CHEM 254 EXPERIMENT 7. Phase Diagrams - Liquid Vapour Equilibrium for two component solutions

Transcription

1 pressure CHEM 254 EXPERIMENT 7 Phase Diagrams - Liquid Vapour Equilibrium for two component solutions The partial pressures of the components of an ideal solution of two volatile liquids are related to the composition of the liquid by Raoult s Law 0 and where P A and P 0 B are the vapour pressures of pure A and B respectively and x A and x B are the mole fractions of A and B in the liquid phase respectively The total vapour pressure of the mixture is (1) The partial pressures of each component can also be calculated using Dalton s Law which gives the relation between the mole fractions in the vapour phase (y A and y B ) and the partial pressures. and (2) ( ) (3) The dependence of total vapour pressure of an ideal solution on the mole fraction of A in the entire system is shown in Figure 1. p A * liquid a b p B * Liquid +vapour x A vapour mole fraction of A Figure 1. Pressure versus composition diagram for a two component ideal solution The composition of solutions can be determined by various techniques. One of these techniques is refractormetry. A refractometer device is for the measurement of refractive index. The refractive index, n, of a medium is defined as the ratio of the velocity of a wave in a vacuum to the phase velocity, v p in the medium itself: n = c / v p y A

2 Figure 2. Scheme for Abbe Refractometry. When the interface between the two mediums of different light speed (u) is flat, the angles of incidence ( i, in medium 1) and refracted ( r, in medium 2), angle between the ray and a surface normal are related through Snell's law: sin i / sin r =n 2 /n 1 Surface angle of angle of incidence reflection incident ray i Interfa air Sample medium angle of refraction r n n refracted ray Figure 3. Refraction and Reflection Purpose: In this experiment liquid - vapor equilibrium of a two component system will be studied.

3 Refractive Index Apparatus and Chemicals Apparatus: Refractometer, boiling point apparatus with electrical resistance and condenser, variac, labeled stoppered test tubes, thermometer, graduated cylinder, soft absorbent paper Chemicals: Benzene, acetone. Procedure 1. Place 15 ml of pure benzene into the distillation flask and set up the apparatus. 2. Open the condenser. Heat the sample very slowly. Adjust the transformer so that liquid boils vigorously at a constant rate (3-4 mv AC). Continue to the boiling with the same rate until the thermometer reading becomes constant. 3. Record the boiling point (Report Sheet-Table 1). 4. Turn off the heater. After waiting for about one minute take about 1 ml of samples from the distillate and the residue. 5. Transfer the samples into labeled test tubes for refractive index measurements. 6. Measure refractive index of both solutions. Caution: Do not touch the surface of the prism of Abbe refractrometer with any glass or metal object. Do not scratch the prism surface. Clean the surface of the prism using soft absorbent paper to keep it always free from dust and dirt. 7. Repeat the steps 2-6 after successive additions of 1.0, 3.0, 5.0 and 7.0 ml of acetone to the residue in the distillation flask through the sampling port. 8. Repeat the experiment using acetone instead of benzene and benzene instead of acetone. Treatment of data 1. a. Using the calibration curve given in Figure 2, find mole fractions of acetone and benzene in the residue and distillate 1,44 1,43 1,42 1,41 1,4 1,39 1,38 1,37 1,36 1,35 y = -0,1794x + 1,5341 R² = 0,9979 0,5 0,6 0,7 0,8 0,9 1 1,1 Mole Fraction of Acetone Figure 4. Calibration curve. Variation of refractive index as a function of mole fraction of acetone in acetone-benzene mixtures.

4 b. Plot temperature versus composition graphs for both distillate and residue on the same graph. 2.a. Calculate theoretical vapour pressures using the following equations (Report Sheet, Table 2). log p A 0 = [ 1161 / (224 + T( C)) ] for acetone log p B 0 = [ 1211 / ( T( C)) ] for benzene b. Calculate mole fractions in the liquid and vapor phases using Raoult s Law. c. Plot temperature versus composition graphs for both distillate and residue on the same graph. QUESTIONS 1. Define Raoult s Law and Dalton s Law. 2. What is the purpose of using Refractive index in this experiment? 3. What is the reason of seperating residue and distillate in this experiment?

5 DATA SHEET Group Number: Student Name: Date: Experiment 6- Liquid Vapour Equilibrium for Two Component Solutions Assistant name and signature: Table1. Experimental data and mole fractions determined from the calibration curve T( C) n R n D x A (D) x A (R) p A 0 p B 0 15 ml benzene + 1 ml acetone + 3 ml acetone + 5 ml acetone + 7 ml acetone 15 ml acetone + 1 ml benzene + 3 ml benzene + 5 ml benzene + 7 ml benzene 1.b. Plot temperature-composition graph. 2.a. Calculate theoretical vapour pressures at each temperature and fill Table 1 log p A 0 = [ 1161 / (224 + T( C)) ] for acetone log p B 0 = [ 1211 / ( T( C)) ] for benzene 2.b. Calculate mole fractions in the liquid and vapour phases using Raoult s law and fill the following Table 2.

6 Table 2. Theoretical mole fractions T( C) p A p B x A y A 15 ml benzene + 1 ml acetone + 3 ml acetone + 5 ml acetone + 7 ml acetone 15 ml acetone + 1 ml benzene + 3 ml benzene + 5 ml benzene + 7 ml benzene 2.c. Plot temperature versus composition diagram.