Problem 19 - A Complete Resource Book in Chemistry for JEE Main

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

Step-by-Step Solution

Answer

The false statement is (d). Statement (d) incorrectly claims no substitution occurs.

Step 1: Understand Zeolite Composition

Zeolites are minerals composed of aluminum, silicon, and oxygen. They possess a three-dimensional framework composed mainly of \( \text{SiO}_4^{4-} \) tetrahedra and \( \text{AlO}_4^{5-} \) tetrahedra linked by shared oxygen atoms, often combined with cations like Na⁺ or K⁺ to balance the charge.

Step 2: Assess Statement (a) on Zeolite Framework

Statement (a) claims that zeolites have an open structure capable of taking up small molecules. This is true, as zeolites are known for their porous nature, which allows them to adsorb and filter small molecules.

Step 3: Evaluate Statement (b) on Ion Exchange

Statement (b) states that zeolites can act as cation exchangers. This is correct because zeolites can exchange their cations with those in surrounding solutions, a property utilized in water softening.

Step 4: Verify Statement (c) on Three-Dimensional Network

Statement (c) claims that zeolites are aluminosilicates with a three-dimensional network. This true statement describes the primary structure of zeolites, which form a stable and complex framework.

Step 5: Analyze Statement (d) on Substitution

Statement (d) claims that none of the \( \text{SiO}_4^{4-} \) units are replaced by \( \text{AlO}_4^{5-} \) units, which is false. In zeolites, some \( \text{SiO}_4^{4-} \) units are in fact replaced by \( \text{AlO}_4^{5-} \), creating the negative charge balanced by cations. Note: \( \text{AlO}_6^{9} \) does not typically occur in zeolites, keeping the focus on \( \text{AlO}_4^{5-} \).

Step 6: Determine the False Statement

Since steps 1 to 5 have determined that Statements (a), (b), and (c) are true, whereas Statement (d) is incorrect, the false statement is clearly Statement (d).

Key Concepts

Ion Exchange

Zeolites are fascinating minerals known for their ability to perform ion exchange. This means they can swap one type of ion for another within their structure. When a zeolite encounters an aqueous solution, it can exchange its internal cations like sodium (Na⁺) or calcium (Ca²⁺) for other cations in the water. This property is particularly valuable in water softening, where calcium and magnesium ions that cause water hardness are exchanged for sodium ions, resulting in softer water. These cation exchanges are influenced by the size and charge of the cations, as well as the temperature and pH of the environment.

• Widely used for water purification and softening.
• Enables the removal of unwanted ions and pollutants.

Ion exchange is a reversible process, making it highly effective for repeated use in industrial and environmental applications.

Aluminosilicates

Aluminosilicates form the backbone of zeolites and other geological formations. They are composed of networks of aluminum, silicon, and oxygen. In zeolites, the specific combination creates a porous structure, which is the hallmark of these minerals. The "alumino" part refers to the incorporation of aluminum (\( \mathrm{Al} \), as \( \mathrm{AlO}_4^{5-} \)) in tetrahedral coordination with oxygen atoms, whereas the "silicate" refers to silicon (\( \mathrm{Si} \), as \( \mathrm{SiO}_4^{4-} \)).

This composition allows for the substitution of some silicon atoms by aluminum, introducing a negative charge imbalance.

• The negative charge is balanced by exchangeable cations like Na⁺, K⁺, and Ca²⁺.
• This structural feature is key to the ion exchange capabilities of zeolites.

Aluminosilicates thus play a crucial role in the unique properties and applications of zeolites.

Zeolite Structure

The structure of zeolites is what grants them their unique properties and makes them extremely useful in industrial applications. Zeolites are crystalline and have a framework similar to a honeycomb. This framework consists of \\( \text{SiO}_4^{4-} \) and \\( \text{AlO}_4^{5-} \) tetrahedra linked together by shared oxygen atoms, forming a three-dimensional network. Their framework contains open channels and cavities of various sizes.

• These porous spaces allow zeolites to function as molecular sieves.
• The network determines what types of molecules can enter or be adsorbed.

Thanks to millions of small cavities, zeolites can selectively take up and release substances. By designing zeolites with specific pore sizes, scientists and engineers can tailor them for specific applications such as gas separation, catalysis, and water purification.

Cation Exchange

Zeolites' ability to exchange cations is one of their most valuable characteristics. In cation exchange, a zeolite will swap one kind of cation it contains for another from an external medium. The ease of this exchange relies on factors including the charge density and ionic radius of the cations involved. For example, a larger cation may not easily fit into the pores of a zeolite, making the exchange more challenging.

This ion exchange capacity is pivotal in areas like agriculture and environmental science. In agriculture, zeolites can be added to soil to enhance nutrient delivery by gradually releasing essential cations like potassium. In waste treatment, harmful metal ions can be removed from water by being exchanged with harmless cations found naturally in soils. Thus, this property not only optimizes environmental practices but also aids in sustainable resource management.

Three-Dimensional Network

Zeolites are characterized by a robust three-dimensional network formed from alternating silicon-oxygen and aluminum-oxygen tetrahedra. This network is similar to a rigid cage made up of numerous interconnected chambers. The geometry and connectivity of the framework creates a highly ordered and stable structure that supports the mineral's many functions.

• The three-dimensional network allows for high stability and resistance to heat and chemicals.
• Such a network is pivotal for applications in catalysis and filtration.

This framework not only determines the stability but also enables zeolites to selectively adsorb molecules by trapping them in their cavernous spaces. Overall, the three-dimensional network is integral to the diversity of applications in which zeolites can be employed.