1) why lewis acid character follows this order :  BI3 > BBr3  >  BCl3  >  BF3

 

ans - this is on the basis of tendency of halogens to back donate their lone pair of electrons to the B atom . size of vacant 2p orbital of boron  and 2p

orbital of F are almost identical. so the l.p of electrons on fluorine  are donated towards the boron . so electron deficiency on B decreases.

 

 

2) during electrolytic reduction of alumina to get Al  , electrolyte is covered  with a layer of coke. why ?

 

ans - to prevent oxidation and loss of heat due to radiation.

3) B-X  bond length in BX3 is shorter than theoretically expected . why ?

 

ans -  due to     pP - p P           back bonding  of the fully filled p orbital  of halogen  into  empty p orbital of boron.

4)  on moving down a group , size is expected to increase. then why radius of Ga <  Al ?
 
Ans -  this is due to filling of electrons in d-orbitals. in between Al  and Ga there are 10 elements having electrons in d orbitals . as d orbitals are large in

size , they do not shield the nucleus effectively .  so the electrons of Ga experience a greater force of atrraction by nucleus than Al .

5) why boron has a high m.p ???

 

ans - bcoz of its crystal structure

6) B  doesn't form ionic compounds . why ?

 

ans - bcoz of small size and high sum of first three I.E , boron doesn't lose electrons to form ionic compounds.

7)  boron halides do not form dimers . why ?

 

ans - B is small in size and unable to accomodate large halogen atoms arrond it.

8) trihalides of grp 13 fume in moist air . why ?

 

ans - bcoz they r hydrolysed by water
           MX3 + 3H20 ->  M(OH)3 + 3HX

9)TlCl is more stable than TlCl3  in aqueous solution .why ?
   
ans  - due to inert pair effect