2022-330-EIII+ans

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Chemistry 33000 Physical Chemistry I: Thermodynamics Fall, 2022 CUNYFirst Class 23729 WITH ANSWERS Section L-25-LEC Regular Exam III: Thermodynmics (T.D.) of Phase Equilibria and Liquid-Liquid Solutions The attached exam consists of five questions, most of which include several independent parts that require verbal explanations and/or numerical calculations. The parts within a problem can often be answered most readily in the sequence they are asked, but if you miss one section it is usually possible to complete the next . Try to budget your time according to the number of points allotted to each question ; expect a class average of ~60 and a curve to estimate the corresponding letter grades. Please write legibly ON THE EXAM SHEET and with sufficient coherence that I may infer your reasoning in those instances where your numerical result is incorrect. Note any assumptions or approximations that you make. If you need to clarify how to interpret a question, ask me! Before you begin to sweat and scribble, write your name on the exam booklet. Hand in the exam questions with your responses and your formula sheet when you have completed your work. Here are some physical constants and relationships that you may find useful. Good luck! R = 8.31 J-mol -1 -K -1 = 0.082 L-atm-mol -1 -K -1 = 0.083 L-bar-mol -1 -K -1 = 1.99 cal-mol -1 -K -1 J = (Pa - m 3 ) / (mol - K) J = (kg-m 2 )/s 2 1 J = 1 N-m g = 9.8 m/s 2 Standard Temperature & Pressure (STP) = 273 K, 1 atm for gases STP (º) = 298 K, 1 bar for chemical formation reactions and phase changes 1 atm = 760 torr = 1 x 10 5 Pa 1 bar 100 cm = 10 dm = 1 m T (K) = t ( 0 C) + 273 1 calorie = 4.18 J U = q + w dU = TdS - pdV dG = -SdT +Vdp dX = 0 for any state function X C p - C v = nR for ideal gases K f for water = 1.86 K-kg-mol -1 dq P = nC P dT dq V = n C V dT K f for naphthalene = 6.94 K-kg-mol -1 Atomic masses: H=1; C=12; O=16; Na=23; Cl=35; Ca=40 2022-EIII-ans 11/27/2022
2022-EIII-ans 12/12/2023 2
This page has been intentionally left blank. Be encouraged to tear off the cover page and refer to it as you work the problems. Also be encouraged to use this page as scratch paper. 1. 2022-EIII-ans 12/12/2023 3
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Pure Substances: Phase Diagrams for Compact T.D. Information (29 points, 4 parts) Plots of pressure vs. temperature, pressure vs. composition, and temperature vs. composition provide a compact but information-rich picture of the phases and chemical mixtures that are present under laboratory conditions of interest. Consider the following data for oxygen (O 2 ), which is familiar to us as a life-supporting gas but can also be present in liquid or solid phases under appropriate circumstances. Consider the following properties: a: triple point: 54.3 K, 1.14 torr b: critical point: 154.6 K and 37,828 torr c: normal melting point: 54.8 K, 760 torr d: normal boiling point: 90.3 K, 760 torr a: triple point: 54.3 K, ln p = ln (1.14 torr) = 0.13 torr b: critical point: 154.6 K, ln p = ln (37,828 torr) = 10.5 torr c: normal melting point: 54.8 K, ln p = ln (760) = 6.6 torr d: normal boiling point: 90.3 K, ln p = ln (760) = 6.6 torr (or mark directly on the diagram) (3 pts) 1b. If ln p = 7.6 torr and T = 60 K, which phase(s) are present ? Explain and mark the phase diagram. e: liquid only in this region (9 pts) 1c. Consider how pressure depends on temperature for liquid and gas phases of O 2 that are in equilibrium with one another. First, answer this question qualitatively by quoting a relationship that describes this dependence and explaining why it has a