Understanding Physisorption and Chemisorption

Physisorption and chemisorption are two types of adsorption processes where molecules adhere to a surface. Physisorption involves weak van der Waals forces, is reversible, and is significant at low temperatures. Chemisorption involves strong chemical bonds, is often irreversible, and requires higher activation energy, becoming important at higher temperatures.

Step 1: Analyze Graph I

Graph I shows that the amount of adsorption decreases as temperature increases. This is typical for physisorption because it is exothermic and favored at lower temperatures. As temperature rises, molecules desorb due to increased kinetic energy.

Step 2: Analyze Graph II

Graph II shows that the amount of adsorption first increases with temperature and then decreases. This indicates chemisorption, which requires activation energy. At low temperatures, the rate is slow; it increases with temperature until an optimum, after which desorption dominates.

Step 3: Analyze Graph III

Graph III shows that the amount of adsorption is low at low pressure and increases rapidly at high pressure, following a curve that may fit the Langmuir isotherm. This behavior is characteristic of chemisorption, which involves monolayer formation and saturates at high pressure.

Step 4: Analyze Graph IV

Graph IV shows that the amount of adsorption increases linearly with pressure at low pressures and becomes less steep at higher pressures, following a trend similar to the Freundlich isotherm. This is typical for physisorption, which can form multilayers and does not saturate as easily.

Final Answer

Based on the analysis:

• I is physisorption (decreases with temperature).
• II is chemisorption (increases then decreases with temperature).
• III is chemisorption (Langmuir-type isotherm).
• IV is physisorption (Freundlich-type isotherm).

Therefore, the correct choices are:

• I is physisorption and II is chemisorption
• IV is physisorption and III is chemisorption (but note: the options say "IV is chemisorption" which is incorrect; IV is physisorption)

Among the given options, only "I is physisorption and II is chemisorption" is correct. The other options incorrectly identify IV as chemisorption or III as physisorption.

Related Topics

• Adsorption Isotherms: Langmuir, Freundlich, BET
• Surface Chemistry
• Activation Energy in Chemisorption
• Desorption Processes

Formulae and Theory

Langmuir Isotherm: $\frac{\theta }{1}=\frac{KP}{1+KP}$, where θ is coverage, K is equilibrium constant, P is pressure.

Freundlich Isotherm: $\frac{x}{m}=k{P}^{1/n}$, where x/m is amount adsorbed, k and n are constants.

Physisorption is exothermic: $\mathrm{\Delta H}<0$, while chemisorption has higher $\mathrm{\Delta H}$ and often requires activation energy.