Introduction: Any system in which two substances that are liquid at normal temperature are mixed is called a two-liquid system. If the two solutions can be dissolved in any proportion, it is called a completely miscible two-liquid system; if it can only be dissolved in a certain proportion range, it is called a partially miscible two-liquid system. The cyclohexane - ethanol binary system is a completely miscible two-liquid system. 1. The purpose of the request (1) Draw a gas - liquid equilibrium phase diagram of the cyclohexane - ethanol two-liquid system at P 0 to understand the basic concepts of phase diagram and phase law. (2) Master the method for determining the boiling point of two-component liquids.  (3) Master the method of determining the binary liquid composition by the refractive index.  2. The basic principles  Any system in which two substances which are liquid at normal temperature are mixed is called a two-liquid system. If the two solutions can be dissolved in any proportion, it is called a completely miscible two-liquid system; if it can only be dissolved in a certain proportion range, it is called a partially miscible two-liquid system. The cyclohexane - ethanol binary system is a completely miscible two-liquid system.  The gas phase composition and liquid phase composition at the time of liquid distillation are not the same. The boiling point of the two-liquid system is usually plotted by geometrical mapping for the composition of the gas phase and the liquid phase. The resulting pattern is called the boiling point (T) composition (x) of the two-liquid system , that is, the T-x diagram. It shows the relationship between the liquid phase composition at the boiling point and the gas phase composition in equilibrium therewith.   Figure II -2-1 Tx diagram of the two-liquid system  Binary Liquid T-x of FIG three ways:  (1) T-x diagram of ideal solution ( Figure II -2 (2 ) T-x diagram with the lowest constant boiling point system (Fig. II -2-1b ) and T-x diagram with the highest constant boiling point system (Fig. II -2) In this experiment, a cyclohexane - ethanol system with the lowest constant boiling point was selected . The composition of the boiling point of a series of different compositions and the composition of the gas-liquid two phases at equilibrium at the boiling point were determined at 101.325 KPa , the T-x diagram was plotted, and the temperature and composition of the constant boiling point were determined from the phase diagram.  The device for determining the boiling point is called a boiling point tester ( Fig. II -2-2) . This is a long neck round bottom flask with a reflux condenser. The bottom of the condenser has a semi-spherical chamber for collecting the condensed gas phase sample. The current is passed through a resistance wire immersed in the solution. This not only reduces the overheating of the solution when it boils , but also prevents bumping. When measuring, the thermometer mercury ball should be half under the liquid surface and half in the gas phase to accurately measure the equilibrium temperature. Solution composition analysis. Since the refractive index of cyclohexane and ethanol differs greatly, and the refractive index is determined by a small amount of sample, the refractive index - composition working curve can be used to measure the two-phase composition of the equilibrium system. Abbe (an Abbe) refractometer principle and physical specifications are detailed in the book Chemistry Experiment â…¢. FIG â…¡ -2-2 Ebulliometer  Dosing thermometer 1. 2. 3. The heating wire port  4. The sampling port was fractionated fractionated liquid  3. Instrument Reagent  Ebulliometer a  Abbe 1 Refractometer station  DC power supply units 1  Sampling tube 10  Cyclohexane ( analytical grade )  Absolute ethanol (analytical grade) Key 4. Experimental steps (1) Determination of refractive index of pure liquid : The refractive index of ethanol and cyclohexane were measured, and repeated 2 to 3 times. (2) Drawing of the working curve: According to the density of ethanol and cyclohexane at room temperature, precisely prepare the ethanol solution with the molar fraction of cyclohexane of 0.1 , 0.2 , 0.3 , 0.4 ... , 1 and close it immediately after the preparation . Determination of refractive index of each solution at room temperature Rate ( the standard solution has been prepared in this experiment, students can directly measure ) , draw a working curve.  It should be noted that the refractive index of the solution is temperature dependent. Strictly speaking, the determination of the refractive index should be carried out under constant temperature conditions.  (3) Determination of boiling point - composition data: 1 Installation ebulliometer: The dried ebulliometer FIG -2-2 â…¡ mounted as well to check whether the rubber stopper with a thermometer (heating resistive wire plugged close to the bottom center, the mercury ball of the thermometer is not in contact resistance. Silk, and each time the solution is changed, ensure that the measurement conditions are as parallel as possible ( including the relative position of the mercury thermometer and the resistance wire ) . 2 Solution preparation: roughly prepare cyclohexane - ethanol solution with a concentration of 0.05 , 0.1 , 0.2 , 0.45 , 0.55 , 0.6 , 0.7 , 0.8 , 0.9, etc., about 50 mL ( already equipped ) . 3 Boiling and balance gas - liquid phases: Remove the stopper, the solution (about 40 mL) was added to be measured, which (preferably in the middle level of the mercury bulb to connect the heating line opens condensed water, and then power. Adjust the DC regulated power supply voltage adjustment knob so that the heating voltage is 10V ~ 15V , and slowly heat. When the liquid boils, adjust the voltage control so that the liquid can condense in the condenser when it boils. Make the vapor reflow in the condenser. Height should not be too high, to The data processing  (1) List the experimental data in the following format:  Room temperature: Atmospheric pressure:  Record format 1 Table â…¡ -2-1 cyclohexane - ethanol refractive index standard solution  Cyclohexane Molar fraction 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.00 Refractive index Cyclohexane different compositions in Table â…¡ -2-2 - ethanol solution refractive index  Approximate composition of the solution Boiling point / fold Light rate group to make gas phase liquid phase Gas phase Liquid phase 1 2 3 average 1 2 3 average (2) Draw the working curve, that is , the relationship between the refractive index and the composition of the cyclohexane - ethanol standard solution. (3) Determine the equilibrium composition of the gas phase and the liquid phase of each solution to be tested according to the working curve, and fill in the table. The composition is the horizontal axis and the boiling point is the vertical axis, and the equilibrium curve between the gas phase and the liquid phase is drawn, that is, the two-liquid phase diagram.  (4) Determine the temperature and composition of the lowest constant boiling point from the graph.  (5) Literature value:  1 The refractive index - composition relationship of the cyclohexane - ethanol system .  Table II -2-3 X- ethanol oxime X- cyclohexane  n 25 D î€ 1.00 0.0 1.35935 0.8992 0.1008 1.36867 0.7948 0.2052 1.37766 0.7089 0.2911 1.38412 0.5941 0.4059 1.39216 0.4983 0.5017 1.39836 0.4016 0.5984 1.40342 0.2987 0.7013 1.40890 0.2050 0.7950 1.41356 0.1030 0.8970 1.41855 0.00 1.00 1.42338 2 Constant boiling point data at standard pressure. Table -2-4 cyclohexane at normal pressure â…¡ - ethanol system phase diagram data constant boiling Boiling point / °C Ethanol mass fraction X cyclohexane 64.9 40 1 64.8 29.2 0.570 64.8 31.4 0.545 64.9 30.5 0.555 6. Notes  (1) Since a part of the thermometer is exposed to the container, the temperature of this part is lower than the temperature of the system to be tested, so it is necessary to correct the stem thermometer for the mercury thermometer. The calibration method can be found in the book III Physical Chemistry Experiment Specification.  (2) The boiling point measured at P 0 is the normal boiling point. Typically ambient pressure is not exactly equal to 101.325 kPa, and should therefore be, the experimental value for the measured pressure correction. The correction formula is derived from the Trouton rule and the Clausius Crablon equation. â–³ t pressure / °C = (273.15 + t A / °C ) / 10. (101325-p / Pa) / 101325 formula in: â–³ t pressure ——- the error due to the pressure not equal to 101.325 kPa ; t A - the experimentally measured boiling point; p - atmospheric pressure under experimental conditions.  (3) The normal boiling point of the corrected system should be  Boiling t = t A + â–³ t + â–³ t pressure dew  7. Thinking questions  The concentration of the test solution of (1) the need for accurate measurement? Why?  (2) This experiment does not measure the boiling point of pure cyclohexane and pure ethanol, but directly uses the data under P 0. What error will this bring ? (3) If the boiling point of pure cyclohexane and pure ethanol is to be measured, the distillation bottle must be washed and dried. When the boiling point and composition of the mixture are measured, the distillation bottle is not washed or baked. Why ? 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