Adsorption is the process where gas molecules accumulate on the surface of a solid. There are two main types: physisorption (physical adsorption) and chemisorption (chemical adsorption). Let's analyze each statement:

Statement 1: Adsorption is always exothermic

This is correct. Both physisorption and chemisorption are exothermic processes. The enthalpy change for adsorption is negative because when gas molecules are adsorbed, they lose energy and the system becomes more stable. For physisorption, ΔH is typically between -20 to -40 kJ/mol, while for chemisorption, it is between -80 to -400 kJ/mol.

Statement 2: Physisorption increases with increasing temperature but chemisorption decreases with increasing temperature

This is incorrect. Physisorption is an exothermic process and, like most exothermic processes, it decreases with increasing temperature due to Le Chatelier's principle. Chemisorption, being also exothermic, generally decreases at very high temperatures, but it first increases with temperature because it requires activation energy. Initially, as temperature rises, the rate of chemisorption increases until an optimum temperature is reached, after which it decreases.

Statement 3: Chemisorption is more exothermic than physisorption, however it is very slow due to higher energy of activation

This is correct. Chemisorption involves the formation of chemical bonds and is therefore more exothermic. However, it has a high activation energy barrier, making it slower at lower temperatures. Physisorption, involving weak van der Waals forces, has low activation energy and occurs rapidly.

Statement 4: Physisorption may transform into chemisorption at high temperature

This is correct. At low temperatures, physisorption occurs first. As temperature increases, the gas molecules may acquire enough energy to overcome the activation energy barrier for chemisorption, leading to a transformation from physisorption to chemisorption.

Related Topics and Formulae

Freundlich Adsorption Isotherm: $\frac{x}{m}=k{P}^{\frac{1}{n}}$ , where x is mass of gas adsorbed, m is mass of adsorbent, P is pressure, k and n are constants.

Enthalpy of Adsorption: Always negative (exothermic).

Activation Energy: High for chemisorption, low for physisorption.

Final Answer: The correct statements are 1, 3, and 4.