Question

Which of the following is wrong: (1) Enthalpy (numerical value) of physisorption is greater than that of chemisorption (2) Physisorption is not very specific but chemisorption is highly specific (3) Chemisorption takes place at relatively high temperatures (4) In physisorption generally multimolecular layers are formed on the adsorbent

Short answer

The incorrect statement is (1).

Step-by-step solution

Understanding Enthalpy of Physisorption and Chemisorption

Enthalpy of physisorption is typically lower than that of chemisorption. Physisorption involves non-covalent van der Waals forces, whereas chemisorption involves the formation of chemical bonds, which require more energy.

Specificity of Physisorption and Chemisorption

Physisorption occurs due to general van der Waals forces and hence is non-specific. Chemisorption, on the other hand, involves specific chemical bond formation, making it highly specific.

Temperature Condition for Chemisorption

Chemisorption typically occurs at higher temperatures because higher temperatures provide the energy needed to break bonds and form new chemical bonds.

Layer Formation in Physisorption

Physisorption can lead to the formation of multimolecular layers on the adsorbent due to the low energy of adsorption.

Identifying the Incorrect Statement

Review the statements based on the above understanding: 1) Incorrect, 2) Correct, 3) Correct, 4) Correct. The incorrect statement is (1) since the enthalpy of chemisorption is greater than that of physisorption.

Key concepts

enthalpy of adsorption

Enthalpy of adsorption, or heat of adsorption, measures the energy change when a substance adsorbs on a surface.

For physisorption, the enthalpy change is relatively low. This is because physisorption involves weak van der Waals forces, which do not require much energy. Values typically range from 20-40 kJ/mol.

In contrast, chemisorption involves stronger chemical bonds between the adsorbate and the surface, resulting in higher enthalpy values, generally between 100-400 kJ/mol.

This difference is crucial. Physisorption is a more superficial, low-energy process, while chemisorption involves deeper, more energetic interactions. Remember, the higher the enthalpy, the more energy is involved in the adsorption process.

specificity in adsorption

Specificity in adsorption refers to how particular an adsorbent is about the types of molecules it attracts and binds to its surface.

Physisorption tends not to be very specific. This is due to its reliance on weak van der Waals forces, which do not favor particular molecules over others. Hence, various molecules can adsorb readily.

Chemisorption, on the other hand, is highly specific. Because it involves the formation of strong chemical bonds, only certain molecules that can interact chemically with the adsorbent surface will be adsorbed. This specificity is often a double-edged sword—it's highly selective but can be limited by the availability of suitable molecules.

temperature effect on adsorption

Temperature plays a different role in physisorption and chemisorption.

For physisorption, increasing temperature can reduce adsorption. This is because higher temperatures give adsorbed molecules enough energy to overcome van der Waals forces and desorb from the surface. Generally, physisorption is more effective at lower temperatures.

Chemisorption behaves oppositely. Higher temperatures often promote chemisorption, as they provide the energy necessary for the adsorbate to break its initial bonds and form new chemical bonds with the adsorbent surface. Thus, chemisorption typically becomes more efficient at higher temperatures.

layer formation in adsorption

The number of molecular layers formed during adsorption also varies between physisorption and chemisorption.

In physisorption, multiple molecular layers can form. Since the forces involved are weak, adsorbate molecules can stack atop one another, creating multimolecular layers. This phenomenon makes physisorption quite versatile but also less specific.

Conversely, chemisorption usually results in monolayer adsorption. The strong chemical bonds formed during chemisorption typically saturate all available bonding sites on the adsorbent with a single layer of adsorbed molecules. This ensures a strong, uniform adsorption but limits the overall capacity compared to physisorption.