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Define adsorption isotherm and explain Freundlich adsorption isotherm of gases on solids. |
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Answer» <P> Solution : Freundlich adsorption isotherm. The behaviour of adsorption with pressure can be expressed by adsorption isotherm as shown in Fig. The inspection of the graph reveals the following FACTS : (i) At low pressure. The graph is almost straight line which indicates that x/m is directly proportional to the pressure. This may be expressed as: `x/m prop p " or" x/m =k p ` .where k is a constant....(i) (ii) At HIGH pressure. The graph becomes almost constant which means that x/m becomes independent of pressure. This may be expressed as: `x/m =` constant ` x/m prop p^0""[ because p^0 = 1]` ` x/m= kp^0`...(ii) (iii) Thus, in the intermediate RANGE of pressure, x/m will depend UPON the power of pressure which lies between 0 to 1 i.e., fractional power of pressure. This may be expressed as:  Fig. Linear graph between log x/m and log p ` x/m prop p^(1//n)` or`x/m = kp^(1//n)`....(iii) where n can take any value between 1 and a larger integer depending upon the pressure. The above relationship is also called Freundlich.s adsorption isotherm. The constants k and n can be determined as explained ahead : Taking logarithms on both sides of Eq. (iii), we get log`x/m = log k+ 1/n log 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. |
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