positive or negative value. Then, make a quantitative estimate by using the given data for points a-d on the O 2 phase diagram. Do a numerical calculation and explain briefly . Qualitatively , dp/dT is positive since S and V are each positive. S is positive since the liquid-gas conversion goes toward a state of greater disorder. V is positive since gases spread out to occupy a larger molar volume than liquids (or solids). Quantitatively , use the data to calculate dp/dT: p ( d ) p ( a ) T ( d ) T ( a ) = ( 760 1.14 ) ( 90.3 54.3 ) = 758.86 36 = + 21.08 Alternatively, use the data to calculate p ( b ) p ( d ) T ( b ) T ( d ) or p ( b ) p ( a ) T ( b ) T ( a ) , also > 0 2022-EIII-ans 12/12/2023 (12 pts) 1a. These data are sufficient to construct the phase diagram shown on the right. (Note: the y-axis shows ln p in order to accommodate the large range of values that are relevant for the three phases of diatomic oxygen.) Use the data to put a, b, c, and d labels on the marked locations of the diagram, including the numerical values that support your choices. 4
(5 pts) 1d. Which values of pressure will ensure that O 2 (s) can sublime to O 2 (g)? Explain or make a sketch to justify your answer. ln p must be below the triple point value of 1.15 torr, as illustrated by the arrow on the graph. 2022-EIII-ans 12/12/2023 5
2. Pure Substances: Condensed Phases in Equilibrium with Gases (16 points, 2 parts) Prior to the discovery that freon-12 (CF 2 Cl 2 ) was harmful to the Earth’s ozone layer, this compound was frequently used as the dispersing agent in cans of hair spray (as well as in refrigerators, air conditioners, and fire extinguishers). Its enthalpy of vaporization at its normal boiling point of 244.0 K is 20.25 kJ mol -1 . (4 pts) 2a. Predict the signs of vap H and vap S for freon, rationalizing your choices with a brief explanation for each quantity you designate. A transformation from liquid to gas always requires input of heat energy, so vap H is endothermic and the value is > 0. A transformation from liquid to gas always increases disorder, so vap S is > 0. (12 points) 2b. Estimate the pressure that a can of hair spray that used freon-12 had to withstand at 313 K (40 ° C), the estimated temperature of a can that has been standing in the sunlight. You may assume that vap H is constant over the described temperature range and equal to its standard value of 20.25 kJ - mol -1 . [adapted from assigned Problem 4.2b. with apologies for the unavoidable blurriness] 2022-EIII-ans 12/12/2023 6
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3. Thermodynamics of Chemical Solutions (31 points, 4 parts) (6 pts) 3a. A liquid mixture of toluene (C 7 H 8 , T) and benzene (C 6 H 6 , B) has a toluene mole fraction of 0.336. At 30 ° C, P* (toluene) = 36.7 torr and P* (benzene) = 118.2 torr as shown on the graph below. Assuming that the mixture behaves ideally, find the total pressure and the partial vapor pressure of each component above the solution at this temperature. (Make a calculation or use the graph.) P B = X B P B * = (0.664) (118.2 torr) = 78.5 torr P T = X T P T * = (0.336) (36.7 torr) =12.3 torr Thus P TOT = P B + P T = 90.8 torr OR: add the values of P B and P T that you can read off the graph for the given mole fraction X B = 0.664. (7 pts) 3b. A different mixture has a total vapor pressure of 100 torr. What is its solution composition ? P TOT = 100 torr = X B p B * + X t p T * = X B p B * + (1- X B ) p T * = 118.2 X B + 36.7 – 36.7 X B 63.3 = 81.5 X B 0.78 = X B and thus X T = 1- X B = 0.22 2022-EIII-ans 12/12/2023 7
