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Hydrides of Nitrogen family-15th Group:

(1) All elements of this group form gaseous hydrides of the type MH3.
NH3     

PH3     
AsH3
SbH3
BiH3
Ammonia
Phosphine
Arsine
Stibine
Bismuthine
             
(2) All the hydrides are strong reducing agents and reacts with metal ions to give phosphides, arsenides or antimonides. Phosphine and other hydrides of heavier members of these groups are highly poisonous.
(3) All the Hydrides are colourless gases.
(4) Except NH3, they are poisonous.
(5) All are covalent in nature and formed by sp3 hybridisation.
(6) Bond angle is directly proportional to the electronegativity of central atom.

Properties of the EH3 Compounds of the Group 15th Elements:

SN
Properties
NH 3
PH 3
AsH 3
SbH 3
BiH 3
1
M.p.  /Kelvin
-77.7
-133.8
-117
-88
271.4
2
B.p. /Kelvin
-33.4
-87.8
-62.5
-18.4
17
3
(E-H) BL (pm)
101.7
141.9
151.9
170.7
-
4
HEH angle °C
107.8
93.7
91.8
91.3
-
5
BE(kj/ mole)
389
322
297
255
-
6
Dipole moment (D)
1.46
0.55
0.22
0.12
-
7
Δ Hf °, kJ mol
-46.11
-9.58
66.44
145.1
277.8

Trends in Properties:

(1) Thermal Stability: For binary compound having smallest Stoichiometric ratio, (Mono atomic anions) the thermal stability order decided by the inter-ionic distance. Larger the inter-ionic distance, lower will be the lattice energy and lower the lattice energy, lower will be the thermal stability. The stability of hydride of 15th Group however decreases down the group.
              NH3 > PH3 > AsH3 >  SbH3 > BiH3
(2) Bond Dissociation Energy: BDE is directly proportional to Stability of hydride.
             NH3 > PH3 > AsH3 >  SbH3 > BiH3
(3) Reducing Character: The order of reducing character is inversely proportional to thermal stability.
             NH3  < PH3  <  AsH3  <  SbH3  <  BiH3
(4) Basic Character: Basic character of hydride is decrease down the group because with increasing atomic structure lone pair at central atom decreases on it and hence attack of Electrophilic (+Ve Charge species) hence basic character decreases down the group.
             NH3 > PH3 > AsH3 >  SbH3 > BiH3
Important Note: [NH3 + H+  à NH4 + ] rate of formation of NH4+ is faster than rate formation of  PH4 +
(5) Bond Angle:  On decreasing electronegativity of central atom Bond Angle decrease.
             NH3 > PH3 > AsH3 >  SbH3 > BiH3
(6) Melting point: Melting point increase on increasing molecular mass (Vander Waal force increase) But Due to Hydrogen bonding melting point of Ammonia is exceptionally very high.
 Expected- NH3  < PH3  <  AsH3  <  SbH3  <<<< BiH3

Actual- PH3  <  AsH3  <  SbH3 < NH3   <<<< BiH3
(7) Boling point: Boiling point increase on increasing molecular mass (Vander Waal force increase) But Due to Hydrogen bonding melting point of Ammonia is exceptionally very high.
 Expected- NH3  < PH3  <  AsH3  <  SbH3  <<<< BiH3

Actual- PH3  <  AsH3  <  NH3  <  SbH3 <<<< BiH3
(8) Dipole Moment: Down the group with decreasing electronegativity of central atom , dipole  moment decreases.
             NH3 > PH3 > AsH3 >  SbH3 > BiH3

Formation of Hydrides:  


Ca3N2
+6H2O
à
Ca(OH)2
+2 NH3
Ca3P3
+6 H2O
à
2PH3
+2NH3
Zn3M3
+6HCl (aq)
à
2MH3(g)
+ZnCl2 (aq)
Where-
M= As, Sb, Bi




Beside these, N2H4 another hydride of nitrogen can be prepared by the action of NH3 and sodium hypochlorite.
             NH3 +NaOCl à N2H4 +NaCl   +H2O
Structure of Hydrides: In these entire hydrides central atom assumes sp3 hybrid state forming four sp3 hybrid orbitals. Bond angle of H – M – H decreases down the group. Down the group thermal stability also decreases.
             NH3 > PH3 > AsH3 >  SbH3 > BiH3
It is because of decrease in M – H bond strength due to increase in the size of central atom. These hydrides behave as reducing agents. And down the group reducing power increases. Boiling point of hydrides increases from PH3 to BiH3 but NH3 has exceptionally high B.P. due to presence of intermolecular hydrogen bonding. The hydrides of group 15, due to availability of lone pair on central atom act as Lewis bases. The basic character decreases down the group. This is due to the decrease in density of electron on the central atom down the group as the volume of central atom increases down the group.
ILLUSTRATIVE EXAMPLE (1): Which of the following is/are not known?
                              PCl3, AsCl3, SbCl3, NCl5, BiCl5
SOLUTION:  NCl5 (due to absence of d-orbitals)
ILLUSTRATIVE EXAMPLE (2): Which of the following is the increasing order of enthalpy of vaporization?
                        (A) NH3, PH3, AsH3                               (B) AsH3, PH3, NH3
                        (C) NH3, AsH3, PH3                               (D) PH3, AsH3, NH3
SOLUTION: (D)
Related Questions:



Trisilyl amine, N(SiH3)3 is planar whereas trimethyl amines N(CH3)3 is pyramidal. Explain why?.















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