P-BLOCK ELEMENTS

Electronic

configuration:- Half-filled (Gr-15) subshell (ns2np3) and fully filled (Gr-18) sub shell (ns2np6) are more stable.

Group 15

Atomic radius:-

Generally atomic radii increase down the group. N P As (end of 3d series followed by Ga(gr 13) , Ge(gr 14) Sb Bi There is small increase of atomic radius from As to Bi. This is due to poor shielding by d-orbitals.

Characteristic of d - Orbitals:-

Greatly diffused, porous in nature and large size so shields poorly. Effect -Atomic radii decreasesEffective nuclear charge on valence electrons increases

Ionization Enthalpy :- I.E. generally decreases down the group N

P I.E.1 < I.E.2 < I.E.3

AsSbBi

Group 14 has much lower Ionization enthalpy than group 15 because of half-filled p- orbital and small atomic size.

Pnicogens Chalcogens

Halogens Noble gas

13 14 15 16 17 18B C N O F Ne

2s22p1 2s22p2 2s22p3

ns2np1 ns2np2 ns2np3 ns2np4 ns2np5 ns2np6

Al Si P S Cl ArGa Ge As Se Br KrIn Sn Sb Te I Xe

Electronegativity :-

Generally decreases down the group.

N

P

As

Sb

Bi

Electronegative difference is less from As to Bi.

Physical properties:-

In group 15 N2 and P4 are polyatomic

N and P are non metals As and Sb are metalloids Bi is metal

Boiling point increases down the group.

Except N all elements show allotropy

Melting pt first increase from N to As and then decreases due to lesser number of covalent bonds formed by them due to inert pair effect.

Chemical properties:- ns2 np3

Common oxidation state-

-3, decreases down the group because of increasing metallic character

+3, increases down the group

+5,decreases down the group

In general tendency of lower oxidation state increases down the group

N +1, +2, +4 +with oxygen

P +1 and +4, + I oxo-acid

As

Sb

Bi

Disproportionation

3HNO2 (+3) → HNO3(+5) + 2NO(-2) + H2O

4H3PO3→ 3H3PO4 +PH3

N has maximum covalency of 4 because of non-availability of d-orbital. Heavier elements can expand their covalence beyond 4 because of

availability of d-orbitals. Eg.- PF6

15P [Ne] 3s2 3p3 3d0 (d-orbital is available)

Anomalous properties of N

1. Small atomic size2. High electronegativity3. High ionization enthalpy4. Absence of d orbital

pπ-pπ multiple bonding tendency –NO2, N≡NThis is due to small size, high electronegativity and effective overlapping of p-orbital. Also, because of multiple bonding it has high boiling point.Q.N2 is diatomic whereas rest are tetraatomic why?Q. N2 is gas at room temperature?

Catenation tendency –Catenation tendency is less in N than P because N-N single bond is weaker than P-P single bond. This weaker bond is due to inter-electronic repulsion of lone pairs due to small size.

Absence of dπ-pπ bond

This is due to absence of d-orbital.

P(C2H5)3 and As(C6H5)3 act as ligands

o As forms dπ-dπ bonds with transition metal elements.

Reactivity towards H2 :-

NH3, PH3, AsH3, SbH3, BiH3 (strongest reducing character)

Thermal stability decreases down the group because E-H bond length increases down the group and B.E. decreases too. Since, B.E. decreases down the group, H is released much easily. Therefore, reducing character increases down the group.

Lewis basicity :

NH3 > PH3 >AsH3 >SbH3 >BiH3

107.8 93.6 91.8 91.3 90

(Basicity depends upon availability of lone pair for donation)

Smaller size of nitrogen makes density of lone pair higher and hence its basicity.

Reactivity towards oxygen :-

E2O3 < E2O5

(+3) (+5)

Acidic character decreases down the group

Eg.- N2O3 (acidic)

Bi203 (basic, because metallic character increases down the group)

N2O3 acidic (Nitrogen forms large number of oxides (+1 to +5) because Pπ- Pπ bonding)

P2O3 (non-metal)

As2O3

amphoteric

Sb2O3

Bi2O3 – basic

Note- higher oxidation states are more electronegative. Therefore, the oxides are more acidic.

