Question

Methylene blue, from its aqueous solution, is adsorbed on activated charcoal at $25^\circ$C. For this process, the correct statement is

• A. the adsorption requires activation at $25^\circ$C
• B. the adsorption is accompanied by a decreases in enthalpy
• C. the adsorption increases with increase of temperature
• D. the adsorption is irreversible

Answer 1

Answer: (B)

the adsorption is accompanied by a decreases in enthalpy

Answer 2

Physical adsorption takes place with decrease in enthalpy thus exothermic change. It is physical adsorption and does not require activation. Thus, (a) is incorrect. Being physical adsorption $\Delta$H < 0 thus, (b) is correct. Exothermic reaction is favoured at low temperature thus (c) is incorrect. Physical adsorption is always reversible, thus (d) is incorrect.