Problem 186 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following characteristics is not correct for physical adsorption? \([\mathbf{2 0 0 3}]\) (a) adsorption on solids is reversible (b) adsorption is spontaneous (c) both enthalpy and entropy of adsorption are negative (d) adsorption increases with increase in temperature

Step-by-Step Solution

Answer

The incorrect characteristic is (d) adsorption increases with increase in temperature.

Step 1: Understanding Physical Adsorption

Physical adsorption, also known as physisorption, involves the adhesion of gas molecules on the surface of solids. This type of adsorption is typically reversible.

Step 2: Analyzing Spontaneity

Physisorption is spontaneous, which means it occurs naturally without needing external energy. This can be related to the Gibbs free energy change (9G being negative) during the process.

Step 3: Evaluating Enthalpy and Entropy Changes

During physisorption, both the enthalpy change (9H) and entropy change (9S) are generally negative. This is due to the interaction forces between the adsorbate and adsorbent, and the decrease in disorder, respectively.

Step 4: Temperature Dependency

Physisorption tends to decrease with an increase in temperature. This is because the process is exothermic; hence, higher temperatures provide energy that discourages molecule adhesion on the surface.

Step 5: Identifying the Incorrect Characteristic

After analyzing the characteristics, the incorrect statement about physisorption is that 'adsorption increases with increase in temperature,' as it actually decreases with higher temperatures.

Key Concepts

Understanding Physisorption

Physisorption, also referred to as physical adsorption, is the process where gas molecules adhere to the surface of a solid without any chemical bond formation. This is a surface phenomenon relying heavily on weak van der Waals forces.

These forces are temporary attractions that occur between molecules due to induced dipoles. Unlike chemical adsorption, or chemisorption, which involves stronger ionic or covalent bonds, physisorption is generally much weaker.

It often acts as the first step before chemisorption occurs, creating a temporary bond that is reversible. This reversal happens easily because of the low energy barrier involved, allowing molecules to detach from the surface effortlessly.

Enticing Role of Enthalpy Change

The enthalpy change (H) during physisorption is a crucial factor. It describes the heat exchange when adsorption occurs. For physisorption specifically, this change is negative. This indicates that the process releases heat, or is exothermic.

The primary reason for this is the establishment of attractive forces between the surface of the solid and the gas molecules, which results in a lower energy state. Although the energy released in physisorption is relatively low compared to chemisorption, it is a key driver that helps the system reach a more stable state.

It's important to note that, typically, the enthalpy change for physisorption is much less than that of chemisorption, justifying why the bonds formed are much weaker.

Spontaneity of Adsorption – The Natural Driver

In the context of physisorption, the concept of spontaneity refers to the natural occurrence of the adsorption process without the need for additional energy. This is influenced by the Gibbs free energy change (G), which determines the spontaneity of the process.

For physisorption, G is negative, signifying that the process is indeed spontaneous. Several factors contribute to this, such as:

• The decrease in free energy as the system approaches equilibrium
• The stability provided by the energy released during the process

Understanding these factors helps in appreciating why adsorption spontaneously occurs, making it a favorable reaction even in the absence of external energy inputs.

Temperature Dependency in Adsorption Dynamics

Temperature plays a pivotal role in the adsorption behavior, particularly in physisorption. This process is highly temperature-dependent, which can be explained by its exothermic nature.

As the temperature increases, the kinetic energy of the gas molecules also rises. This elevation in energy tends to overcome the weak van der Waals forces holding the gas molecules onto the solid surface, effectively reducing the extent of adsorption.

Thus, contrary to what might be initially assumed, an increase in temperature typically leads to a decrease in physisorption.

• The increased kinetic energy enables gas molecules to overcome adsorption forces easily.
• This observation is a common trait in exothermic processes, where higher temperatures promote desorption rather than adsorption.