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Adsorption is a surface of phenomenon of attraction and retention of the molecules of adsorbate on the adsorbent leading to a higher concentration of the adsorbate on the surface of the adsorbent. A theoretical concept introduced at Grade K-11-12 Chemistry classes can be studied as a laboratory experiment. It impresses the concept of adsorption as a surface phenomenon.

The substance adsorbed is termed as an adsorbate while the surface on which the substance is adsorbed is called the adsorbent. An example that demonstrates this phenomena is water vapor adsorbed by dry silica gel. This is in contrast to absorption wherein the substance is uniformly distributed into the bulk of the solid/liquid, for example anhydrous calcium chloride absorbs moisture. Adsorption can be negative or positive - When the concentration of the adsorbate is larger on the surface of the adsorbent than in the bulk, it is positive adsorption. In contrast, if the concentration of the adsorbate on the surface is less relative to its concentration in the bulk, it is called negative adsorption. Like surface tension, adsorption is a consequence of surface energy.

A molecule in the interior of a liquid/solid is completely surrounded by other molecules on all sides and hence the intermolecular forces of attraction are exerted equally in all directions. However, atoms on the surface of the adsorbent are not wholly surrounded by other adsorbent atoms and therefore experience some net inward force of attraction. Because of these unbalanced inward forces, liquids and solids have the property to attract and retain molecules of a gas or a dissolved substance onto their surfaces with which they come in contact.

For adsorption, ∆H, the enthalpy change is negative, as molecules of the adsorbate are held on the surface of the solid adsorbent, entropy decreases i.e. ∆S is also negative. Since ∆G = ∆H - T∆S for the process of adsorption to occur spontaneously, ∆G must be negative which is possible only when ∆H>T∆S in magnitude. This is true in the initially, but as the reaction proceeds, ∆H keeps on decreasing and T∆S keeps on increasing till ∆H = T∆S so that ∆G = 0. This state is called adsorption equilibrium. There are two well established types of adsorption isotherm viz. the Freundlich adsorption isotherm and the Langmuir adsorption isotherm.

The Freundlich equation or Freundlich adsorption isotherm is an adsorption isotherm, which is a curve relating the concentration of a solute on the surface of an adsorbent, to the concentration of the solute in the liquid with which it is in contact. The first mathematical expression for an isotherm was published by Freundlich and Küster (1894) and is a purely empirical formula for gaseous adsorbates.

x/m= Kc 1/n
where,
x is the quantity adsorbed
m is the mass of the adsorbent
c = Equilibrium concentration of adsorbate in solution.

k and n are empirical constants for each adsorbent-adsorbate pair at a given temperature. The function has an asymptotic maximum as pressure increases without bound. As the temperature increases, the constants k and n change to reflect the empirical observation that the quantity adsorbed rises more slowly and higher pressures are required to saturate the surface. The chemistry project on Freundlich’s adsorption isotherm uses activated carbon is used as an adsorbent. Adsorbents must have high abrasion resistance, high thermal stability and small pore diameters, which results in higher exposed surface area and hence high surface capacity for adsorption. Five stoppered bottles containing different concentrations of acetic acid were prepared. To each powdered activated charcoal was added. The mixture was allowed to stand for half and hour with occasional shaking. It was titrated against sodium hydroxide using phenolpthalein as the indicator. A Freundlich adsorption isotherm of x/m vs concentration of acetic acid gives a sigma shaped profile.

Acknowledgements- The experimental work was carried out by Sai Phaltankar and Swadha Das Guru of Grade 12 of R.N. Podar CBSE School Santa Cruz(W).