92168.pdf

282 Problems

Amount of'N 2 adsorbed (VIVm) 0.4 0.8 1.2
p (90 K)/p (77 K) 14.3 17.4 7.8

Calculate an isosteric enthalpy of adsorption for each value of VIVm

and comment on the values obtained.

20. In adsorption studies of a particular system, it was found that
    the adsorption isotherms could be expressed by empirical equations
    of the form, 6 = A (p/mmHg)", where 9 is the fraction of the solid
    surface covered by adsorbed gas at equilibrium pressure, p. Studies at
    two temperatures yielded the following values for the constants of the
    above equation:

TIK A n

400 0.065 0.43

420 0.014 0.55

Calculate enthalpies of adsorption for coverages 0 = 0.1 and & = 0,5,

and comment on the results obtained.

21. Benzene was passed at various temperatures through a gas
    chromatography column packed with silica gel and the following
    specific retention volumes (extrapolated to zero sample size) were
    measured:

TemperaturerC 90 105 120 140

Specific retention volume! 170 97 55 29

(cm^3 at 0°C) g-^1

Calculate an isosteric enthalpy of adsorption of benzene on the silica

gel sample.

22. Use the Kelvin equation to calculate the pore radius which
    corresponds to capillary condensation of nitrogen at 77 K and a
    relative pressure of 0.5. Allow for multilayer adsorption on the pore
    wall by taking the thickness of the adsorbed layer on a non-porous
    solid as 0.65 nm at this relative pressure. List the assumptions upon
    which this calculation is based. For nitrogen at 77 K, the surface
    tension is 8.85 mN m"^1 and the molar volume is 34.7 cm^3 mol"^1.