Q. Arrange trioxides and pentoxides in terms of acidity?

Reactivity towards Halogens :-

1. N does not form pent halides because of non- availability of d- orbital.2. EX3 < EX5 (Covalent Character), because E+5 is more electronegative.

High charge on cation Greater polarizing power (Fajan’s Rule)

3. Only NF3 and no other N halides are stable because other halogens are big enough not to be accommodated around smaller N atom.

4. Trihalide except BiF3(Bi being metal) are predominantly covalent in nature.5. Trihalides are more stable than pentahalides because (i) down the group

stability of +3 O.S increases due to inert pair effect. (ii) with increase in size of halogen strength of P-X bond decreases and also gets sterically hindered.As PCl5 is thermally less stable therefore dissociates on heating PCl5 → PCl3 + Cl2 (also acts as good chlorinating agent)

6. Trihalides of P,As and Sb and all penthalides act as lewis acid due to vacant d orbital and is supported by following reaction:

PF3 + F2 → PF5 PCl5 + Cl- → PCl6-

Reactivity towards metal :-

Group 15 elements combine with metal as they have -3 oxidation state. Example- Ca3N2, Ca3P2, Na3As2, Zn3Sb2, Mg3Bi2

Preparation of N2 -

Preparation in industries – liquefaction and then fractional distillation of Nitrogen.

Preparation in labs-

NH4Cl(aq) + NaNO2 → N2(g) + 2H2O + NaCl2 (aq)

(NH4)2Cr2O7 ∆→

N2 + 4H2O + Cr2O3

NO and HNO3 are also formed as by products.

To obtain very pure N2 –

Ba(N3)2 ∆→

Ba + 3N2

2NaN3 ∆→

2Na + 3N2

Properties of N2 – Inert at room temperature because of high bond energy. N2 gets reactive when temperature is increased

Ex.- 6Li + N2 ∆→

2Li3N (reaction of N2 with metals)

3Mg + N2 → Mg3N2

N2 + H2 773K→

2NH3 (reaction of N2 with non-metals)

N2 + O2 ∆→

2NO

Preparation of Ammonia –NH2CONH2 + 2H2O → (NH4)2CO3¿↔ 2NH3 + H2O +CO2

2NH4Cl + Ca(OH2) → 2NH3 + 2H2O +CaCl2 (ON SMALL SCALE)

(NH4)2SO4 + 2NaOH → 2NH3 + 2H2O + Na2SO4

N2 + 3H2 700K /200atm↔

2NH3 △H = -46.1 KJ/mol (ON LARGE SCALE)

(Haber’s process)

High Pressure and low temp is needed along with Iron oxide + Al2O3 + K2O as catalyst and molybdenum as promoter.

Properties of Ammonia –

High melting and boiling point due H- bonding

Less than tetrahedral angle due to repulsion between lone pair and bond pair.

Highly soluble in water due to H- bonding.

H- bonding

NH3 in water is basic. NH3 +H2O ¿↔ NH4OH

It forms salts with acids. NH3 + HCl → NH4Cl NH3 + H2SO4 → (NH4)2SO4

It precipitates metals as hydroxides ZnSO4 + 2NH4OH → Zn(OH)2 (s) + (NH4)2SO4

(White ppt) FeCl3 + NH4OH → Fe2O3.xH2O(s) + NH4Cl

(Brown ppt) NH3 has lone pair on N hence

1. Shows Lewis basicity.2. Forms coordination compounds.(act as ligand) Ex.- Cu+2 + 4NH3 → [Cu(NH3)4] (aq.) Ag+(aq) + Cl+(aq) → AgCl (white ppt.)

But, Ag+ + 2NH3 →[Ag(NH3)2]+

NH3 acts as a Lewis base due to presence of lone pair of N.

Oxides of N:-

NO2dimerises because it is an odd e- molecule2NO2 ¿↔ N2O4 (even e- species are more stable)

Resonating structure of N2O4

Resonating structure of N2O5

Note: Covalence in both cases remains same i.e 4

Nitric acid (HNO3):- It is an oxoacid of N. other oxoacid of N are H2N2O2(hypo nitrous acid) and HNO2.

