P-BLOCK ELEMENT. prepared by : Mrs. Abha shrivastava KV-3, Bhopal. General electronic configuration : ns² np¹ˉ⁶. GROUP-15 ELEMENTS.
Atomic and ionic radii : increases down the groupIonisation 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
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.PREVIOUS KNOWLEDGE (cont...)
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.GENERAL PROPERTIES OF HYBRIDES OF GROUP
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 + 2BiH3Preparation : By hydrolysis of binary metal compounds:
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 650-800 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 + 3KH2PO2Preparation : By reduction of trichlorides
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 90STRUCTURE :
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.Explanation
The important characteristics of these hydrides are : 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 NH
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.
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.Characteristics of hydrides
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
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.THERMAL STABILITY
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
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 + 3H2OREDUcing character
Ammonia has a higher boiling point than increases from NH3 to BiH3. thus, increasing order of reducing character is as follows: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.
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 derwaals forces. There van derwaals forces increase with increase in molecular size and therefore, boiling points increase on moving from PH3 to BiH3.BOILING and melting points
Ammonia forms hydrogen bonding with water molecules while increases from NH3 to BiH3. thus, increasing order of reducing character is as follows: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 givenSOlubility
Elements of group 15 form gaseous tri- increases from NH3 to BiH3. thus, increasing order of reducing character is as follows:hydrdides 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
(a)Assignment for bright students: P(15) and
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?REMEDIAL MEASURES
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?(b)Assignment for average students:
2.Phosphine has lower boiling point than ammonia. why.
3.Explain Habers process for ammonia .with condition.(c)Assignment for weak students:
2.WRITE TEN REASONING QUESTIONS FROM THE TOPIC.PROJECT :