Discussion
For the adsorption of N2 on activated carbon at 77 K :
Answer: B
For this physisorption, ΔH \(\lt\) 0 and ΔS \(\lt\) 0.
When N2 molecules are adsorbed on the carbon surface they lose freedom of translation: a gas moving in three dimensions becomes confined to a two-dimensional layer, so the entropy decreases. Adsorption at 77 K is nevertheless spontaneous, so ΔG = ΔH - TΔS must be negative. With -TΔS positive, this is only possible if ΔH is negative: adsorption is exothermic, as van der Waals attraction to the surface releases energy.
ΔH = 0 or ΔH \(\gt\) 0 with ΔS \(\lt\) 0 would make ΔG positive and adsorption non-spontaneous. The last option, printed with ΔS twice, describes no change at all.
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