Experiment 2 Phase Diagram of a Binary Liquid-Vapor System 1. Objective 1.1. To draw the phase diagram of cyclohexane-et

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Experiment 2 Phase Diagram Of A Binary Liquid Vapor System 1 Objective 1 1 To Draw The Phase Diagram Of Cyclohexane Et 1 (69.37 KiB) Viewed 10 times

Experiment 2 Phase Diagram Of A Binary Liquid Vapor System 1 Objective 1 1 To Draw The Phase Diagram Of Cyclohexane Et 2 (83.52 KiB) Viewed 10 times

Experiment 2 Phase Diagram Of A Binary Liquid Vapor System 1 Objective 1 1 To Draw The Phase Diagram Of Cyclohexane Et 3 (44.76 KiB) Viewed 10 times

Experiment 2 Phase Diagram Of A Binary Liquid Vapor System 1 Objective 1 1 To Draw The Phase Diagram Of Cyclohexane Et 4 (17.11 KiB) Viewed 10 times

Experiment 2 Phase Diagram of a Binary Liquid-Vapor System 1. Objective 1.1. To draw the phase diagram of cyclohexane-ethanol binary liquid-vapor system and to learn the basic concept of phase diagram and the phase rule. 1.2. To learn the method of determining normal boiling point and the boiling point of a binary liquid system. 1.3. To learn the method of determining the composition of binary liquid systems by refractometry. 2. Introduction A two-component liquid system is composed of a mixture of two kinds of liquids. If the two components are miscible in all proportions, it is called a totally miscible liquid- liquid system. At a given pressure, the boiling point of a two-component liquid depends not only on the ambient pressure but also on the composition of the liquid. According to the phase rule, F-C-P+2 where C is the number of components; P is the number of phase; F is the number of degrees of freedom. 2 represents the temperature and the pressure of system. For a liquid of two components, when the gas phase and liquid phase is in equilibrium, P = 2, F=1, the equilibrium temperature of system is not a boiling point but a boiling range, which is related with the composition of liquid, I' is the function of the x proportion of the liquid. For a totally miscible binary liquid system at given pressure, the proportion of gas is different with liquid because the vapor pressure of two liquids are different. Measure the equilibrium temperature of a given concentration solution, and determine the proportions of the liquid phase and gas phase respectively, thus one set of experimental data of T-x is obtained. To measure another concentration solution, next set could be gained. Finally, a liquid-vapor phase diagram could be drawn according to these sets of data. For a totally miscible binary liquid system, there are three kinds of the plot of T-x as shown in the Fig. 2-1a, Fig. 2-1b, and Fig. 2-1c. ata a 120 b C 66 P-53.33 AP SPA 35 2006 64) P-101.325 kP Fig. 2-1. The plot of 7-x of binary liquid-vapor system. The Fig. 2-1a is the liquid-vapor phase diagram of a binary liquid-vapor system satisfying Raoult's law. The lower curve, indicating the relationship of the liquid-vapor equilibrium temperature and liquid composition is liquid line; the upper curve,
indicating the relationship of the liquid-vapor equilibrium temperature and vapor composition is vapor line. If at a given temperature the vapor pressure of a solution is higher than that predicted by Raoult's law, the system would show a positive deviation from Raoult's law. In many cases the deviations are large enough to result in minima or maxima in the T-x curve, as shown in Fig. 2-1b and Fig. 2-1c. These extrema are called azeotropic points, at which the composition of the liquid and the vapor are the same. At ambient pressure, the mixture of cyclohexane and ethanol was heated in a special ebulliometer. When the temperature is maintained constant, record the temperature and measure the compositions of liquid phase and gas phase respectively, then a set data of T-x is obtained. Measure all the solutions to get a series of T-x data. From each set of data, we get two phase points on the phase diagram: a gas point and a liquid point. Connect all gas points to plot a smooth line, and connect liquid points to plot another one. Thus an integrated binary liquid-vapor phase diagram is completed. The refractive indexes of cyclohexane and ethanol are sufficiently different from each other, the composition of a mixture of cyclohexane and ethanol could be measured by use of a standard curve of refractive index to composition. 3. Apparatus and Reagents Ebulliometer, Abbe refractometer; Long pipette and normal pipette; Cyclohexane (A. R.); Anhydrous ethanol (A. R.); Cyclohexane-ethanol standard sample (cyclohexane composition, w/w %: 5%, 15%, 30%, 45%, 55 %, 65%, 70 %, 75%, 85%, and 97%). 4. Procedure 4.1. Measuring the boiling points of cyclohexane-ethanol samples Fill the dry and clean flask of ebulliometer about one half full with the solution of cyclohexane-ethanol mixture. Then turn on the cooling water and the power. Increase the output voltage of the ebulliometer gradually to heat the solution slowly. When the mixture is boiling, make the gas condensate full reflux through slight readjustment of output voltage and cooling water flow rate. After the reading of thermometer is steady. keep this state for more 3-5 min until the whole system reaches equilibrium. During this process, the condensed liquid in the hemisphere is poured back to the flask when it is full. Record the reading of thermometers. 4.2. Sampling and measuring the liquid The power is turned off and the heating is stopped before sampling. Use a dry pipette to suck all the condensed liquid in the hemisphere via the top of the condenser and drop it directly onto the ground prism of the refractometer for a quick measurement of refractive index. Then take the liquid sample by another pipette and measure the refractive index after cleaning the prism. These two samples can be considered as the vapor and liquid phases of the system in equilibrium. When the measurement is finished, the remaining solution should be poured into the original solution bottle. 4.3. Repeat the procedures mentioned above and measure the boiling points and refractive indices of a series of cyclohexane-ethanol mixtures. If all the procedures are
correct, the solutions can be recycled and reused by other students, and the ebulliometer need not to be dried between measurements. 4.4. After accomplishing the experiment, turn off the power of ebulliometer and the cooling water. Clean Abbe refractometer prism. Read the room temperature. 5. Data Analysis 5.1. Fill out the table 1-1 with both the original data and the calculated results. Table 2-1. The boiling points and refractive indices of cyclohexane-ethanol system gas phase liquid phase mole. 25 °C) no(l. It) Mp(1. 25 °C) w% (cyclohexane) 0% 5% 15% 30% 45% 55% #ºC no(c.mt) Koca 70% 75% 85% 97% "rt: room temperature; no (25 °C)-mo(t)+4-10 (-25), the composition can be obtained from the corresponding calibration curve of refractive index against composition (Appendix Table 2-2). 5.2. Plot the T-x diagram of cyclohexane-ethanol through the measured temperature against gas and liquid equilibrium compositions, then find out the composition of the minimum constant boiling mixture and the temperature of minimum constant boiling point from the diagram. 6. Notes 6.1. The voltage transformer's output voltage should be slowly raised until the liquid boil. 6.2. Record the temperature after the reading of thermometer is steady. 6.3. The electric heater should be turned off before sampling. 6.4. The liquid and gas phase samples should be sucked by two different pipettes. 6.5. All the gas phase sample in hemisphere must be sucked. 6.6. Each time before the measurement of samples, the glass surface of refractometer prism must be blown to dry by rubber suction bulb. Be careful not to touch the prism with any hard and/or sharp object. 7. Exercises
7.1. Is there influence of the volume of hemisphere on the results of measurement? 7.2. What is the influence on the phase diagram if the atmosphere pressure is changed during the experiment? 7.3. Whether it will affect the experiment when one solution composition is slightly changed?