Question

Which one of the following statements about the zeolite is false? (a) They have open structure which enables them to take up small molecules (b) They are used as cation exchangers (c) Zeolites are aluminosilicates having three dimensional network (d) None of the \(\mathrm{SiO}_{4}^{4-}\) units are replaced by \(\mathrm{AlO}_{4}^{5-}\) and \(\mathrm{AlO}_{6}{ }^{9}\) ions in zeolites

Short answer

Option (d) is false because in zeolites, \\( ext{SiO}_4^{4-}\\) units are replaced by \\( ext{AlO}_4^{5-}\\).

Step-by-step solution

Analyze Option (a)

Zeolites have an open framework structure comprised of pores and channels that allow them to trap and accommodate small molecules. This is a true statement, as it describes the property of zeolites being able to act as molecular sieves.

Analyze Option (b)

Zeolites have the ability to exchange cations within their structure, which is a feature widely utilized in water softening and purification processes. Therefore, this statement is true.

Analyze Option (c)

Zeolites are composed of a three-dimensional network of aluminosilicate tetrahedra. This structural makeup is accurate, making this statement true.

Analyze Option (d)

In zeolites, some of the \( ext{SiO}_4^{4-}\) tetrahedra are indeed replaced by \( ext{AlO}_4^{5-}\) tetrahedra, and this replacement creates cation exchange capacity as a result of the overall negative charge introduced by aluminum substitution. Thus, this statement, saying no such replacements occur, is false.

Key concepts

Aluminosilicates

In the world of materials science, aluminosilicates play a crucial role due to their unique properties. These compounds are made from aluminum, silicon, and oxygen, forming a 3D framework of tetrahedral {SiO_4^{4-}} and {AlO_4^{5-}} units. This structure is intertwined in various patterns to create a stable, intricate network.

One of the most fascinating applications of aluminosilicates is found in zeolites. These natural or synthetic minerals consist of a vast crystalline structure that allows them to trap impurities and small molecules within their frameworks. This makes them ideal for use in filtration and as catalysts or adsorbents in industrial processes. Their porosity and ability to exchange ions give them versatility in usage, enhancing the reaction processes in which they are involved.

Zeolites exhibit unique characteristics like thermal stability and resistance to chemical changes, which further expands their applications. They are extensively used in odor control, gas separation, and even in pet litter due to their ability to trap gases.

Cation Exchange

Cation exchange is a pivotal process in science, especially when discussing materials like zeolites. This capability of zeolites stems from their structural composition, which includes charged frameworks due to the presence of {AlO_4^{5-}} tetrahedra. When aluminum replaces some silicon in the framework, it leaves a negative charge that needs counterbalancing.

This negative charge is balanced by cations, which are positively charged ions like sodium ( {Na^+}), potassium ( {K^+}), or calcium ( {Ca^{2+}}). The exciting part is that these cations are not rigidly fixed in place. They can be exchanged with others from the surrounding medium without altering the zeolite structure. This makes zeolites incredibly useful in softening water by exchanging hardness ions like calcium and magnesium for sodium ions, thus preventing scale formation.

Molecular Sieves

Molecular sieves are a fascinating application of zeolites, owed to their ability to act selectively based on molecule size. This property arises from the unique pore structures within the zeolite framework. The pores and channels are of precise and regular size, allowing the passage of smaller molecules while blocking larger ones.

This selectivity is why zeolites are frequently employed in gas separation processes, like in the petrochemical industry where different hydrocarbons are separated based on size. Their ability to discriminate between molecules not only assists in purification processes but also makes them excellent drying agents. By absorbing moisture while allowing gases or solids to pass, they ensure the surroundings remain dry and contamination-free.

Two-Dimensional Network

While zeolites are known for their three-dimensional network, the two-dimensional aspect is incredibly vital in understanding the overall framework of aluminosilicates. Though not a physical characteristic of zeolites, understanding the concept of dimensions is essential when discussing crystalline structures.

Primarily, a two-dimensional network in the context of material science refers to plane-like arrangements where each unit connects to others in only two directions. While zeolites are more firmly characterized by their three-dimensional, cage-like structures, understanding the two-dimensional concept helps appreciate the difference and complexity of their structure.

This concept contributes to analyzing how the network influences properties such as porosity, flexibility, and ion-exchange capability. By considering both two-dimensional and three-dimensional frameworks, we gain insight into how molecules interact within these structures, optimizing their use in industrial applications.