Transcription

1 Chapter No. 2 EXPERIMENTAL TECHNIQUES IN CHEMISTRY LONG QUESTIONS Analytical Chemistry: The science of chemical characterization is called analytical chemistry. OR The branch of chemistry which deals with qualitative as well as quantitative analysis of a sample is called analytical chemistry Qualitative Analysis: Detection or identification of elements in a compound is called qualitative analysis.e.g. Detection of radicals of salt Quantitative Analysis: The determination of the relative amounts of elements in a compound is called quantitative analysis.e.g Combustion analysis It involves following steps i. Sampling ii. Separation of desired components iii. Measurements iv. Calculation of results v. Drawing the conclusion Experimental Techniques: During the laboratory work a student deals with following techniques i. Filtration ii. Crystallization iii. Sublimation iv. Solvent Extraction v. Chromatography 1

2 i. FILTRATION The process in which insoluble particles are separated from a liquid is called filtration. For filtration we can use a Filtration paper or Filter crucible. Filter Paper: The filtration by filter paper is a slow process. In this process solvent passes through filter paper and insoluble particles are left behind on the filter paper. Many types of filter papers are available. They have different porosites (pore sizes). A suitable filter paper is chosen. Its size should be large so that it should be to full of precipitates at the end of filtration. The funnel should be large so that its rim should extend 1-2 cm above the filter paper. The stem of funnel should be long. It should touch the side of beaker. Moreover stem of funnel should remain full of liquid during the filtration. The filter paper should be fitted properly otherwise it is time wastage. Folding of Filter Paper: i. Filter paper should be folded twice. ii. The first fold should be along the diameter. iii. In second fold the edges should not be quite matched. iv. The folded filter paper should be opened on the larger section. v. A cone is formed by three fold side and one fold side. vi. The apex angle should be greater than 60 o. vii. Then filter paper is placed into the 60 o funnel. viii. Finally wet the 2

3 filter paper and press it firmly. ix. The rate of filtration can be increased by gentle suction. (Fluted Filter Paper) x. The rate of filtration can also be increased by fluted filter paper. Fluted Filter Paper: A Filter Paper Raving many elevations and depressions is called fluted filter paper. A filter paper folded by fan like arrangement with alternate elevations and depressions given fluted filter paper. Filter Crucibles: We use filter crucible for an easy filtration. There are two types of filter crucibles. i. Gooch Crucible ii. Sintered glass Crucible i. Gooch Crucible: It is made of porcelain. Its bottom is perforated covered with paper pulp. The Gooch crucible is placed on suction apparatus. It is shown in figure. It is used to filter those precipitates which are to be heated at high temperature. Some solutions like HCl, KM n O 4 react with paper. So their filtration is done by covering Gooch crucible with asbestos [CaMg 3 (SiO 3 ) 4 ]. 3 (Gooch Crucible Fitted on Filtring Apparatus)

4 ii. Sintered Glass Crucible: It is made of glass. Its bottom is sealed with sintered (porous) glass disc. It needs no preparation. All solutions can be (Sintered Glass Crucible) filtered by it. ii. Crystallization The process of crystals formation is called crystallization. When a hot saturated solution of a substance is cooled, then crystals of solid substance separate out. It is called crystallization. This process is used for purification of solid compounds. E.g. Purification of naphthalene. The process of crystallization involves the following steps. i. Choice of Solvent: A suitable solvent is chosen by hit and trial method. An ideal solvent has following properties i. It should dissolve maximum amount of solute. ii. It should not react with solute. iii. It should not dissolve the impurities. iv. On cooling, it should form pure crystals v. It should be cheap (in-expensive) vi. It should be easily removable. vii. Its use should be safe viii. An inflammable solvent should be heated on water bath. The most common solvents are water, rectified spirit(95% alcohol), absolute alcohol(99% alcohol), ether, chloroform, acetone, acetic acid, carbon tetra chloride. ii. Preparation of Saturated Solution: The substance is dissolved in minimum amount of solvent. To dissolve more and more solute, the solution should be heated directly or on water bath. The stirring of solution is necessary. 4

5 Precaution:There are certain solutions which are volatile and these should be heated with the help of water bath. Diagram iii. Filtration: The hot saturated solution is filtered to remove insoluble impurities. Sometimes premature crystals can form. It can be prevented by using hot water funnel. Diagram.. iv. Cooling: The hot filtered solution is cooled. The moderate cooling gives the medium sized crystals and slow cooling gives bigger crystals. Therefore slow cooling is avoided because it contains solvent as an impurity. Sometimes a few crystals of pure solid are added in solution to start the crystallization process. v. Collecting the Crystals: When the crystallization is complete, then mixture of crystals and mother liquor is filtered through Gooch crucible using a vacuum pump. The mother liquor is removed completely by full suction. Then the filter cake is pressed to remove the rest mother liquor. Finally the crystals are washed with small amount of cold solvent. This process is repeated many times till pure crystals are formed. By evaporation of the mother liquor we can get a fresh crop of crystals. The mixture through which crystals are formed called mother liquor. 5

