Question

Which of the following is true in respect of adsorption? (a) \(\Delta \mathrm{G}<0 ; \Delta \mathrm{S}>0 ; \Delta \mathrm{H}<0\) (b) \(\Delta \mathrm{G}<0 ; \Delta \mathrm{S}<0 ; \Delta \mathrm{H}<0\) (c) \(\Delta \mathrm{G}>0 ; \Delta \mathrm{S}>0 ; \Delta \mathrm{H}<0\) (d) \(\Delta \mathrm{G}<0 ; \Delta \mathrm{S}<0 ; \Delta \mathrm{H}>0\)

Short answer

Option (b) is true for adsorption: ΔG < 0, ΔS < 0, ΔH < 0.

Step-by-step solution

Understanding Adsorption

Adsorption is a process in which molecules are attracted to and adhere to the surface of a solid material from a gas or liquid. It is generally an exothermic process because bonds are formed between the adsorbate (substance being adsorbed) and adsorbent (surface on which adsorption takes place). Consequently, the enthalpy change (ΔH) is usually negative.

Gibbs Free Energy in Adsorption

For a process to be spontaneous, the Gibbs free energy change (ΔG) must be negative. Adsorption leads to a decrease in the system's energy, thus ΔG for adsorption is negative, indicating a spontaneous process.

Entropy Change Consideration

In adsorption, as the adsorbate molecules get attached to the surface, there is a decrease in randomness or disorder, particularly for the adsorbate molecules, leading to a decrease in system entropy (ΔS). Therefore, ΔS is usually negative in adsorption processes.

Analyzing Each Option

We have identified that for adsorption, ΔG < 0, ΔH < 0, and ΔS < 0. Now, let's check: - (a) ΔG < 0, ΔS > 0, ΔH < 0: Incorrect due to ΔS > 0 - (b) ΔG < 0, ΔS < 0, ΔH < 0: Correct, matches all conditions - (c) ΔG > 0, ΔS > 0, ΔH < 0: Incorrect due to ΔG > 0 - (d) ΔG < 0, ΔS < 0, ΔH > 0: Incorrect due to ΔH > 0

Conclusion

Option (b) correctly describes the adsorption process since it reflects all the thermodynamic characteristics typical of adsorption: ΔG < 0, ΔS < 0, ΔH < 0.

Key concepts

Thermodynamics

Thermodynamics is a branch of physics that deals with the principles and laws governing energy and its transformations. When you encounter processes like adsorption, thermodynamics makes it easier to understand how energy and matter interact. Adsorption, the process where molecules adhere to a solid surface, is primarily guided by the laws of thermodynamics. There is the aspect of energy conservation, often manifested through exothermic reactions.
Exothermic reactions occur when a process releases energy, usually in the form of heat. This release is an integral part of adsorption, as molecules transition from a higher energy state in gas or liquid form to a lower energy state when adhered to a surface. This energy release, expressed negatively in terms of enthalpy (\( \Delta H \)), is a direct application of thermodynamics in explaining why adsorption occurs spontaneously. Understanding these fundamentals lays the foundation for deeper exploration into specific parameters like Gibbs free energy and entropy.

Gibbs Free Energy

Gibbs free energy is a concept representing the measure of the maximum reversible work that a thermodynamic system can perform. It is significantly valuable in determining the spontaneity of a process. A negative change in Gibbs free energy (\( \Delta G < 0 \)) indicates that a process can occur without any additional energy input, making it spontaneous.
In adsorption, the transition of molecules from a disordered state (in a gas phase) to an ordered state (on a surface) results in a decrease in the system's energy. As such, \( \Delta G \) becomes negative, signifying that adsorption is a favorable and spontaneous process.

• A spontaneous process means that the system can naturally proceed without external intervention due to its energy advantage.
• Adsorption changes the system's energy towards stability, which is achieved under spontaneous conditions when \( \Delta G \) is negative.

With Gibbs free energy, it becomes clearer how natural processes such as adsorption are driven by the quest for minimizing energy to attain stability.

Enthalpy

Enthalpy is a thermodynamic property that measures the total heat content of a system. When considering processes like adsorption, enthalpy changes (\( \Delta H \)) provide insight into whether heat is being absorbed or released. Typically, adsorption is an exothermic process, meaning that it releases heat, corresponding to a negative \( \Delta H \).
As gas or liquid molecules get adsorbed onto a solid surface, they transition from a higher energy state to a lower one. This transition is often accompanied by a release of energy into the surrounding environment.

• In the context of adsorption, the negative \( \Delta H \) is crucial—it shows that energy is released, a common indicator of bond formation between adsorbate and adsorbent.
• Understanding how enthalpy relates to the energy exchanges in adsorption helps underscore why this process is typically spontaneous.

Enthalpy gives a complete picture of how energy transactions play a role in thermodynamic processes, including adsorption.

Entropy

Entropy is a measure of the disorder or randomness in a system. In thermodynamics, it is often associated with the distribution of energy across states. Adsorption typically causes a decrease in entropy, noted as \( \Delta S < 0 \), because it results in rearranging molecules from a more random state to an ordered one on a surface.
When a substance transitions from a less organized state (like a gas) to an organized one (onto a solid surface), the randomness decreases. This decrease is indicative of the restrictive movement and arrangement of the adsorbed molecules. Despite this decline in entropy:

• The process remains spontaneous due to the overcoming energy advantage, as indicated by the negative \( \Delta G \).
• The reduction in disorder correlates to thermodynamically more stable systems, where the overall energy is minimized while arranging the molecules in a specific pattern on a surface.

Understanding entropy within adsorption helps clarify the delicate balance between energy and disorder in thermodynamic processes.