Animated Solution for Chemistry - Surface Chemistry: The correct option(s) related to adsorption processes is(are)

Physisorption vs Chemisorption

Welcome to the fascinating microscopic world of surface chemistry! Imagine a solid surface exposed to a gas. The gas molecules, constantly in random motion, occasionally strike the surface and decide to stay. This accumulation of molecules on a surface is what we call adsorption.

But not all molecules stick the same way. Depending on the nature of the interaction between the gas (adsorbate) and the solid (adsorbent), adsorption is broadly classified into two categories: Physisorption and Chemisorption. Understanding the fundamental differences between these two is the key to unlocking this problem.

Let's start by looking at Chemisorption. As the name suggests, this process involves the formation of actual chemical bonds between the gas molecules and the atoms on the solid surface.

Think of the solid surface as a parking lot with a fixed number of parking spaces (valencies). Once a gas molecule parks in a spot and forms a chemical bond, that spot is taken. When the entire surface is covered with a single layer of molecules, there are no more exposed surface atoms to form new bonds. Therefore, chemisorption strictly results in a unimolecular layer. This makes statement (A) absolutely correct.

On the other hand, Physisorption relies on weak van der Waals forces. Because these forces don't require specific chemical bonds, molecules can stack on top of each other, forming multilayers.

Now, let's talk about energy. When gas molecules, which were previously flying around freely, get trapped on a surface, their randomness decreases. In thermodynamic terms, the change in entropy is negative ().

For any process to occur spontaneously, the Gibbs free energy change must be negative (). According to the famous equation:

Since is negative, the term becomes positive. To ensure that remains negative, the enthalpy change () must be highly negative. This means that adsorption—whether physical or chemical—is inherently an exothermic process. Heat is always released! This immediately tells us that statement (C) is incorrect.

What about the magnitude of this heat? Because physisorption involves only weak van der Waals forces, the heat released is quite low, typically in the range of to . Chemisorption, involving strong chemical bonds, releases much more heat, usually between to . Therefore, statement (B), which claims physisorption has an enthalpy change of to , is incorrect.

Finally, let's look at how temperature affects these processes. We've established that physisorption is an exothermic equilibrium:

According to Le Chatelier's principle, if we lower the temperature, we are essentially removing heat from the system. The system will try to counteract this change by shifting the equilibrium in the forward direction to produce more heat.

Thus, lowering the temperature favors the forward reaction, meaning it favors physisorption. This makes statement (D) correct.

By systematically applying fundamental principles of chemical bonding and thermodynamics, we can confidently conclude that the correct statements are (A) and (D).