6 Diagram vi. Drying of Crystals: There are three methods for drying the crystals. a) Press the crystals between the folds of filter paper. Repeat this process many times but by this method crystals are crushed and fibers of filter paper can mix with them. b) Dry the crystals at 100 o C in an oven. Here solid should not melt or decompose at 100 o C. This method is also not reliable because there are many solids which have low very low m.p can not be dried. c) Spread the crystals over watch glass and keep in vacuum desiccators for many hours. The drying agents used in desiccators are CaCl 2, P 2 O 5, Silica gel. This is the most reliable method Diagram. vii. Decolourization of Undesirable Colours: Sometimes crystals become coloured due to impurities. To prevent it, the substance is boiled with animal charcoal. Animal charcoal absorbs the coloured impurity. The pure colourless crystals are formed by cooling the filtrate. iii. Sublimation The process in which a solid, on heating directly changes into vapours without passing through liquid state is called sublimation. The hot vapours are then condensed to form the solid again. It is called sublimate. The process of sublimation is used to purify 6

7 the solids. E.g. Naphthalene, NH 4 Cl, Iodine and benzoic acid etc. the solid substance is taken in a watch glass. A funnel is inverted on it. The solid is heated slowly over a sand bath. The funnel is cooled by wet cotton. The pure solid deposits on the inner sides of funnel and impurities are left behind in watch glass. Limitation: Those substances can be sublimed which have high v.p than their m.p iv. Solvent Extraction We can separate a solute from a solution by shaking it with an immiscible solvent in which solute is more soluble. It is called solvent extraction. It is mainly used to separate organic compounds from water. It is done by an apparatus called. i. Separating Funnel: It is shown in figure. The example of solvent extraction is ether extraction. In this method an aqueous solution containing an organic compound is taken in separating funnel. The ether is also put into the funnel. It is stoppered, shaken well and allowed to stand. After sometime two layers are formed. The organic compound goes to ether layer and 7

8 Inorganic impurities remain in water layer. The ether layer is separated. It is evaporated on water bath to get organic compound. The repeated extraction using small portions of solvent (ether) are more efficient than a single extraction using large volume of solvent. The solvent extraction is an equilibrium process. So it is based upon Distribution law or Partition law. This law states that a solute distributes itself between two immiscible liquids in a constant ratio of concentration irrespective of its added amount. The ratio of concentration of a solute dissolved in two immiscible solvents is a constant. It is called distribution coefficient. It is given as For example distribution of I 2 in water and Carbon tetrachloride. The I 2 is not very soluble in water but readily soluble in aqueous KI. In water I 2 is soluble in the form of tri Iodide ion ( ). When ccl 4 is added to aqueous solution of ions, then equilibrium takes place. When two solvents are gently shaken, then their area of contact increases and transfer of I 2 molecules improves. Thus a good separation is achieved. The result is that brown colour of in aqueous layer fades and purple colour of free I 2 in CCl 4 layer appears. When equilibrium is established, then rate at which iodine passes from CCl 4 to water equals the rate at which it passes from water to CCl 4. The distribution coefficient is given as k = [I2(CCL4)] [I3(water)] v. Chromatography The word chromatography is derived from Greek word Khromatos which means colour writing. A technique by which we can separate components of a mixture due to their different distribution between stationary and mobile phase is called chromatography. The 8

9 phase over which mobile phase flows is called stationary phase. It may be a solid or liquid. The phase which flows over the stationary phase is called mobile phase. It may be a liquid or gas. Principle: The principle of chromatography depends upon two points. i. Relative affinity (solubility) of components in two phases. ii. Distribution coefficient of components in two phases. The distribution coefficient K is given as A component with smaller value of K remains in stationary phase. A component with grater value of K remains in mobile phase. Types of Chromatography The chromatography has two types (i) Adsorption Chromatography (ii) Partition Chromatography i. Adsorption Chromatography: The Chromatography in which solid substance is used as stationary phase is called adsorption chromatography. e.g. Column Chromatography. ii. Partition Chromatography: The Chromatography in which a liquid substance is used as stationary phase is called partition chromatography. e.g. paper chromatography. Paper Chromatography It is a kind of partition chromatography. Here stationary phase is liquid (H 2 O) adsorbed on paper. The mobile phase is an organic liquid which passes over the paper. The solutes are distributed between two liquids. There are three kinds of paper chromatography. i. Ascending paper chromatography ii. Descending paper chromatography 9

10 iii. Radial / Circular paper chromatography Here will explain only ascending type. Ascending Paper Chromatography The chromatography in which solvent front or mobile phase travels upwards by capillary action is called ascending paper chromatography. Method: A solvent mixture is taken in a chromatographic tank. A 20 cm long strip of whatmann s paper no.1 is taken and a pencil line is drawn on it at 2.5 cm from one end. A spot point of mixture solution is put on the line. Two other spots of pure components are also put on the line as standard. Now spots are dried and paper is suspended into the tank such that its lower end dips 5-6 mm into the solvent. The glass cover is put on the tank. The solvent passes the spots. The solutes begin to move upward according to their distribution coefficients. When the solvent has covered 10 length of paper, then strip is removed, position of solvent is marked and paper is dried. The pattern on the is called chromatogram. If spots are coloured, then they can be easily identified. If spots are colourless, the chromatogram is developed by physical or chemical

11 methods. The retardation factor (R f ) of each component is calculated as Each component has its specific R f value. For example the shown chromatogram tells that mixture solution (A) has two components (B, C). Their R f values are given as Rf(B)= x/y (x is distance covered by component B, Z is distance covered by component C and y is distance covered by solvent) Rf(C)= z/y Think! Why R f value can never be greater than 1 Uses of Chromatography: i. The chromatography is useful in separation of organic compounds. ii. It is used for identification of compounds. iii. It is useful in qualitative and quantitative analysis. iv. It is used in checking purity of a compound. 11