Chapter 7 The p Block Elements Solutions
Question - 61 : - Why are halogens coloured?
Answer - 61 : -
Almost all halogens are coloured.This is because halogens absorb radiations in the visible region. This resultsin the excitation of valence electrons to a higher energy region. Since theamount of energy required for excitation differs for each halogen, each halogendisplays a different colour.
Question - 62 : - Write the reactions of F2 andCl2 with water.
Answer - 62 : - (i)
(ii)
Question - 63 : - How can you prepare Cl2 fromHCl and HCl from Cl2? Write reactions only.
Answer - 63 : -
(i) Cl2 canbe prepared from HCl by Deacon’s process.
(ii) HClcan be prepared from Cl2 on treating it with water.
Question - 64 : - What inspired N. Bartlett forcarrying out reaction between Xe and PtF6?
Answer - 64 : - Neil Bartlett initially carried out a reaction betweenoxygen and PtF6. This resulted in the formation of a redcompound, 
Later, he realized that the first ionization energy ofoxygen (1175 kJ/mol) and Xe (1170 kJ/mol) is almost the same. Thus, he tried toprepare a compound with Xe and PtF6. He was successful and ared-coloured compound,
was formed.
Question - 65 : - What are the oxidation states ofphosphorus in the following:
(i) H3PO3 (ii) PCl3 (iii) Ca3P2
(iv) Na3PO4 (v) POF3?
Answer - 65 : - 
(i) H3PO3
(ii) PCl3
(iii) Ca3P2
(iv) Na3PO4
(v) POF3
Write balanced equations for thefollowing:
Answer - 66 : -
(i) NaCl is heated with sulphuric acid in the presence of MnO_2.
(ii) Chlorine gas is passed into a solution of NaI in water.
Answer
(i)
(ii)
Question - 67 : - How are xenon fluorides XeF2,XeF4 and XeF6 obtained?
Answer - 67 : -
XeF2, XeF4, andXeF6 are obtained by a direct reaction between Xe and F2. Thecondition under which the reaction is carried out determines the product.
Question - 68 : - With what neutral molecule is ClO− isoelectronic?Is that molecule a Lewis base?
Answer - 68 : -
ClO− isisoelectronic to ClF. Also, both species contain 26 electrons in all as shown.
Total electrons ClO− =17 + 8 + 1 = 26
In ClF = 17 + 9 = 26
ClF acts like a Lewis base as itaccepts electrons from F to form ClF3.
Question - 69 : - How are XeO3 andXeOF4 prepared?
Answer - 69 : -
(i) XeO3 canbe prepared in two ways as shown.
(ii) XeOF4 canbe prepared using XeF6.
Question - 70 : - Arrange the following in theorder of property indicated for each set:
Answer - 70 : -
(i) F_2, Cl_2, Br_2, I_2 - increasing bond dissociation enthalpy.
(ii) HF, HCl, HBr, HI - increasing acid strength.
(iii) NH_3, PH_3, AsH_3, SbH_3, BiH_3 − increasing base strength.
Answer
(i) Bonddissociation energy usually decreases on moving down a group as the atomic sizeincreases. However, the bond dissociation energy of F2 is lower than that ofCl2 and Br2. This is due to the small atomic size of fluorine. Thus, theincreasing order for bond dissociation energy among halogens is as follows:
I2 < F2 2 < Cl2
(ii) HF< HCl < HBr < HI
The bond dissociation energy ofH-X molecules where X = F, Cl, Br, I, decreases with an increase in the atomicsize. Since H-I bond is the weakest, HI is the strongest acid.
(iii) BiH3 ≤ SbH3 3 < PH3 < NH3
On moving from nitrogen tobismuth, the size of the atom increases while the electron density on the atomdecreases. Thus, the basic strength decreases.