prepared by : Mrs. Abha shrivastava - e-CTLT

Report
P-BLOCK ELEMENT
PREPARED BY :
MRS. ABHA SHRIVASTAVA
KV-3, BHOPAL
General electronic configuration :
ns² np¹ˉ⁶
GROUP-15 ELEMENTS
PREVIOUS KNOWLEDGE
Atomic and ionic radii : increases down the group
Ionisation Enthalpy : 1st I.E. Of group 15 elements
are higher than group 14 elements and on going
down the group, IE decreases.
 Electronegativity : Value of Group 15 are higher
than group 14 and on going down the group, value
decreases.
 Metallic Character : On going down the group
metallic character increases from N to Bi.

PREVIOUS KNOWLEDGE (CONT...)
Catenation : group 15 show a tendancy towards
catenation, but to a much smaller extent than
Carbon.
 Allotropy : Except N and Bi , all elements of the
group show allotropy.
 Oxidation States : They exhibit oxidation states
from -3 to +5. The tendancy to exhibit -3 oxidation
state decreases down the group and the stability
of +5 oxidation state decreases while that of +3
increases due to inert pair effect.

GENERAL PROPERTIES OF HYBRIDES OF GROUP
Reactivity towards hydrogen (formation of hydrides)
All the elements of group 15 form gaseous tri
hydrides of the formula EH3 (where E=N, P, As, Sb or
Bi) such as :
NH3
PH3 AsH3
SbH3
BiH3
The lighter elements also form hydrides of the fomula
M2H4 such as N2H4 (hydrazine), P2H4 (diphosphine)
and HN3 known as hydrogen azide or hydrazoic acid.
PREPARATION : BY HYDROLYSIS OF BINARY METAL COMPOUNDS:
The tri hydrides can be easily obtained by the hydrolysis of
their binary metal compounds with water of dilute acids :
Mg3N2 + 6H2O ------- 3Mg(OH)2 + 2NH3
Ca3N2 + 6H2O ------- 2Ca(OH)2 + 2NH3
Ca3P2 + 6H2O
-------- 3Ca(OH)2 + 2PH3
Zn3E2 + 6HCl
-------- 3ZnCl2 + 2EH3
Zn3As2 + 6HCl --------- 3ZnCl2 + 2AsH3
Mg3Bi2 + 6HCl --------- 3MgCl2 + 2BiH3
PREPARATION : BY REDUCTION OF TRICHLORIDES
The tri chlorides of these elements except that of bismuth give the
corresponding hydrides on reduction with Zn/acid or LiAlH4
ECl3 + 3LiAlH4
------EH3 + 3LiCl + 3AlH3 (E=N,P, As, Sb)
Ammonia (H3) is the most important trihydride of the group and is
extensively used in the manufacture of nitric acid (HNO3) and important
chemical fertilizers such as ammonium sulphate, urea, calcium ammonium
nitrate, calcium cyanamide, etc. it is prepared on an industrial scale by
Haber process. In this process, nitrogen combines with hydrogen at 650800 K under a pressure of 200-350 atm in the presence of iron catalyst
and Mo as promoter.
N2 + 3H2
------------ 2NH3; H = -92.4 kJ mol-1
Phosphine (PH3) can be prepared by heating white phosphorus with
concentrated caustic alkali in an inert atmosphere of oil gas :
P4 + 3KOH + 3H2O --------- PH3 + PH3 + 3KH2PO2
STRUCTURE :
All these hydrides are covalent in nature and have pyramidal
structure. These involves sp3 hybridization of the central atom
and one of the tetrahedral position is occupied by a lone pair.
The structure of NH3 molecule is shown in Fig. 2. Due to the
presence of lone pair, the bond angle in NH3 is less than the
normal tetrahedral angle. It has been found to be 107. as we go
down the group the bond angle decreases as :
NH3
PH3
AsH3
SbH3
BiH3
107.8
93.6
91.8
91.3
90
EXPLANATION
In all these hydrides, the central atom is surrounded by four
electron pairs, three bond pairs and one lone pair. Now, as we
move down the group from N to Bi, the size of the atom goes on
increasing and its electro negativity decreases. Consequently,
the position of bond pair shifts more and more away from the
central atom in moving from NH3 to BiH3. for example, the bond
pair in NH3 is close to N in N-H bond than the bond pair in P-H
bond in PH3. As a result, the force of repulsion between the
bonded pair of electrons in NH3 is more than in PH3. in general,
the force of repulsion between bonded pairs of electrons
decreases as we move from NH3 to BiH3 and therefore, the
bond angle also decreases in the same order.
CHARACTERISTICS OF HYDRIDES
The important characteristics of these hydrides are :
I) Basic Strength :
All these hydrides have one lone pair of electrons on their central
atom. Therefore, they act as Lewis bases. The can donate an electron
pair to electron deficient species (Lewis acids). As we go done the
group, the basic character of these hydrides decreases. For example,
NH3 is distinctly basic; PH3 is weakly basic; AsH3, SbH3 and BiH3 are
very weakly basic.
Explanation
Nitrogen atom has the smallest size among the hydrides. Therefore
the lone pair is concentrated on a small region and electron density
on its is the maximum. Consequently, its electron releasing tendency
is maximum. As the size of the central atom increases down the
family, the electron density also decreases. As a result, the electron
donor capacity or the basic strength decreases down the group.
THERMAL STABILITY
Thermal stability of the hydrides of group 15 elements
decreases as we go down the group. Therefore, NH3 is
most stable and BiH3 is least stable. The stability of the
hydrides of group 15 elements decreases in the order :
NH3 > PH3 > AsH3> SbH3 > BiH3
Explanation :
This is due to the fact that on going down the group, the
size of the central atom increases and therefore its
tendency to form stable covalent bond with small
hydrogen atom decreases. As a result the M-H bond
strength decreases and therefore thermal stability
decreases.
REDUCING CHARACTER
The reducing character of the hydrides of group 15 elements
increases from NH3 to BiH3. thus, increasing order of reducing
character is as follows:
NH3 < PH3 < AsH3 < SbH3 < BiH3
Explanation :
The reducing character depends upon the stability of the hydride. The
greater unstability of group 15 hydrides decreases from NH3 to BiH3,
hence the reducing character increases. For example, NH3 being
most stable among the group 15 hydrides is not a good reducing
agent. The other hydrides being unstable act as good reducing
agents. However, it may be noted that ammonia at high temperature
reduces copper oxide to copper :
3CuO + 2NH3 --------- 3Cu + N2 + 3H2O
BOILING AND MELTING POINTS
Ammonia has a higher boiling point than phosphine and then the boiling
point increases down the group because of increase in size.
Molecule
NH3
PH3
AsH3
SbH3
BiH3
Boiling Point (K)
238.5
185.5 210.6 254.6
290
Melting Point (K)
195.2
139.5 156.7 185.0
----Similar behavior is observed for melting points.
Explanation :
The abnormally high boiling point of ammonia is due to its tendency to form
hydrogen bonds. The electro negativity of N(3.0) is much higher than that
H(2.1). As a result N-H bonds are polar and therefore NH3 undergoes
intermolecular hydrogen bonding.
In PH3 and other hydrides, the intermolecular forces are van der waals
forces. There van der waals forces increase with increase in molecular size
and therefore, boiling points increase on moving from PH3 to BiH3.
SOLUBILITY
Ammonia forms hydrogen bonding with water molecules while phosphine
and other hydrides do not form hydrogen bonding with water. Therefore NH3
is soluble I water while PH3 ad other hydrides are insoluble I water.
The trends in the properties of hydrides of group 15 are given
Elements of group 15 form gaseous trihydrdides of the formula EH3
These hydrides are covalent in nature and
have pyramidal structure.
These involve sp3 hybridisation of the central
atom and one of the tetrahedral position is
occupied by a lone pair.
Bond angle decreases down the group due to
presence of lone pair and increase in size of
atom.
Basic strength decreases down the group
due to increase in size and the electron
donor capacity decreases.
Thermal stability decreases down the group
due to increase in size.
Reducing character increases down the
group since the stability decreases from NH3
to BiH3 .
Ammonia has high BP due to its tendency to
form H-bond.
All hydrides are insoluble except NH3 due to
H-bonding.
RECAPITUALATION
HOMEWORK





