PUT A SQUARE BOX AROUND ALL ANSWERS

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1 UNIVERSITY OF TORONTO FACULTY OF APPLIED SCIENCE AND ENGINEERING CHE 112F PHYSICAL CHEMISTRY MID-TERM EXAM, OCTOBER :10 pm to 8:00 pm EXAMINER P.V. Yaneff 1. Answer all questions. The marks add up to 90. The marks for each question are indicated in [square brackets]. 2. The mid-term counts for 20% of your final grade. 3. University of Toronto Approved calculators only. 4. LOOK OVER THE USEFUL INFORMATION BELOW TO ASSIST IN SOLVING THE QUESTIONS BEFORE YOU BEGIN. 5. ALL WORK IS TO BE DONE IN EXAM BOOKLETS 6. PUT YOUR NAME AND STUDENT ID # ON YOUR EXAM BOOKLET AND IF USING MORE THAN ONE INDICATE THE TOTAL (EG., 1 OF 2, 2 OF 2 ON THE COVER. PUT A SQUARE BOX AROUND ALL ANSWERS Some Useful Data: SATP = 0 o C, 1 bar; STP = 0 o C, 1 atm; 1 bar = 10 5 Pa = atm; 1 atm = kpa = 760 mmhg; 1 J = 1 Pa/m 3 R = J mol -1 K -1 = L torr mol -1 K -1 = L bar mol -1 K -1 = L atm mol -1 K -1 Molar mass (g/mol): C (12.01); H (1.01); He (4.00); N (14.01); O (16.0); Cl (35.46): K (39.1); S (32.06) Acceleration due to gravity: g = m. s -2 Power is the rate of doing work; 1 Watt = 1J s -1. Density of water is 1000 kg m 3 Some Useful Formulae: P i = x i P; P= P A + P B + P C W mech = mgδz; W PV = -P ext ΔV syst ; W rev = -nrt ln(v 2 /V 1 ) vdw: [P+ a(n/v) 2 ](V-nb) = nrt; Virial: Pv/RT = 1 + B(T)/v + C(T)/v 2 ΔU= Q + W; ΔU v = Q; Q= CΔT = ncδt ; Q= mcδt, where c = specific heat capacity, c p = c v + R ΔH p = ΔU + PΔV; ΔH p = Q p = C p ΔT = nc p ΔT ; ΔH p = mc p ΔT, where c p = specific heat const P; ΔH fus = nδh fus ; Specific heat of fusion: ΔH fus = mδh fus Page 1 of 11

2 to question 3 is at the end. 1. Phosphorus trichloride (PCl3) reacts with water to form phosphorous acid (H3PO3) and hydrochloric acid: PCl3(liq) + 3H2O(liq) H3PO3(aq) + 3HCl(aq) (a) Which is the limiting reactant when 12.4 g of phosphorus trichloride is mixed with 10.0 g of water? (b) What masses of phosphorous acid and hydrochloric acid are formed? (a) PCl3 = (35.45) = g mol 1 H2O = 2(1.008) = g mol 1 H3PO3 = 3(1.008) (16.00) = g mol 1 HCl = = g mol 1 From the stoichiometry of the reaction, 3 moles of H2O are required for every mole of PCl3. No. moles of PCl3 added to mixture = No. moles of H2O added to mixture = 12.4 g g / mol 10.0 g g/ mol = mol = mol! moles of H The actual ratio of H2O to PCl3 is # 2 O$ & " moles of PCl 3 % actual = = > 3 There is too much water; i.e., there is not enough PCl5 for the amount of H2O present, so all the PCl3 will be reacted, but not all the H2O; PCl3 is the limiting reactant. PCl3 is the limiting reactant (b) From the stoichiometry of the reaction, Moles H3PO3 produced = moles of PCl3 consumed = mol Page 2 of 11

3 Use Word 6.0c or later to view Macintosh picture. Use Word 6.0c or later to view Macintosh picture. ( ) g H 3 PO 3 Therefore, mass of H3PO3 produced = mol H 3 PO 3 H3PO3 Similarly, ( ) = g mol H 3 PO 3 Moles of HCl produced = 3 times the number of moles of PCl3 reacted; therefore: ( ) 3 Mass of HCl produced = mol PCl 3 ( ) g HCl mol HCl mol PCl 3 ( mol HCl) = g HCl 7.40 g H3PO g HCl 2. A company sells oxygen- enriched air, which they claim contains at least 50% by volume pure oxygen. To test their claim, a 1.00 liter glass bulb was filled with the gas to a total pressure of 750 Torr at 20.0 C. The weight of the gas sample was found to be grams. What is the volume percent of oxygen gas in the mixture? Assume ordinary air consists of 20.0% by volume O2 and 80.0% by volume N2. Molar masses: O = 16.00, N = Molar masses: O2 = = g mol 1 N2 = = g mol 1 Method 1: Total moles n of gas in bulb: n = PV RT = ( 750 ) ! ( ) ( ) ( ) = mol Let there be x moles of O2, and ( x) moles of N2 in the sample. Total mass = mass of O2 + mass of N g = ( x mol) ( g mol) ! x mol ( mol) ( ) g Solving: x = mol Therefore, % O2 = x n !100% =!100% = 52.98% Page 3 of 11

