Chemistry Project(1) 4oj55

INTRODUCTION Since solids exist because of intermolecular forces between the repeating units that make up the lattice, those intermolecular forces are unsaturated or unsatisfied at the surface of the solid. In the interior of the solid, each molecule (if it is a molecular solid) is surrounded on all sides by identical molecules. At the surface of such a solid, however, each molecule is only partially surrounded by identical molecules; when it is not surrounded by identical molecules, any available molecule or ion is adsorbed to its surface. The solid phase is called the adsorbent. The molecules that are absorbed on the absorbent are collectively called the absorbed phase or absorbate. The absorbate is either a gas(molecules) or a solute(molecules or ions) in a solution. In this experiment, we will investigate the adsorption of acetic acid in aqueous solution on activated charcoal.

Adsorption by a solid is not a very important process unless the solid has a very large surface compared to its mass. Consequently, charcoal made from bone, blood, or coconut shells is specially effective because it has a highly porous structure. Charcoal is activated by 1

being heated to quite high temperatures in a vaccum or in a stream of dry air. This stream desorbes the hydrocarbons that are adsorbed when the charcoal is first produced. Charcoal , a covalently bonded solid, is more effective at absorbing molecules than ions. Silver chloride, on the other hand, forms as a precipitate of nearly colloidal dimensions if precipitated rapidly; in colloidal form it has a very high surface-to-mass ratio and readily adsorbes ions from solutions, often to the dismay of analytical chemists.

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Meaning of Adsorption

Adsorption is the adhesion of atoms, ions, biomolecules or molecules of gas, liquid, or dissolved solids to a surface. This process creates a film of the adsorbate (the molecules or atoms being accumulated) on the surface of the adsorbent. It differs from absorption, in which a fluid permeates or is dissolved by a liquid or solid. The term sorption encomes both processes, while desorption is the reverse of adsorption. It is a surface phenomenon. Similar to surface tension, adsorption is a consequence of surface energy. In a bulk material, all the bonding requirements (be they ionic, covalent, or metallic) of the constituent atoms of the material are filled by other atoms in the material. However, atoms on the surface of the adsorbent are not wholly surrounded by other adsorbent atoms and therefore can attract adsorbates. The exact nature of the bonding depends on the details of the species involved, but the adsorption process is generally classified as phsiosorption (characteristic of weak van der wall forces) or chemisorption (characteristic of covalent bonding). It may also occur due to electrostatic attraction.

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Application of Adsorption

Adsorption is present in many natural physical, biological, and chemical systems, and is widely used in industrial applications such as activated charcoal, capturing and using waste heat to provide cold water for air conditioning and other process requirements (adsorption chillers), synthetic resins, increase storage capacity of carbide-derived carbons for tunable nanoporous, and water purification. Adsorption, ion exchange, and chromatography are sorption processes in which certain adsorbates are selectively transferred from the fluid phase to the surface of insoluble, rigid particles suspended in a vessel or packed in a column. Lesser known, are the pharmaceutical industry applications as a means to prolong neurological exposure to specific drugs or parts thereof.

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Birth of Freudlich Equation

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 . In 1909, Freundlich gave an empirical expression representing the isothermal variation of Adsorption of a quantity of gas adsorbed by unit mass of solid adsorbent with pressure. This equation is known as Freundlich Adsorption Isotherm or Freundlich Adsorption equation. There are basically two well established types of adsorption isotherm: the Freundlich adsorption isotherm and the Langmuir adsorption isotherm. Here the amount of mass that is adsorbed is plotted against the temperature which gives an idea about the variation of adsorption with temperature.

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Freundlich Equation

The first mathematical fit to an isotherm was published by Freundlich and Küster (1894) and is a purely empirical formula for gaseous adsorbates,

where x is the quantity adsorbed, m is the mass of the adsorbent, P is the pressure of adsorbate and 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.

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Freundlich Adsorption Isotherm

The Freundlich Adsorption Isotherm is mathematically expressed as

It is also written as

or

It is also written as

where x = mass of adsorbate. m = mass of adsorbent. p = Equilibrium pressure of adsorbate. c = Equilibrium concentration of adsorbate in solution. K and n are constants for a given adsorbate and adsorbent at a particular temperature. At high pressure 1/n = 0 Hence extent of adsorption is independent of pressure But at high pressure it is dependent on pressure.

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Freundlich Adsorption Isotherm Graph

Example of the Freundlich isotherm, showing the amount adsorbed, q (e.g., in mol/kg), as a function of equilibrium concentration in the solution, c (e.g., in mol/L). The graph is for the values of the constants of K=4 and 1/n=0.

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Adsorption of Acetic Acid on Charcoal The amount of adsorption, given the symbol Y, has units of moles adsorbate per mass adsorbent. If the adsorbate is a gas, Y may have units of volume adsorbate per mass adsorbent. The amount of adsorption Y increases with the concentration c of the adsorbate. The increase is very rapid at first, when the surface of the adsorbent is relatively free. As the surface fills with the adsorbate, the rate of adsorption d Y/ d c decreases.