Meaning of oxoacid- at least one =O and one –OH connected to central atom.

Related questions –

1. Why do NO2 dimerise? (it is an odd e- molecule)2. Name three oxoacids of N3. What is the oxidation state of N in HNO2 ? (+3)

Preparation of HNO3 –

In lab : NaNO3 + H2SO4 ∆→

NaHSO4 + HNO3

Large scale : Through Ostwald process (catalytic oxidation of NH3 by

amphoteric O2).

1. 4NH3(g) + 5O2(g)

PtRh500K ,9 ¯¿→

¿¿

4NO(g) + 6H2O

2. 2NO(g) + O2 ¿↔ 2NO2(g)3. 3NO2(g) + H2O ¿→ 2HNO3(aq) + NO(g)

Properties of HNO3 :-

HNO3 in gaseous state exists as a planar molecule H O

O N O

In aqueous solution HNO3 is strong acid. HNO3 +H2O ¿→ H3O+ + NO3

-

Reaction with metals-

Conc. HNO3 is good oxidation agent (oxidises all metals except Au and Pt).Note:- different products are formed with different metals and different concentration.Ex. - Cu + dil. HNO3 ¿→ 3Cu(NO3)2 + 2NO + 4H2O Cu + conc. HNO3 ¿→ Cu(NO3)2 + 2NO2 + 2H2O 4Zn + dil. HNO3 ¿→ 4Zn(NO3)2 + N2O + 5H2O Zn + conc. HNO3 ¿→ Zn(NO3)2 + 2NO2 + 2H2ONote: Nature of oxides of nitrogen differs with conc. and nature of metal.

Cr and Al form passive layer on reaction with HNO3, therefore do not dissolve.

Reaction with non metals-

I2 + 10HNO3 ¿→ 2HIO3 + 10NO2 + 4H2O

C + 4HNO3 ¿→CO2 + 2H2O + 4O2

S8 + 48HNO3 ¿→ 8H2SO4 + 48NO2 + 16H2O

P4 + 20HNO3 ¿→ 4H3PO4 + 20NO2 + 4H2O

Brown ring test –

Reduced

NO3- + 3Fe+2 + H+ ¿→ 3Fe+3 + NO + 2H2O

Oxidised

NO + [Fe(H2O)6]+2 ¿→ [Fe(H2O)5NO]+2 + H2O

(Brown ring) Phosphorus

Types- White Red (Three main forms out of many) Black

i. White phosphorus :

Properties-

i. White waxy solidii. Translucentiii. Poisonousiv. Insoluble in water

v. Soluble inCS2

vi. Glows in dark(chemiluminescence)vii. Highly reactive

Dissolves in NaOH in inert atmosphere. P4 + 3NaOH + 3H2O → PH3 + 3NaH2PO2 (sodium hypophosphite)

Tetrahedral P4

Angle far less than tetrahedral angle, thereforei. Experience angular strainii. Less stableiii. Highly reactiveiv. Catches fire (P4 + 5O2 → P4O10)

Q. Nitrogen is diatomic but phosphorous is tetratomic. Why?

A: It is due to Pπ-Pπ multiple bonding in case of N2.

ii. Red phosphorus -

Properties-

i. Odourless having iron grey lusture.ii. Non poisonous iii. Insoluble in water

iv. Insoluble inCS2

v. Less reactive compared to white Pvi. Do not glow in dark

White P

△ at 573k inert atmospherefor several days

¿→

¿¿¿

Red P △→

series of phases of black P

Phosphine – PH3

Preparation:

Ca3P2 + 6H2O ¿→ 3Ca(OH)2 + 2PH3

Ca3P2 + 6HCl ¿→ 2CaCl2 + 2PH2

In lab :

P4 + 3NaOH + 3H2O CO 2→

PH3 + 3NaH2PO2 (sodium hypophosphite)

PH3- when pure is non inflammable becomes inflammable due to presence of P4 and P2H4 vapoursTo purify - PH3 + HI ¿→ PH4I PH4I + KOH ¿→ KI + H2O + PH3(pure)

Properties –

Colourless gas Rotten fish smell Highly poisonous Explodes when contacts with oxidised agents like- HNO3, Cl2, Br2.

