Answer the questions based on the passage given below:

Adsorption is a surface phenomenon and it differs from absorption which occurs throughout the body of the substance that absorbs. In physisorption the attractive forces are mainly van der Waals forces while in chemisorption ionic / covalent bonds are formed between particles of adsorbent and adsorbate. The catalytic activity of finely divided iron in Haber's process of ammonia manufacture can be explained by adsorption theory. Adsorption being an exothermic process, the heat of adsorption is utilized in enhancing the rate of the reaction. Adsorption has many applications being used in gas masks, control of humidity, chromatograph separation, curing diseases etc.

The modern adsorption theory lists the following steps for heterogeneous catalysis

A. Adsorption of reactant molecules on the surface of a catalyst.

B. Occurence of chemical reaction on the surface of a catalyst

C. Desorption of products form a catalytic surface

D. Diffusion of reactants to the surface of the catalyst.

Choose the correct sequence of steps from the options given below:

D, A, B, C

The correct answer is option 4. D, A, B, C.

Here is a detailed explanation of each step in the modern adsorption theory of heterogeneous catalysis and their correct sequence:

D. Diffusion of Reactants to the Surface of the Catalyst

Reactant molecules from the bulk phase (such as a gas or liquid) must move towards and reach the surface of the catalyst. This movement is driven by concentration gradients and occurs through diffusion. For a reaction to occur on a catalytic surface, the reactants need to be in close proximity to the surface where the catalytic action happens. This step is crucial because without the reactants reaching the catalyst surface, no reaction can take place.

A. Adsorption of Reactant Molecules on the Surface of the Catalyst

Once the reactant molecules arrive at the surface, they adsorb onto it. Adsorption involves the interaction between the reactant molecules and the surface of the catalyst, which can be either physical (physisorption) or chemical (chemisorption).

Physical Adsorption: Weak van der Waals forces are involved. It is typically reversible and does not involve a chemical change in the adsorbed molecules.

Chemical Adsorption: Stronger interactions occur due to the formation of chemical bonds between the reactant molecules and the surface. This process often involves a change in the chemical structure of the adsorbed molecules.

Adsorption is essential because it brings the reactant molecules into an optimal position for the catalytic reaction. It often also lowers the activation energy required for the reaction.

B. Occurrence of Chemical Reaction on the Surface of the Catalyst

Once adsorbed on the catalyst surface, the reactant molecules undergo a chemical reaction. The catalyst provides an active site where the reaction can proceed more efficiently than in the bulk phase. The catalyst offers an alternative reaction pathway with a lower activation energy. This can involve the breaking and making of chemical bonds at the surface of the catalyst. The actual transformation of reactants into products takes place at this stage. The efficiency of the catalyst is determined by how well it facilitates this reaction.

C. Desorption of Products from the Catalytic Surface

After the reaction, the product molecules formed need to be released (desorbed) from the surface of the catalyst. This allows the catalyst to be ready for the next cycle of reactions. If the products do not desorb, they could block the active sites on the catalyst, reducing its effectiveness and leading to catalyst deactivation. Effective desorption is crucial for maintaining the catalyst's activity over time.

Sequence:

Step 1 (Diffusion): Reactants must first diffuse to the catalyst surface before any other interaction can occur.

Step 2 (Adsorption): Once at the surface, reactants need to adsorb to be in the right position for the reaction.

Step 3 (Reaction): With the reactants adsorbed, the chemical reaction can occur on the surface.

Step 4 (Desorption): After the reaction, products must desorb to free up the catalyst for further reactions.

Thus, the correct sequence of steps according to modern adsorption theory is: 4. D, A, B, C.