Paper: CSIR-NET December 2016 · Subject: Physical Chemistry · Chapter: Surface Chemistry · Topic: Surface Chemistry – General · Marks: 2 · Difficulty: Medium
The volume of a gas adsorbed on a solid surface is 10.0 mL, 11.0 mL, 11.2 mL, 14.5 mL and 22.5 mL at 1.0, 2.0, 3.0, 4.0 and 5.0 atm. pressure, respectively. These data are best represented by
(a)Gibbs's isotherm
(b)Langmuir isotherm
(c)Freundlich isotherm
(d)BET isotherm
Answer
Answer: D ✓ checked by 4AB · confidence medium
The source book printed C; on checking, D is correct — see the explanation.
Adsorbed volume nearly levels off (10.0→11.0→11.2 mL, monolayer) then rises steeply (14.5, 22.5 mL) at higher pressure — multilayer adsorption, a type-II curve described by the BET isotherm. Freundlich (V ∝ p^(1/n), n>1) is concave downward and cannot produce this upturn; Langmuir saturates.
Explanation
Increments in adsorbed volume: 1.0, 0.2, 3.3, 8.0 mL per atm. The volume first nearly saturates (≈ 11 mL, monolayer completion) and then rises steeply at higher pressure, i.e. further layers are adsorbed on the monolayer.Langmuir (monolayer) predicts saturation, and Freundlich (V = kp^(1/n), n > 1) gives a curve that bends downward; neither shows the upturn. Gibbs's isotherm relates surface excess to surface tension of solutions.Monolayer followed by multilayer growth (type-II isotherm) is described by the BET isotherm. Answer: (D).