Define : (a) Schottky defect (b) Ferromagnetism(ii) Write the name of catalyst used in preparation of HDP.(iii) Draw structure of isoprene unit of natural rubber.

Answer:

(i)(a) Schottky defect: A point defect in an ionic solid in which an equal number of cations and anions are missing from their lattice sites, so that electrical neutrality is maintained. It lowers the density of the crystal. It occurs in ionic compounds with cations and anions of similar size and high coordination number, e.g. NaCl, KCl, CsCl, AgBr.

(b) Ferromagnetism: The property of substances that are strongly attracted by a magnetic field and can be permanently magnetised (they retain magnetism even after the field is removed). It arises because the unpaired electron spins in small regions called domains are aligned in the same direction. Examples: Fe, Co, Ni, CrO₂.

(ii) High density polythene (HDP) is prepared using the Ziegler–Natta catalyst: triethylaluminium, (C₂H₅)₃Al, with titanium tetrachloride, TiCl₄ (at about 333–343 K and 6–7 atm).

(iii) Natural rubber is cis-1,4-polyisoprene. Its repeating isoprene unit is

–CH₂–C(CH₃)=CH–CH₂–

with the two –CH₂– groups (chain links) on the same side of the C=C double bond (cis), i.e. the polymer is –[CH₂–C(CH₃)=CH–CH₂]_n–. The monomer is isoprene, 2-methylbuta-1,3-diene, CH₂=C(CH₃)–CH=CH₂.