CBSE Class 12 Chemistry P Block Elements Notes Set 01

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1.Account for the following: (Group 15 elements)

(i) There is a considerable increase in covalent radius from N to P. However, from As to Bi only small increase in covalent radius is observed.

Ans: This is due to the presence of completely filled d and/or f orbital in heavier members which have low screening effect.

(ii) Ionization enthalpy decreases down the group 15.

Ans: Due to gradual increase in atomic size.

(iii) The ionization enthalpy of the group 15 elements is much greater than that of group 14 elements in the corresponding periods.

Ans: Because of the extra stable half-filled p orbital electronic configuration and smaller size.

(iv) Nitrogen exists as gas and phosphorus as solid.

Ans: Because N2 is Diatomic molecules, hence weak Vander Waal’s force of attraction thus is a gas whereas P4 is tetra atomic hence Stronger Vander Waal’s force of attraction thus it is solid.
Symbol

(v) NH3 is basic while BiH3 is only feebly basic.

Ans: NH3 is basic due to smaller size & high electron density on lone pair of Nitrogen.

(vi) The HNH angle value is higher than HPH, HAsH, and HSbH angles.

Ans: Due to increase in repulsion by the lone pair with increase in size of lone pair from N to Sb.

(vii) R3P=O exist but R3N=O does not.

Ans: Due to the absence of d orbitals in valence shell of nitrogen. Because of inability of Nitrogen to expand its covalency beyond four, nitrogen cannot form d π–p π bond.

(viii) Nitrogen shows catenation properties less than phosphorus.

Ans: Because N-N bond is weaker than the single P-P bond.& strong pπ–pπ overlap in Nitrogen.

(ix) Ammonia has higher boiling point than Phosphine.

Ans: Ammonia (NH3) form hydrogen bond but Phosphine (PH3) does not.

(x) H3PO3 is diprotic acid.

Ans: Due to presence of two ionisable –OH group. (Draw the structure of H3PO3).

(xi) Oxides of nitrogen have open chain structures while those of phosphorus have closed chain or cage structures

Ans:Nitrogen has ability to form multiple bonds involving p-p overlap while Phosphorous does not.

(xii) A nitrogen atom has five valence electrons but it does not form the compound NCl5.

Ans: Because of absence of d-orbitals it can’t expand its covalency from 3 to 5.. 

(xiii) Nitrogen is fairly inert gas. 
 
Ans: Nitrogen exists as triply bonded diatomic non polar molecule. Due to short internuclear distance between two nitrogen atoms the bond strength is very high. It is, therefore, very difficult to break the bond. 
 
(xiv) All the bonds in the molecules of PCl5 are not equal. 
 
Ans: PCl5 has a trigonal bipyramidal shape in the gas space. A trigonal bipyramidal is an irregular structure in which some bond angles are 90 degree and others of 120 degree resulting in unequal P-Cl bond lengths 
PCl5 + H2O → POCl3 + 2HCl & PCl5 →  PCl3 + Cl2 
According to VSEPR theory, axial bonds experience more repulsion and are thus longer than equatorial bonds. 
 
(xv) In solid state PCl5 exists as Ionic compound. 
Ans: Since solid phosphorous pentachloride exists as [PCl4]+ [PCl6]- and hence exhibit some ionic character. [PCl4]+ is tetrahedral and the anion, [PCl6]–octahedral.
 
(xvi) Nitrogen does not form pentahalides. 
 
Ans: Nitrogen with n = 2, has s and p orbitals only. It does not have d orbitals to expand its covalence beyond four. That is why it does not form pentahalide.
 
(xvii) NH3 is a good complexing agent.
 
Ans: Because nitrogen has lone pair of electrons which it can donate to form co-ordinate bond 
(xviii) Nitrogen shows anomalous behaviour. 
 
Ans: Nitrogen differs from the rest of the members of this group due to its smaller size, high electronegativity, high ionisation enthalpy and non-availability of d orbitals 
 
(xix) Pentavalent Bismuth is a strong oxidizing agent. 
 
Ans: Bi3+ is more stable than Bi5+ due to inert pair effect. 
d 
 
(xxi) The first ionization energy of nitrogen is greater than oxygen. 
 
Ans: Because of the extra stable half-filled p orbitals electronic configuration of nitrogen. 
 
(xxii) NCl3 gets hydrolysed easily while NF3 does not. 
 
Ans: In NCl3, Cl has vacant d-orbital but in NF3, F does not have to accept lone pair of electrons donated by Oxygen atoms of H2O 
 
(xxiii) PH3 has lower boiling point than NH3. 
 
Ans: PH3 molecules are not associated through hydrogen bonding in liquid state. That is why the boiling point of PH3 is lower than NH3. 
 
(xxiv) Pentahalides of group 15 are more Covalent than trihalides 
 
Ans :Higher the positive oxidation state of central atom, more will be its polarizing power which, in turn, increases the covalent character of bond formed between the central atom and the other atom. 
 
(xxv) N2 is less reactive at room temperature.
 
Ans: Because of strong pπ–pπ overlap Nitrogen has triple bond between two nitrogen atoms N≡N which has high bond dissociation enegy. So it is less reactive. 
 
(xxvi) NH3 act as Lewis base. 
 
Ans: Nitrogen atom in NH3 has one lone pair of electrons which is available for donation. Therefore, it acts as a Lewis base. 
 
(xxvii)Bond angle in PH4+ is higher than that in PH3. 
 
Ans: Both are SP3 hybridised. In PH4+ all the four orbitals are bonded whereas in PH3 there is a lone pair of electrons on P, which is responsible for lone pair-bond pair repulsion in PH3 reducing the bond angle to less than 109° 28′ as in PH4+ 
 
(xxviii) NO2 dimerises to form N2O4 
 
Ans:NO2 contains odd number of valence electrons. It behaves as a typical odd molecule. On dimerisation, it is converted to stable N2O4 molecule with even number of electrons.
 
(xxix) NO2 is coloured but N2O4 is colourless. 
 
Ans. NO2 has unpaired electrons therefore it absorbs light from visible and radiate brown colour whereas N2O4 does not have unpaired electrons so it does not absorb light from visible region.
 
(xxx) H3PO2 and H3PO3 act as good reducing agents while H3PO4 does not. 
 
Ans: In H3PO2, two H atoms are bonded directly to P atom & in H3PO3 one H atom is bonded directly to P atom which imparts reducing character to the acid, whereas in H3PO4 there is no H atom bonded directly to P atom 
 
(xxxi) NH3 form hydrogen bond but PH3 does not. 
 
Ans: Due to smaller size & high electro-negativity of Nitrogen. 
 
(xxxii) CN¯ ion is known but CP¯ ion is not known. 
 
Ans: Due to inability of P to form π bond while N can. 
 
(xxxiii) Nitrogen exists as diatomic molecule and phosphorus as P4. 
 
Ans: Because N has π bond forming capacity while P does not. 
Account for the following: (Group 16 elements) 
(i)Elements of group 16 generally show lower value of first ionization enthalpy compared to the corresponding periods of group 15. 
Ans: Due to extra stable half-filled p orbitals electronic configurations of Group 15 elements, lesser amount of energy is required to remove electron by Group 16 elements. 
(ii) Tendency to show –2 oxidation state diminishes from Sulphur to polonium in group 16. 
 
Ans: The outer electronic configuration of group 16 elements is ns2 np4. These elements therefore have the tendency to gain two electrons to complete octet. Since electro-negativity and I.E. decrease on going down the group, tendency to show –2 oxidation state diminishes.

 

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