Problem 100 - A Complete Resource Book in Chemistry for JEE Main

Question

According to the adsorption theory of catalysis, the speed of the reaction increases because (a) adsorption produces heat which increases the speed of the reaction (b) adsorption lowers the activation energy of the reaction (c) the concentration of reactant molecules at the active centres of the catalyst becomes high due to adsorption (d) in the process of adsorption, the activation energy of the molecules becomes large

Step-by-Step Solution

Answer

The correct answer is (b) adsorption lowers the activation energy of the reaction.

Step 1: Understanding Adsorption in Catalysis

Adsorption in catalysis involves the adhesion of reactant molecules onto the surface of a catalyst. This is a critical process in facilitating chemical reactions as it provides a surface where reactions can occur more efficiently.

Step 2: Exploring the Concept of Activation Energy

Activation energy is the minimum energy required for reactants to undergo a chemical reaction. Lowering this energy can speed up the reaction as it allows more molecules to have sufficient energy to react.

Step 3: Analyzing the Effect of Adsorption on Activation Energy

In the context of catalysis, adsorption often lowers the activation energy. This means reactant molecules can react at a faster rate since they require less energy to reach the transition state on the catalyst's surface.

Step 4: Investigating the Concentration of Reactant Molecules

Adsorption increases the concentration of reactant molecules at the active centers of the catalyst. Higher concentration at these centers enhances the chances of reactions between the molecules.

Step 5: Evaluating the Possible Options

Given that adsorption reduces activation energy and increases reactant concentration, options (b) and (c) are considered. However, option (b) is more directly related to why the reaction speed increases according to adsorption theory.

Key Concepts

Activation Energy

Activation energy is a crucial concept when it comes to understanding how chemical reactions occur. Think of activation energy as a barrier that reactant molecules need to overcome to transform into products. This barrier prevents molecules from spontaneously reacting. The lower the activation energy, the easier it is for reactions to take place.

In the context of the Adsorption Theory of Catalysis, catalysts work by lowering this barrier. They provide an alternative pathway for the reaction with a lower activation energy. As a result, more reactant molecules can overcome this barrier and proceed to form the products, thus increasing the reaction rate.

Lowering the activation energy doesn't change the energy balance of the overall reaction, but it makes the path to reaching this balance much easier and quicker for the molecules.

Chemical Reaction Kinetics

Chemical reaction kinetics is the study of the speeds or rates at which chemical reactions occur. Imagine it as a clock for reactions, showing not just when but how fast reactions take place. Kinetics helps us understand the effect of various conditions on the rate, such as concentration, temperature, and the presence of a catalyst.

In reactions involving catalysts, kinetics becomes very dynamic. Catalysts work by increasing the rate without being consumed in the process. They achieve this primarily by decreasing the activation energy, which allows more molecules to react in the same unit of time.

Adsorption is a key part of this process in catalysis. It involves the adherence of reactant molecules on the catalyst surface, which enhances the efficiency of collisions that lead to product formation. By facilitating the formation of intermediate compounds more rapidly, catalysts significantly impact the kinetics of a chemical reaction.

Catalyst Active Sites

Catalyst active sites are the specific places or regions on the surface of a catalyst where the reaction actually takes place. Think of these sites as tiny workstations where molecules gather to undergo the transformation into products.

These active sites are crucial because they determine the efficiency and effectiveness of the catalyst. The arrangement and nature of these sites often govern how easily the reactant molecules can adsorb and how efficiently the catalysis can occur.

Adsorption increases the concentration of reactants at these active sites, making it easier for molecules to collide in the right orientation and energy needed for reaction. The more efficiently a catalyst can attract and hold onto reactant molecules at these active sites, the faster and more effective the overall reaction becomes.

• Active sites provide necessary orientation for molecules.
• They lower the energy barrier for the reaction.
• Concentration of molecules at these sites increases the reaction rate.