CHEM 254 EXPERIMENT 8. Phase Diagrams, Solid - Liquid Phase Equilibrium for Two Component System and Missibility Gap

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1 Temperature, T Temperature, T CHEM 254 EXPERIMENT 8 Phase Diagrams, Solid - Liquid Phase Equilibrium for Two Component System and Missibility Gap Solid and liquid phases may both be present in a system at temperatures below the boiling point. Consider the temperature composition phase diagram for two almost immiscible solids and their completely miscible liquids shown in Figure 1. a. b. liquid a 1 a 2 a 1 a 2 liquid cooling b3 a 3 a 3 B precipitating liquid + A(s) e liquid + B(s) a 4 a 4 eutectic freezing Solid A and B a 5 a5 Solid cooling 0.0 mole fraction of B, x B 1.0 time Figure 1. a. The temperature composition phase diagram b. Cooling curve During cooling of a sample w/composition (a 1 to a 5 on Fig. 1a) various phase changes are observed as temperature decreases the foloowing proceses do occur. a 1 a 2 : system enters the two phase region liquid (A and B) and solid B. Here B solidifies, and the liquid is richer in A. a 2 a 3 : more B solidifies. The relative amounts the solid and liquid can be determined by lever rule. The liquid phase is richer in A and mole fraction of B in the liquid phase is b 3. a 3 a 4 The composition of the liquid is given by e. This liquid freezes to give two phase system of pure A and pure B. e is called the eutectic composition. A liquid with the eutectic composition freezes at a single temperature. A solid with the eutectic composition melts without a change of composition at the lowest temperature of any mixture. The corresponding cooling curve starting at point a 1 down to a 5 is shown in Figure 1.b.

2 Missibility gap: A mixture of liquids is a homogeneous distribution of two or more substances, whereby all components have a definite vapour pressure. Two liquids may either be completely miscible or only partly miscible. When the van der Waal forces between the two components are smaller than those between molecules of the same type, then an increase in the vapour pressure results. The molecules can leave their arrangement more easily than with equally large attractive forces. With sufficiently high deviation from Raoult s law: where The components of a binary system no longer continuously mix, but instead have the tendency to again unmix. A miscibility gap can be observed. This is a range of concentrations in which the two liquids form two phases. The molar mixing enthalpy is positive. Unmixing means in this case the transition to a lower energy condition. Systems with limited miscibility can be presented as isobars in temperature / mass content and temperature / quantity diagrams. In these separation curves, the compositions of the two coexisting liquid phases, which form from the homogeneous mixture when a certain temperature has been reached, are plotted as functions of temperature. The coexisting liquid phases are described as conjugated solutions. They are saturated solutions of the one component in the other. The line connecting the coexisting liquids is designated as the tie line. Normally the mutual solubility of liquid components increases with increasing temperature. The coexisting solutions are identical at a critical dissolving temperature. Above the critical dissolving temperature the components are miscible with one another in any ratio. The compositions of the coexisting solutions at certain temperature are constant and independent of the mass ratios or the two components. Lever Rule: If the mass content w is used as the concentration variable the mass ratio of the two liquid phases can be determined using the socalled rationality law. This states that the masses of phases a and b are inversely proportional to the distance of their composition from the composition of the original mixture c, from which it follows that The relationship is shown in Fig.2 Fig.2 Solubility Diagram of Phenol Water Mixture

3 Purpose: In this experiment, heterogenous equilibrium between solid and liquid phases of a two component system will be investigated. The method involves the measurement of temperature for solutions of two components with definite proportions (thermal analysis). A number of different mixtures of phenol and water are prepared and heated until complete miscibility is achieved. As the mixtures cool, two-phase systems form at certain temperatures which are recognisable by the appearance of turbidity. Plotting separation temperatures against compositions of the mixtures gives the separation curve Figure 3. Experimental set up Apparatus and Chemicals Apparatus: Test tubes (20 ml capacity), thermometer or thermocouple, rubber stoppers, wire stirrer, constant temperature bath, stop watch. Experimental set-up is shown in Figure 3. Chemicals: Naphtalene (N) and diphenyl amine (DPA), water, phenol. Procedure I. Naphtalene + diphenyl amine 1. Prepare a hot water bath. 2. Place 5.0 g of naphthalene into a test tube. 3. Place the tube into the hot water bath and let it melt completely. 4. Take out the test tube from the hot water bath.

4 5. Place thermocouple into the test tube. Record temperature at each 10 seconds until the 6th measurement below freezing ( Report Sheet Table ). 6. Add 0.50 g diphenyl amine, DPA, into the test tube and repeat steps Add 1.0 g DPA into the test tube and repeat steps 3-5 (total amount of DPA is 1.5 g). 8. Add 1.0 g DPA into the test tube and repeat steps 3-5. (total amount of DPA is 2.5 g). II. Diphenyl amine + naphtalene 1. Repeat procedure given in Part I using diphenyl amine instead of naphthalene and naphthalene instead of diphenyl amine. record Record temperature at each 30 seconds until the 6th measurement below freezing (Report Sheet Table ). III. Missibility gap: mixture Weigh the respective phenol portions into appropriately numbered test tubes 2. Seal the test tubes with rubber stoppers and heat them in a temperature controlled bath to 90 C. 3. During heating remove the rubber stoppersfrom time to time to release excess pressure and shake the mixtures. 4. When clear solutions have formed in all test tubes remove the tubes from the temperature bath and start the cooling function. 5. Record the temperatures at which the turbidity caused by separation becomes visible. Caution: DON T INHALE PHENOL

5 Treatment of Data 1. Calculate the mole fraction of naphthalene for all mixtures. 2. Plot cooling curve according to the data obtained for all compositions. Find the freezing points. 3. Compare the experimental freezing points of pure naphthalene and pure diphenyl amine with the theoretical values and discuss the possible sources of errors at the discussion part. 4. According to the experimental freezing point determined and mole fractions, plot the phase diagram. 5. Find eutectic point and indicate the temperature and composition it found. 6. Calculate for all mixtures 7. Draw solubility phase diagram (misibility gap) for phenol and water mixture by plotting separation temperatures against the composition of the mixtures as weight percentage 8. Find upper critical temperature. Questions 1. Discuss the meaning of eutectic point. 2. Explain the meaning of upper critical temperature and discuss the behaviour of the solution above and below this temperature.

6 DATA SHEET Experiment 9. Phase Diagrams Solid - Liquid Phase Equilibrium for Two Component System Group Number: Date: Assistant name and signature: Fill the following tables. Pure naphthalene N g DPA N g DPA N g DPA Time(s) T( C) Time(s) T( C) Time(s) T( C) Time(s) T( C)

7 Pure DPA DPA g N DPA g N DPA g N Time(s) T( C) Time(s) T( C) Time(s) T( C) Time(s) T( C) Calculate mole fractions. 2. Fill the following table using experimental data. Pure naphthalene X naphthalene Freezing point N g DPA N g DPA N g DPA DPA g N DPA g N DPA g N Pure DPA

8 3. Plot cooling curve. 4. Draw phase diagram. Missibility gap: 1. Fill the following table by using experimental values: Phenol- water mixture T( o C) Plot solubility diagram of water-phenol mixture