Slightly soluble in water PH3 decomposes in water in the presence of light

PH3 + H2O =light→

red P + H2O

PH3 and Metal salts –

2CuSO4 + 2PH3 ¿→ Cu3P2 + 3H2SO4

3HgCl2 + 2PH3 ¿→ Hg3P2 + 6HCl

Corresponding phosphides Basic character of PH3 –

Weak basic like ammonia.PH3 + HBr ¿→ PH4Br (phosphonium bromide)

Uses of PH3 – As smoke sirens Holme’s signals

Mixture of CaC2 + Ca3P2

Containers are pierced and thrown into the sea, gases evolve and burn and serve as a signal.

3PH3 + 4O2 ¿→ P2O5 + 3H2O

(fumes)

Structure –

Tetrahedral

Bond angle of PH3 < PH4+ (no lone pair in case of PH4

+)

Phosphorus trichloride –(PCl3)Preparation :

White P4 + 6Cl2 ¿→ 4PCl3White P4 + 8SOCl2 ¿→ 4PCl3 + 4SO2 + 2S2Cl2 (Thionyl chloride)

Properties :

Colourless oily liquid.

Hydrolysis of PCl3 –

PCl3 + 3H2O ¿→ H3PO3 + 3HCl

(Due to above reaction PCl3 fumes in moisture)

Reaction of PCl3 with organic compounds – 3CH3COOH + PCl3 ¿→ 3CH3COCl + H3PO3

3CH3CH2OH + PCl3 ¿→ 3C2H5Cl + H3PO3

Structure/shape and hybridization of PCl3 :

i. Pyramidal in shapeii. Sp3 hybridization

Phosphorus Pentachloride (PCl5) –

Preparation –

White P4 + 10 Cl2 ¿→ 4PCl5

P4 + 10 SO2Cl2 ¿→ 4PCl5 + 10 SO

Properties –

Yellowish white powder Hydrolysis of PCl 5

PCl5 + H2O ¿→ POCl3 + 2HCl (Partial hydrolysis) POCl3 + 3H2O ¿→ H3PO4 + 3HCl (Complete hydrolysis)

Q. write the chemical equation for hydrolysis of PCl5 with heavy waterA. PCl5 + D2O ¿→ POCl3 + 3DCl (POCl3 + 3D2O ¿→ D2PO3 + 3DCl)

Q. why does PCl3 fume in air?A. PCl3 + 2H2O ¿→ H3PO3 + 3HCl

Sublimation and decomposition of PCl5 :

PCl5

△¿→

¿¿¿

PCl3 + Cl2

Reaction of PCl5 with organic compounds (containing -OH)C2H5OH + PCl5 ¿→ C2H5Cl + POCl3 + HClCH3COOH + PCl5 ¿→ CH3COCl + POCl3 + HCl

Reaction of PCl5 with metals- 2Ag +PCl5 ¿→ 2AgCl + PCl3Sn + 2PCl5 ¿→ SnCl4 + 2PCl3

Structure of PCl5 –In liquid and gas phase-

Trigonal bypiramidal In solid phase – Ionic form

PCl4+ PCl6-

Tetrahedral Octahedral

Oxoacids of phosphorus:

(Tribasic) H3PO4 (tetra basic) H4P2O7

Orthophosphoric acid pyrophosphoric acid

(dibasic) H3PO3 (monobasic) H3PO2

Orthophosphoric acid hypophosphoric acid

Cyclo trimetaphosphoric acid

In above, there is at least- One P=O One P-OH

Intermediate oxidation states(+3) disproportionate 4H3PO3 ¿→ 3H3PO4 + PH3

(+3) (+5) P-H containing molecules have reducing property(no role in basicity)

4AgNO3 + 2H2O + H3PO2 ¿→ 4Ag + 4HNO5 + H3PO4