Problem 29 - A Complete Resource Book in Chemistry for JEE Main

Question

Shape selective catalysts are known by this name because (a) due of the shape of catalyst (b) The specificity of catalyst (c) The size of pores of the catalyst can trap only selective species (d) none of these

Step-by-Step Solution

Answer

Option (c): The size of pores of the catalyst can trap only selective species.

Step 1: Understanding the Concept of Shape Selective Catalysts

Shape selective catalysts are materials that catalyze reactions selectively, not solely based on the chemical composition of the reactants, but primarily based on the shape or size of the molecules that can enter the catalyst's internal structure. This selectivity is determined by the size and shape of the pores in the catalyst.

Step 2: Analyzing Each Option

We need to evaluate each option to see if it aligns with the property of shape selective catalysts. - Option (a): "due to the shape of catalyst," implies the catalyst's own external shape, which is not relevant to shape selectivity. - Option (b): "The specificity of catalyst," is vague and doesn't specifically address pore size or shape. - Option (c): "The size of pores of the catalyst can trap only selective species," accurately describes how only certain molecules can fit into or pass through the pores, allowing for selectivity. - Option (d): "none of these," is not suitable as option (c) appears correct.

Step 3: Selecting the Correct Option

Since option (c) directly describes the functional mechanism of shape selective catalysts—using pore size to restrict access to certain molecules—it is the best choice.

Key Concepts

Catalysis

Catalysis is a process that increases the rate of a chemical reaction by adding a substance known as a catalyst. A catalyst does this without being consumed in the process. Understanding catalysis helps us see why certain reactions are eligible for acceleration while others are not. A catalyst provides an alternative pathway for the reaction with lower activation energy, making it easier for the reaction to proceed. As a result, the reactants are converted to products more swiftly. Catalysts are central to many chemical processes in industries, including the production of fuels, pharmaceuticals, and chemicals. They are also pivotal in biological systems, where enzymes act as natural catalysts, speeding up reactions necessary for life.

Pore Size

Pore size is a critical factor in determining the selectivity of shape selective catalysts. The catalyst's pore size can act like a sieve, allowing only molecules of a particular size and shape to enter and participate in the reaction. This unique trait enables certain catalysts, such as zeolites, to favor the production of specific products. The pores within these catalysts can be engineered or naturally occur to have precise dimensions.

• Smaller pores can limit the entry of larger molecules, thus selectively promoting reactions with smaller molecules.
• Larger pores can facilitate reactions by accommodating bigger molecules, though this often reduces selectivity.

Alteration of pore size can therefore shape the outcome of the catalytic process, influencing not only the reaction rate but also the nature of the products.

Selective Reactions

Selective reactions are reactions where a catalyst favors the formation of a specific product over others. Shape selective catalysts are paramount in guiding these reactions. The specificity is achieved by influencing which reactants have access to the catalyst's active sites, often dictated by pore size and structure. In industrial applications, such selectivity is invaluable:

• It ensures minimal waste and reduces the need for additional purification steps.
• It allows for the production of high-value or desired products efficiently.

Selectivity is not a guaranteed trait in all catalysts but is a defining feature in shape selective ones, as they can discern which molecular shapes are permissible for reaction.

Chemical Specificity

Chemical specificity refers to a catalyst's ability to act on a particular molecule or type of bond within a molecule. For shape selective catalysts, chemical specificity is often tied to the physics of their structure. Their ability to offer specific reaction pathways is leveraged through structural designs that favor certain reactants over others, based on size and shape compatibility. This specificity allows these catalysts to

• Enhance yield and efficiency for desired chemical reactions.
• Promote sustainability by reducing by-products and energy consumption.

In essence, chemical specificity in shape selective catalysts roots from engineered constraint, embodying a harmonized interaction between physical form and chemical function.