arsenic and antimony form pentahalides due to the presence of vacant d orbitals in their valence shells. Nitrogen does not form pentahalides due to the absence of a d orbital in its valence shell. Pentahalides are more covalent than the corresponding trihalides. Moreover, the covalent character of halides decreases from nitrogen to bismuth.
 
All the elements of group 15 react with metals to form their binary compounds showing -3 oxidation state with the general formula M 3E 2. Here, M stands for metals while E stands for an element of group 15. EX: calcium nitride, calcium phosphide etc.

     3M   +    2E        →     M 3E 2
                                      Binary compound
     3Ca  +     N 2        →    Ca 3N 2
                                      Calcium nitride
     6Ca  +     P 2        →    2Ca 3P 2
                                      Calcium phosphide
     6Zn  +     4Sb       →    2Zn 3Sb 2
                                      Zinc antimonide
     6Mg  +     4Bi       →    2Mg 3Bi 2
                                       Magnesium bismuthide

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