Chemistry NCERT Exemplar Solutions Class 12th Chapter Five

This is a Long Answers Type Questions as classified in NCERT Exemplar

Ans: Catalytic reaction which depends on the pore structure of catalyst and size of the reactant, product molecules is known as shape-selective catalysis. Zeolites are considered as good shape-selective catalysts due to their honeycomb-like structures. They are aluminosilicates with three-dimensional network of silicates wherein some of the silicon atoms are replaced by aluminium atoms giving Al-O-Si frame work. The reactions taking place in zeolites depends upon size and shape of reactant and product molecules, the pores and cavities of the zeolites also. They are found in nature and synthesised for catalysts selectivity. Zeolites are widely used as catalysts in petrochemical industries for cracking of hydrocarbons and isomerisation.

This is a Long Answers Type Questions as classified in NCERT Exemplar

Ans: In heterogeneous catalysis reactants are generally in gas phase and catalyst are in solid phase. The activity of a catalyst depends upon the strength of chemisorption to a large extent. The reactants must get adsorbed reasonably strongly on to the catalyst to become active. However, they must not get adsorbed so strongly that they are immobilized and other reactants are left with no space on the catalyst's surface for adsorption. It has been found that for hydrogenation reaction, the catalytic activity increases from Group 5 to Group 11 metals with maximum activity being shown by groups 7-9 elements of the periodic table. Catalyst also directs a reaction to yield a particular product. For example, starting with H₂ and CO, and using different catalysts, we get different products.