Eventually the surface of the adsorbent becomes full and further increases in the concentration cause no further increase in the amount adsorbed as shown in the figure below. The amount adsorbed when the surface is just covered with a mono molecular of the adsorbate is called Ymax. At a given concentration the amount adsorbed decreases with increasing temperature. In this purely empirical equation (Y= kc1/n), the units of Y are moles adsorbate per gram adsorbent, c is the concentration(mole/ L), And k and n are experimentally determined constants. Since above equation is valid only for a given adsorbed phase and adsorbent at a constant temperature, it is sometimes called as feundlich isotherm. To test the validity of the freundlich isotherm, take the logarithm of both sides of the equationLog10Y= Log10k+1/n Log10C If Log10 Y is plotted against Log10 C, Straight lines results with the slope= 1/n and the intercept =Log10k.

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Limitation of Freundlich Equation

Experimentally it was determined that extent of adsorption varies directly with pressure till saturation pressure Ps is reached. Beyond that point, rate of adsorption saturates even after applying higher pressure.

Thus freundlich adsorption isotherm failed at higher pressure. To correct this the following theory was followed. The postulates of this theory are as follows:-

1. The adsorbed phase forms a layer of molecules, one molecule deep. 2. The system is in a state of equillbrium such that the rate of adsorption equals the rate of desorption. 3. The rate of adsorption is proportional to the concentration and the fraction of the surface that is vacant. 4. The rate of desorption is proportional to the fraction of the surface that is already covered.

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Determination of The Specific Area of the Adsorbent

If the adsorption of the adsorbate leads to a maximum of a single molecular layer when the adsorption is complete, it is possible to calculate the area of the adsorbent. When a mono molecular layer is adsorbed, it may be assumed that the area of the adsorben is equals total area of adsorbed molecules. The determination of the area of an area of an adsorbed molecule is simple in the case of small adsorbed gas molecules such as helium, hydrogen and nitrogen. When the adsorbed molecule is a carboxylic acid , however, the structure is clearly more complexed. Nevertheless, the studies of adsorption of straight-chain aliphatic monocarboxlic acids indicate that the number of moles of acid adsorbed per gram adsorbent is independent of chain lenghth( Hansen and Craig, 1954). This suggest that the acid molecules are adsorbed vertically, with the aliphatic chain up and the carboxyl group down and attach to the adsorbent. The cross-sectional area Aa of a straight chain acid may be taken to be about 21*10-20m2. The specific area of the absorbent S(m2/g) is then given by S=AaNAYmax where NA is Avogadro number.

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APPARATUS USED

This experiment requires about 20 grams activated charcoal(from blood); 12 glass- stopper; 125 ml Erlenmeyer flask; 5 -,10-,25-,and 50- ml pipettes; medium –fine filter paper; 600ml 0.4M acetic acid; 500 ml 0.1M sodium hydroxide (standardized); a phenolphthalein indicator; and a 50 ml burette.

Erlenmeyer flask

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activated charcoal

Safety Considerations

If it is necessary to prepare the 0.4M acetic acid from glacial acetic acid, take care not to breathe the fumes, and carry out the dilution in a ventilated hood.

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Experimental Procedure

Weigh about 1.5 g charcoal into each of the dry glass- stoppered Erlenmeyer flask. Record the weight to 1 mg. prepare a series of acetic acid solutions of various concentrations according to the table below. Add 100 ml of each solution to each charcoal sample. Swirl the flasks vigorously and let them stand overnight. Filter the solutions and titrate a suitable size aliquot of each filtrate with standard 0.1 M sodium hydroxide. Use progressively larger aliquots for the more dilute solutions. Run each concentration in duplicate.

Suggested Volumes of 0.4M Acetic Acid to Dilute to 100.00 ml

Samplea analysis(ml)

0.4 M Acetic Acid (ml)

1. 2. 3. 4. 5. 6.

a

Prepare samples in duplicate.

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100 75 50 25 10 5

Aliquot for

10 10 10 25 25 50

Results and Calculations

From the titration data determine the concentrations of the original acetic acid solutions and of the acid solutions in equilibrium with the adsorbent. From the volume of the solutions, their equilibrium concentrations, and the original acid concentrations, calculate Y, the number of moles of the acid adsorbed per gram of adsorbent. Prepare suitable tables of the quantities needed to test the validities of freundlich isotherm for the aqueous acetic acid charcoal system. Plot the duplicate runs independently(do not average it before plotting). Calculate Freundlich parameters and discuss the observed results.

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Applications of Activated Charcoal

1. It is used in the water treatment for removing factory waste from water.

2.

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It is used in the medical treatment.

3. It is used in electronics in double layer capacitors and hard disks.

4. It is used in mineral recovery.

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5. It is also used in solvent recovery.

6. It is used in depolarization of industrial chemicals, eg. sugar refinement etc.

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Bibliography

1. www.Google.com 2. www.Wikipedia.com 3. www.idoub.com 4. Ncert class XII Chemistry 5. Britannica

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