(a) Schottky Defect: This defect is caused if some of the lattice points are unoccupied. The points which are unoccupied. The points which are unoccupied are called lattice vacancies. The number of missing positive and negative ions is the same in this case and the crystal remains neutral. The existence of two vacancies, one due to a missing Na+ ion and the other due to a missing Cl− ion in a crystal of NaCl. It appears in ionic crystals in which positive and negative ions do not differ much in size.
Frenkel Defects: This defect arise when an ion occupies an interstitial position between the lattice points. This defect occurs generally in ionic crystals in which the anion is much larger in size than the cation.
AgBr is an example for this type of defect. One of the Ag+ ion occupies a position in the interstitial space rather than its own appropriate site in the lattice.
The crystal remains neutral since the number of positive ions is the same as the number of negative ions.
(b) Adsorption theory explains the mechanism of heterogenous catalysts. Here, the catalyst functions by adsorption of the reacting molecules on its surface.
In general there are four steps involved in the heterogenous catalysis.
A(g)+B(g)catalyst−−−−→C(g)+D(g)
Step 1: Adsorption of reactant molecules:
The reactant molecules A and B strike the surface of the catalyst. They are held up at the surface by weak Vanderwall's forces by partial chemical bonds.
Step 2: Formation of activated complex:
The particles of the reactants adjacent to one another join to form an intermediate complex (A−B) . Tee activated complex is unstable.
Step 3: Decomposition of Activated Complex:
The activated complex breaks to form the products C and D. The separated particles of the products hold to the catalyst surface by partial chemical bonds.
Step 4: Desorption of products:
The particles of the products are desorbed or released from the surface.