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Methylene blue, from its aqueous solution, is adsorbed on activated charcoal at 25 degrees C. For this process, which of the following statements is correct?

1. The adsorption requires activation at 25 degrees C
2. The adsorption is accompanied by a decrease in enthalpy
3. The adsorption increases with increase of temperature
4. The adsorption is irreversible

Correct answer: The adsorption is accompanied by a decrease in enthalpy

Solution

Adsorption is always exothermic because the adsorbate forms bonds/interactions with the surface, releasing energy (delta_H < 0). Adsorption generally decreases with temperature, is reversible (physisorption), and does not require activation at 25 degrees C for activated charcoal. The correct statement is that adsorption is accompanied by a decrease in enthalpy.

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