Question

Which one of the following is correct for the adsorption of a gas at a given temperature on a solid surface ?

• A. \(\Delta H>0, \Delta S>0\)
• B. \(\Delta H<0, \Delta S>0\)
• C. \(\Delta H>0, \Delta S<0\)
• D. \(\Delta H<0, \Delta S<0\)

Hint:

Adsorption is always exothermic and always results in a decrease in entropy. Think of it as gas molecules losing their "freedom" to move in 3D space.

Answer 1

The Correct Option is D

The problem presented explores the thermodynamics of gas adsorption on a solid surface at a given temperature. Several key concepts in thermodynamics come into play: enthalpy change \((\Delta H)\) and entropy change \((\Delta S)\). 

1. Understanding Adsorption: Adsorption is the process where gas molecules adhere to the surface of solids, creating a layer of adsorbate on the adsorbent. This process is usually exothermic (releases heat), impacting enthalpy and entropy.
2. Enthalpy Change \((\Delta H)\):
   • During adsorption, gas molecules lose freedom as they bind to the surface.
   • This process typically releases energy; hence, \(\Delta H\) is negative, indicating an exothermic process.
3. Entropy Change \((\Delta S)\):
   • Entropy, a measure of disorder, usually decreases in adsorption. Initially, gas molecules are more free-moving in the gaseous phase.
   • When these molecules are adsorbed onto the solid surface, their freedom is significantly reduced, thus reducing entropy. Hence, \(\Delta S\) is negative.
4. The Correct Option Analysis:
   • The correct thermodynamic conditions for adsorption where both \(\Delta H\) and \(\Delta S\) are negative match the option: \(\Delta H<0, \Delta S<0\).
   • The other options do not fit this logical pattern for adsorption:
     • \(\Delta H>0, \Delta S>0\): Would imply an endothermic process and increasing disorder, which is not typical for adsorption.
     • \(\Delta H>0, \Delta S<0\): An endothermic process with reducing entropy is less feasible.
     • \(\Delta H<0, \Delta S>0\): An exothermic process with increasing disorder is not characteristic of adsorption.

Therefore, the correct choice for the condition of a gas adsorbing on a solid surface at a given temperature is: \(\Delta H<0, \Delta S<0\).