Understanding Adsorption: Physisorption vs. Chemisorption

The question asks us to identify the type of surface phenomenon, specifically adsorption, based on the observed standard enthalpy for CH4 as an adsorbate. The given enthalpy value is -21 kJ/mol.

Adsorption is the accumulation of molecular species at the surface rather than in the bulk of a solid or liquid. Adsorption processes are generally exothermic, meaning they release heat, so the change in enthalpy ($\Delta H$) is negative.

There are two main types of adsorption:

• Physisorption (Physical Adsorption): This type of adsorption involves weak intermolecular forces, such as van der Waals forces, between the adsorbate and the adsorbent surface. It is similar to the liquefaction of gases.
• Chemisorption (Chemical Adsorption): This type involves the formation of chemical bonds (like covalent or ionic bonds) between the adsorbate molecules and the adsorbent surface.

Adsorption Enthalpy Values: Key Difference

The magnitude of the enthalpy of adsorption is a key factor in distinguishing between physisorption and chemisorption.

• For Physisorption, the enthalpy of adsorption is typically low, in the range of approximately \(-20 \text{ to } -40 \text{ kJ/mol}\). This is comparable to the enthalpy of liquefaction of gases.
• For Chemisorption, the enthalpy of adsorption is much higher, generally in the range of approximately \(-80 \text{ to } -240 \text{ kJ/mol}\). This is because chemical bonds are formed, which involves a significant release of energy.

Analyzing the Given Enthalpy for CH4 Adsorption

The observed standard enthalpy of adsorption for CH4 is given as -21 kJ/mol.

Let's compare this value to the typical ranges:

• Physisorption range: \(-20 \text{ to } -40 \text{ kJ/mol}\)
• Chemisorption range: \(-80 \text{ to } -240 \text{ kJ/mol}\)

The value -21 kJ/mol falls squarely within the typical range for physisorption. It is a low enthalpy value, characteristic of interactions based on weak van der Waals forces rather than strong chemical bonds.

Conclusion: Identifying the Type of Adsorption

Based on the observed standard enthalpy of -21 kJ/mol, the adsorption of CH4 on the surface is an example of physisorption, also known as physical adsorption.

The options provided are:

1. Physisorption
2. Chemisorption
3. Physical adsorption
4. Chemical adsorption

Options 1 and 3 both correctly describe the type of adsorption indicated by the enthalpy value.