p-Block Elements (Group 13 & 14) · Question 29

Which one of the following statements about the zeolite is false?

1. They are used as cation exchangers.
2. They have open structure which enables them to take up small molecules.
3. Zeolites are aluminosilicates having three dimensional network.
4. Some of the SiO44- units are replaced by AlO45- and AlO69- ions in zeolites.

Answer

(4) Some of the SiO44- units are replaced by AlO45- and AlO69- ions in zeolites.

Solution

As per NCERT, if aluminium atoms replace a few silicon atoms in the three-dimensional network of silicon dioxide, the overall structure is an aluminosilicate carrying a negative charge, balanced by cations such as Na+, K+ or Ca2+; zeolites are such aluminosilicates.

(1) is true: the loosely held cations can be exchanged, so zeolites (permutit) are used as cation exchangers in softening hard water.

(2) is true: the open, porous network takes up small molecules, so zeolites act as molecular sieves (e.g. ZSM-5 converts alcohols directly into gasoline).

(3) is true: they have a three-dimensional aluminosilicate network.

(4) is false: Al replaces Si only at the tetrahedral positions (as AlO4 units); octahedral AlO69- units are not part of the zeolite framework.

Answer: (4)