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Tuesday, July 2, 2019

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 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:

Which is more basic N(CH3)3 or N(SiH3)3 , explain?

Are all the five bonds of PCl5 equivalent? Justify your answer.

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















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Monday, July 1, 2019

Halides of Nitrogen family- 15th Group:

Two types of halides are available for this group. One is MX3 and another is MX5 type.
(A) MX3 Type or Trihalides:
MX3:

NCl3, PCl3, AsCl3, SbCl3 and BiCl3
Characteristic of Tri halides:
(1) All possible trihalides of all the elements of nitrogen family are known except NCl3, NBr3, and NI3.
(2) These are (NCl3, NBr3, and NI3) unstable due to the low polarity of the (N – X) bond. And Weakness of (N - X) bond due to large size difference.
(3)  All trihalides are covalent except BiF3 which is ionic.
(4)  Like hydrides these trihalides have pyramidal structure and a central atom is sp3 hybridized.

Structure of PCl3:
(5)  These trihalides can be easily hydrolyses by water except NX3.

SN
Halides
 Hydrolysis

NF3
Do not Hydrolysed at room temperature
1
NX3 (X= Cl, Br, I)
NX3 +3H2O à NH3 + 3HOX          (At RT)
2
PX3 (X= F Cl, Br, I)
PCl3 +  3H2O à H3PO3 + 3HCl
2
AsCl3
AsCl3  + H2O à AsO3 + 6HCl    or
AsCl3  + 3H2O à H3AsO3 + 3HCl
4
SbCl3
SbCl3 + H2O à SbOCl (White turbidity) + 2HCl
5
BiCl3
BiCl3 + H2O à BiOCl  (White turbidity) + 2HCl

Imp note:
(i) NF3 Hydrolysed only at high temperature (300 0 C) and give N2O3 which is unstable and dissociate to produced NO and NO3 gas.
                           2NF3 + 3H2O  à N2O3 +6HF à NO (gas) + NO2 (gas) + 6HF
(ii)When an aq. solution of BiCl3 is prepared, after some time turbidity appears which is milky in appearance and finally a white ppt is formed due to formation of BiOCl.
(6)  The trihalides of P, As and Sb acts as Lewis acids and combine with Lewis bases.
PF3 +F2  
à
PF5
SbF3 +2F
à
[SbF5]2-

 (7) Lewis Base Order of trihalides follows as
        NF3 < NCl3 <NBr3 < NI3

 Imp note: As electronegativity of halides decrease and availability of loan pair over nitrogen atom increases hence Lewis Base character increase
(B) MX5 Type or Pentahalides:
MX5:
        PCl5, AsCl5, SbCl5
Characteristic of Pentahalides:
(1) Except N and Bi all form Pentahalides, N does not form due to absence of d-orbital while Bi does not form due to inert pair effect.
(2) Phosphorous form Pentahalides of all halogens.
(3) Bismuth forms only BiF5.
(4) As and Sb form Pentafluorides and pentachlorides only.
(5) The central atom of Pentahalides attains sp3d hybridization and forms five covalent bonds with five chlorine atoms.
Structure of  PCl5  :
(6) Penta halides have less thermal stability as compared to trihalides. All Penta halides act as Lewis acids.
PF5 +F-   
à
[PF6]-
SbCl3 +Cl- 
à
[SbCl4]-

(7) PCl5 acts as an effective chlorinating Agent so it decomposes into PCl5 à PCl3 + Cl2
(8) PCl5 exists as molecule in gaseous state but in solid it exists as [PCl4]+ [PCl6]- and is ionic in nature. PBr5, PI5 also exist in the ionic form in solid state.
SN
Halides(Gaseous state)
Solid State
1
PCl5
[PCl4]+[PCl6]- [sp3 and sp3d]
2
PBr5
[PBr4]+[Br]-
3
PI5
[PI4]+[I]-

Structure of PCl5  in solid state:
ILLUSTRATIVE EXAMPLE (1): Are all the five bonds of PCl5 equivalent? Justify your answer.
SOLUTION: PCl5 has trigonal bipiramidal structure. It has three equivalent equatorial and two equivalent axial P – Cl bonds. However, due to greater bond pair – bond pair repulsions, the axial P – Cl bonds are longer and hence different from the three equatorial bonds.

ILLUSTRATIVE EXAMPLE (2): Phosphorus can form PCl5 but nitrogen can not form NCl5 why?
SOLUTION: Phosphorus forms PCl5 due to availability of vacant d – orbital though white P can extend its oxidation state but this is not applicable in case of N due to unavailability of d – orbitals.

ILLUSTRATIVE EXAMPLE (3): Why NCl3 can not be hydrolysed?
SOLUTION: Due to unavailability of vacant d – orbitals.

EXERCISE (1): Among the trihalides of nitrogen, which one is the least basic?
                        (A) NF3                                                 (B) NCl3
                        (C) NBr3                                                (D) NI3
EXERCISE (2): Which of the following halides is most acidic?
                        (A) PCl3                                                (B) SbCl­3
                        (C) BiCl3                                               (D) CCl4
EXERCISE (3): Which of the following is not hydrolysed?
                         (A) AsCl3                                              (B) PF3
                         (C) SbCl3                                               (D) NF3

SN
EXERCISE
ANSWER KEY
1
1
(A)
2
2
(A)
3
3
(D)


           


Related Questions:

Which is more basic N(CH3)3 or N(SiH3)3 , explain?

Are all the five bonds of PCl5 equivalent? Justify your answer.

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















Kumar Sir at No comments:
Topic:
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