(1) All elements of this group form
gaseous hydrides of the type MH3.
Trisilyl amine, N(SiH3)3 is planar whereas trimethyl amines N(CH3)3 is pyramidal. Explain why?.
Why bond angle of hydrides of group 15, decreases down the group?
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 size, its lone pair density decreases at central atom hence attack of Electrophilic (positive 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:
No comments:
Post a Comment