Note

If you cannot find what you are looking for. Please visit our sitemap

Saturday, 26 September 2015

FSc Notes Chemistry Part 1 Chapter 2 Experimental Techniques in Chemistry Lecture 3

FSc Notes Chemistry Part 1 Chapter 2 Experimental Techniques in Chemistry

(4) Sublimation:

The process in which a substance is converted from solid into gaseous state with out going into liquid state by heating is known as sublimation. This process is used for the purification of a solid subliming substance such as iodine (I2) naphtha tine, Ammonium chloride, etc. In this process the substance is taken is a watch glass covered with an inverted funnel. The watch glass is then heated gently over a sand bath while the funnel is cooled by covering its other surface with wet cotton. Thus after sometime we get pure crystals of the solid upon the inner surface of the funnel which are then scratched and stored. Actually firstly the solid substance converts from solid to gaseous form and then the gaseous vapors get back into solid form because the funnel is cool. The impurities are left behind at the watch glass.

(5) Distillation:

The process in which the components of a solution are separated on the basis of difference in their boiling points through evaporation and condensation is called as distillation. In this process a distillation flask is used which contains the mixture (solution). A thermometer is fixed in its neck such that its bulb is dipped in the solution. This flask is then connected with a water condenser. The condenser consists of a narrow glass tube covered by a glass jacked with two openings one at Lowe and other at the upper side. A continuous flow of water should be applied through the gasket of the condenser which keeps the inner narrow tube cooled. The other end of the condenser is connected with a flask known as receiving flask. The receiving flask receives one of the components of the solution in the low b.p liquid. For example if we want to separate ink from water, the mixture is placed in the distillation flask and heating is started. We will see that at 100oc, the temperature will steps, meaning that water starts evaporation. When the vapors pas through the condenser, they are cooled down and thus they condenser out. Thus we get the drops of pure distilled water in the receiving flask. When the temp starts rising the burner is removed because all the water has been transferred from the dist flask into the receiving flask. Thus the ink and water are separated out.

(6) Solvent Extraction:

According to this technique a solute can be separated from solution by shaking the solution with a solvent in which the solute is more soluble such that the added solvent does not mix with the solution. This process is carried out in a funnel known as “separating funnel”. The most common laboratory example of solvent extraction is “Ether extraction”. The extraction is used for separating on organic compound from water i.e. aqueous solution. The aqueous solution of the organic compound is placed with ether in a separating funnel and shaken. Then they are allowed to separate because water and ether are immiscible. The organic compound gets dissolved in the ether layer which all the impurities are left in the water layer. It is because of the fact that the organic compound is more soluble in the ether then in water but the in organic impurities are insoluble in ether. Then by controlled opening of the stopper of the separating funnel, the two layers are separated out. Then we get the solid organic compound by evaporation ether. Solvent extraction is an equilibrium process and is controlled by “distribution law”. This law states that a solute distributes it self between two immiscible liquids in a constant ration of concentration irrespective of the amount of solute added.” For example consider the distribution of iodine between two immiscible solvents i.e.; water having some K.I dissolved in it and carbon tetrachloride (Ccln1). Here I2 reacts with I-1 ions to form trite disk ions (I-3) in a reversible manner i.e.
I2 + I-(aq) I-s(aq)
Here the rates of forward and reverse reactions becomes equal. If we added ccl4 to an aqueous solution of I-3 ions, the I2 will transfer from aqueous layer (H2o) into organic layer (ccl4). Thus the brown color of the I-3 ions gets faded and a purple color of the free iodine (I2) appears in the organic layer.
For further good separating the two solvents are gently shaken to increase their area of contact and improve the chances of transferring I2 molecules to organic layer. It is important to note that no matter, how much iodine is used, the ration of the final concentration at equilibrium is constant. The constant is called “Distribution co-efficient”, (K) and is given by;
K = [I2(ccl4)]/ [I2as I-3 (aq)]
Soluble in
CCl4
Soluble in
H2O

Chromatography

It is an important application of the distribution phenomenon. The word chromatography is actually derived from a Greek work “Khromatos” which means color writing. Chromatography is a technique which is used for the separation of various colored components of a mixture.
Here two phases are used for the separation of the components of a mixture. One is known as stationary phase and the other is mobile phase. The stationary phase may be solid or liquid supported as a thin film on the surface of an inert solid.
The mobile phase may be a liquid or a gas. In chromatography, substances are separated due to their relative affinities for the stationary and mobile phases. The distribution of the components of a mixture between the two phases is governed by distribution co-efficient (K). i.e. K = Concentration of a component in the moving / Concentration of that component in the stationary phase. That components of the mixture which has small value of “K” mostly remains in the stationary phase as the moving phase flows over it. The component with greater value “K” remains largely dissolved in the mobile phase, and passes over the stationary phase quickly. Types: There are two main types.

(i) Adsorption chromatography:

In this type of chromatography a substance leaves the mobile chromatography the stationary phase is liquid (for example water) which is adsorbed on paper. Where as the mobile phase usually on organic liquid. The following are three common ways to carry paper chromatography.
  1. Ascending
  2. Descending
  3. Radial / Circular
We discuss over only ascending type:
Ascending paper chromatography:
In this technique the solvent is in a pool at the bottom of vessel in which the paper is supported and the solvent travels upwards by the capillary action.
Procedure:
A solvent mixture specially composed according to the nature of sample to be separated. This liquid is poured into the chromatographic tank. The tank is converted from its top to homogenize its inner atmospheric what Mann s chromatographic paper No.1 is taken which has about 20 cm in length and a line is drawn with pencil from one end. A point is spotted with sample mixture phase to become adsorbed on the surface of the solid phase. Here the stationary phase is solid.

(ii) Partition chromatography:

In this type of chromatography, the substances being separated are distributed through out both the stationary and mobile phases. Here the stationary phase a liquid. There several chromatographic techniques; like paper chromatography, TLC (Thin Layer Chromatography),etc.
Paper Chromatography:
It is the technique of partition chromatography. Actually all the chromatography are based upon the principal of selection distribution of the components of a mixture between two phase, stationary and moving or mobile phase. In paper solution on the pencil line. When the spot home dried the paper is suspended with clips. As the solvent front passes the spots, the solute begin to none upward as the solvent front rises to about 3/4 of the length of the paper the step is removed. The solvent from is marked with pencil. The strip is allowed to dry. The dried paper is called “Chromatogram” after the process. Now if the components are colored, they can be identified visually. If they are colorless, the chromatogram has to be denoted by chemical or physical methods, to identify the spots. Each component has a specific retardation factor called Rf value. The Rf value is given by Rf = distance traveled by a component / distance traveled by solvent.

Written by: Asad Hussain

No comments:

Post a Comment