Chapter 7 The p Block Elements Solutions
Question - 41 : - Illustrate how copper metal cangive different products on reaction with HNO3.
Answer - 41 : -
Concentrated nitric acid isa strong oxidizing agent. It is used for oxidizing most metals. The products ofoxidation depend on the concentration of the acid, temperature, and also on thematerial undergoing oxidation.
Question - 42 : - Give the resonating structuresof NO2 and N2O5.
Answer - 42 : - (i)
(ii)
Question - 43 : - The HNH angle value is higherthan HPH, HAsH and HSbH angles. Why? [Hint: Can be explained on thebasis of sp3 hybridisation in NH3 andonly s−p bonding between hydrogen and other elements of thegroup].
Answer - 43 : -
Hydride NH3 PH3 AsH3 SbH3
H−M−H angle 107° 92° 91° 90°
The above trend in the H−M−H bondangle can be explained on the basis of the electronegativity of the centralatom. Since nitrogen is highly electronegative, there is high electron densityaround nitrogen. This causes greater repulsion between the electron pairsaround nitrogen, resulting in maximum bond angle. We know thatelectronegativity decreases on moving down a group. Consequently, the repulsiveinteractions between the electron pairs decrease, thereby decreasing the H−M−Hbond angle.
Question - 44 : - Why does R3P=O existbut R3N=O does not (R = alkyl group)?
Answer - 44 : -
N (unlike P) lacks the d-orbital.This restricts nitrogen to expand its coordination number beyond four.Hence, R3N=O does not exist.
Question - 45 : - Explain why NH3 isbasic while BiH3 is only feebly basic.
Answer - 45 : -
NH3 isdistinctly basic while BiH3 is feebly basic.
Nitrogenhas a small size due to which the lone pair of electrons is concentratedin a small region. This means that the charge density per unit volume is high.On moving down a group, the size of the central atom increases and the chargegets distributed over a large area decreasing the electron density. Hence, theelectron donating capacity of group 15 element hydrides decreases on movingdown the group.
Question - 46 : - Nitrogen exists as diatomicmolecule and phosphorus as P4. Why?
Answer - 46 : -
Nitrogen owing to its small sizehas a tendency to form pπ−pπ multiple bonds withitself. Nitrogen thus forms a very stable diatomic molecule, N2. Onmoving down a group, the tendency to form pπ−pπ bondsdecreases (because of the large size of heavier elements). Therefore,phosphorus (like other heavier metals) exists in the P4 state.
Question - 47 : - Write main differences betweenthe properties of white phosphorus and red phosphorus.
Answer - 47 : -
| White phosphorus | Red Phosphorus |
| It is a soft and waxy solid. It possesses a garlic smell. | It is a hard and crystalline solid, without any smell. |
| It is poisonous. | It is non-poisonous. |
| It is insoluble in water but soluble in carbon disulphide. | It is insoluble in both water and carbon disulphide. |
| It undergoes spontaneous combustion in air. | It is relatively less reactive. |
| In both solid and vapour states, it exists as a P4 molecule. | It exists as a chain of tetrahedral P4 units. 
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Question - 48 : - Why does nitrogen show catenationproperties less than phosphorus?
Answer - 48 : -
Catenation is much more common inphosphorous compounds than in nitrogen compounds. This is because of therelative weakness of the N−N single bond as compared to the P−P singlebond. Since nitrogen atom is smaller, there is greater repulsion of electrondensity of two nitrogen atoms, thereby weakening the N−N single bond.
Question - 49 : - Give the disproportionation reactionof H3PO3.
Answer - 49 : -
On heating, orthophosphorus acid(H3PO3) disproportionates to give orthophosphoric acid (H3PO4)and phosphine (PH3). The oxidation states of P in various speciesinvolved in the reaction are mentioned below.
Question - 50 : - Can PCl5 act asan oxidising as well as a reducing agent? Justify.
Answer - 50 : -
PCl5 can only actas an oxidizing agent. The highest oxidation state that P can show is +5. InPCl5, phosphorus is in its highest oxidation state (+5). However, itcan decrease its oxidation state and act as an oxidizing agent.