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Surface Chemistry

Question
CBSEENCH12006546
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The adsorption of gases on solids can be described by Frundlich’s reaction:
xm=kp1/4

(i) How are the values of k and n determined experimentally?
(ii) Explain why the value of n should be equal to or greater than one?

Solution
Answer:

Freundlich gave anempirical relationship between the quantity of gas adsorbed by unit mass of solid adsorbent and pressure at a particular temperature. The relationship can be expressed by the following equation:

xmkp1/n

where x is the mass of the gas adsorbed on mass m of the adsorbent at pressure P, k and
n are constants which depend on the nature of the adsorbent and the gas at a particular
temperature.




Fig. Linear graph between log x/m and log p. Taking logarithms on both sides, we get

log xm = log k + 1nlog p.

Thus, if we plot a graph between log (x/m) and log p, a straight line will be obtained. The slope of the line is equal to 1 / n and the intercept is equal to log k. Thus the values of k and n are determined.

Freundlich isotherm explains the behaviour
of adsorption in an approximate manner. The
factor 1 n can have values between 0 and 1
(probable range 0.1 to 0.5). Thus,  above equation

When 1/n = 0, x/ m= constant, the adsorption is independent of pressure.
When 1/n = 1, x/m= k p, i.e. x/m∝ p, the adsorption varies directly with pressure.

 

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