Write the electronic configuration of the given
element P(15) and Sb (51)?
Explain why ammonia is basic in nature.
How is ammonia manufactured industrially?
Discuss the trend in chemically reactivity of
group 15.
Why ammonia is a good complexing agent.
REMEDIAL MEASURES
(a)Assignment for bright students:
1.How dose ammonia react with a solution of Cu2+.
2.PCl5 is ionic in solid state .why .
3.Give the shape of the following: P4 , PF5 , NF3.
4. The HNH angle value is higher than HPH, HASH
and HSbH angles. Why ?
5. PH3 forms bubbles when passed slowly in water
but NH3 dissolves. Why?
(B)ASSIGNMENT FOR AVERAGE STUDENTS:
1.Discuss the hydrides of group 15 elements.
2.Draw the structure of the NH3 & PH3
3.Why NH3 is basic in nature.
4.Why NH3 is more soluble in water.
5.Which hydride of group 15 element has lowest
boiling point and why?
(C)ASSIGNMENT FOR WEAK STUDENTS:
1.Write the electronic configuration of group 15
element.
2.Phosphine has lower boiling point than
ammonia. why.
3.Explain Habers process for ammonia .with
condition.
PROJECT :
1.TO PREPARE STRUCTURAL
DIAGRAM OF HYDRIDE OF
NITROGEN AND PHOSPHOROUS.
2.WRITE TEN REASONING
QUESTIONS FROM THE TOPIC.

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