(6 pts) 3c. A third mixture of toluene and benzene has a vapor that is equimolar in the two components. What are the solution composition and total vapor pressure for this mixture ? (The graph of Problem 3a is replicated for your viewing convenience.) If the vapor has y B = y T = 0.5, then p i = y i p TOT has the same value for benzene and toluene. Thus p B = p T where the lines cross and can be read from the graph as p i 29 torr (each tick mark on the y-axis is 40 torr). The corresponding solution composition read from the graph is X B 0.23 (each tick mark on the x-axis is 0.25). The total vapor pressure is then 2 p B or 2 p T = 58 torr. Solution compositions (X i ) relate to P i *, whereas vapor compositions (y i ) relate to P TOT ! (Analytical methods of solution are also possible.) (12 pts) 3d. Finally, return to the mixture of benzene and toluene that is described in Problem 3a, for which the mole fraction of toluene is 0.336. Again assuming ideal behavior, calculate G, H, and S (for the system), when 1 mole of each liquid is mixed at 30 ° C. Do an analogous calculation for benzene and toluene in the gas phase , noting any similarities and differences . mix G = nRT ( X b ln X b + X t ln X t ) = (2) (8.314 J/mol-K) (303 K) [(.664) ln (.664) + (.336) ( ln (.336)] = 5038 [ - 0.272 - 0.366 ] (6 pts) = -3217 J/mol mix S = - mix G /T = +10.62 J / mol-K mix H = mix G + T mix S = 0 The calculation would be identical for a mixture of ideal gases with the same mole fractions, except that in practice, T would be higher unless we chose p to retain T at 303 K. 2022-EIII-ans 12/12/2023 8
4. Colligative Properties (10 points) The addition of 5.00 g of an unknown solid compound to 250 g of liquid naphthalene lowered the freezing point of this solvent by 0.780 K. Find the molar mass of the compound . The cryoscopic constant for naphthalene is 6.94 K-kg-mol -1 . [Adapted from assigned Ex. 5.6b] ∆T (T f * - T f ) = K f m B (or K f b B per your textbook), where K f = the cryoscopic constant for the solvent, m B or b B = molality moles of the solute per kg of solvent m B = ∆T K ( f ) = 0.780 K 6.94 K kg mol 1 = 0.112 mol B kgnaphthalenesolvent Since 5.00 g of the unknown compound is dissolved in 250 g naphthalene, this value corresponds to 20.00 g per 1.000 kg of that solvent. Then considering 1 kg naphthalene for a basis in each case, 20.00 g 0.112 mol = 178 g/mol for the molecular weight of the unknown compound. Watch your units, definition of molality (m B or b B ), and distinctions between solute and solvent! 2022-EIII-ans 12/12/2023 9
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5. Phase Equilibria: the case of a liquid ( l ) with a gas (g) ( 14 points ) Consider the derivation of the Clapeyron equation that was shown in class and in your text. Provide a brief verbal statement and/or equation to justify each step . (The extensive variables (G, S, V) per mole are related as shown, but the bars above the symbols or ‘m’ subscripts have been omitted for simplicity.) dG g = - S g dT + V g dp combined 1 st & 2nd Laws of T.D. for gas ; G = U -TS + pV to transform variables from (S,V) to (T, p) dG l = - S l dT + V l dp combined First & Second Laws of T.D. for liquid ; G = U -TS + pV to transform variables from (S,V) to (T, p) - S g dT + V g dp = - S l dT + V l dp gas and liquid are in equilibrium, so dG g = dG l or μ ( g ) = μ ( l ) dp (V g – V l ) = dT (S g – S l ) separate (sequester) variables (p & T) dp dT = [ S ( g ) S ( l ) ] [ V ( g ) V ( l ) ] formulate slope that signifies P vs. T dependence = ∆ S ∆V for l g define ∆ X as X f – X i for a T.D. process e.g., ∆ S as S g – S g for l g 2022-EIII-ans 12/12/2023 10
= ∆ H T ∆V ∆ S≡ dqrev T = dH T = ∆ H T for a phase transition at constant T 2022-EIII-ans 12/12/2023 11