4 53.0 % The company is telling the truth! Method 2: Let the mass of O2 in the bulb be m grams; therefore the mass of the N2 is (1.236 m) grams Total moles n of gas in bulb: n = PV RT = ( 750 ) ! ( ) ( ) ( ) = mol Therefore n O 2 + n N 2 = i.e., m g (1.236! m) g g / mol g / mol = m (1.236! m) (32.00)(28.02) = m (1.236! m) = ( )(32.00)(28.02) m = m = ! = Therefore, n O 2 = m = = mol and the % by volume O2 is n O 2 n !100% =!100% = 52.99% % 4. Calculate the total work for one cycle consisting of the isothermal expansion (A B) and compression (B A) of one mole of ideal gas at K between one atm and atm. The expansion involves lifting 5.0 kg through 22 m. The compression is accomplished by a mass of 40 kg falling through 22 m. Friction may be neglected. The acceleration due to gravity is m s 2 Page 4 of 11

5 Take the gas as the system: When the gas expands, it delivers work equal to wab = mg z = (5.0)(9.807)(22) = J When the gas is compressed, work must be done on the gas; the amount is wba = mg z = (40)(9.807)(22) = J More work is done to compress the gas than is delivered by the gas when it expands. The net work done on the gas is thus wnet = = J = kj Net work input of 7.55 kj 5. One kilogram of O2 is heated in a frictionless cylinder- piston apparatus at a constant pressure of one atm from 20 C to 30 C. Assuming ideal gas behaviour, calculate: (a) H for the process, (b) U for the process, and (c) cv for oxygen gas. The molar mass and molar heat capacity of O2 are, respectively, g mol 1 and cp = J mol 1 K g (a) No. of moles of O2: n O 2 = = mol g / mol Therefore H = ncp T = (31.25)(29.355)(30 20) = J kj (b) U = H (PV) = H (nrt) = H nr T = (31.25)(8.3145)(30 20) = = J J (c) For an ideal gas, even if the volume changes, U = ncv T Page 5 of 11

6 Therefore, cv =!U n!t = (31.25)(10) = J mol 1 K J mol 1 K 1 Alternatively, cv = cp R = = J mol 1 K 1 [as above] L of nitrogen gas at 100 kpa pressure and 300 K is located in a cylinder/piston assembly as shown. The piston weighs 100 kg and the surrounding atmosphere is at a pressure of 100 kpa. The cross- sectional area of the piston face is 100 cm 2. For the following calculations, nitrogen may be assumed to behave as an ideal gas and any friction may be neglected. The acceleration due to gravity is 9.8 m s 2. (a) The retaining pin is removed and the gas is spontaneously compressed until the piston comes to rest and the system re- attains thermal equilibrium at 300 K. How much work will have been done on the gas? P atm = 100 kpa m = 100 kg 100 kpa 300 K 4.00 L Area = 0.01 m 2 pin (b) If the gas were compressed isothermally and reversibly between the same initial and final states as in (a), how much work would have to be done on the gas? (a) The piston will compress the gas until the final pressure of the gas (P2) is the same as the external pressure (Pext ), which is the sum of the atmospheric pressure plus the force exerted by the mass of the piston: Thus P2 = Pext = Patm + Ppiston = Patm + mg A = 100! (100)(9.80) 0.01 = ! 10 5 = 1.98! 10 5 Pa Therefore: V = V2 V1 = P 1 V 1 P 2 V1 = (10 5 )(0.004) 1.98! = = m 3 Page 6 of 11

7 W = Pext V = (1.98! 10 5 )( ) = J The positive sign indicates that work must be done on the gas J (b) At constant temperature: P 1 V 1 = P 2 V 2 = nrt " W =!nrt ln $ V 2 % " ' =!P1 V # & 1 ln $ P 1% ' " =!(10 5 )(0.004)ln $ 10 5 % # & # 1.98(10 5 & = J V 1 P J 7. The molar heat capacity at constant pressure for ammonia (NH3) vapour is c P = T, where cp is in J K 1 mol 1 and T is in kelvin. Assuming ammonia behaves as an ideal gas, calculate the change in internal energy U when 10.0 moles of ammonia vapour is heated at a constant pressure of bar from 25.0 C to 75.0 C. T1 = = K T2 = = K For an ideal gas, U = n c V dt! T 2 T 1 cv = cp R = ( T) = T ( ) ( ) Therefore, U = n " T 2! T 1 T ! T % # $ 1 & ' = (10.0) " ( ! ) ( ! % # )& $ 2 ' = (10.0) { } = (10.0){ } = J kj Alternatively: H = J U = H (PV) = H (nrt) = H nr T = (10)(8.314)(50.0) = = J = kj [As before.] Page 7 of 11

8 8. A 350 g metal bar initially at 1000 K is removed from a furnace and quenched by quickly immersing it in a well- insulated closed tank containing kg of water initially at 300 K. The heat capacities of the metal and the water are constant at 0.45 J g 1 K 1 and 4.18 J g 1 K 1, respectively. Neglecting any heat loss from the tank or water lost by vaporization, what is the final equilibrium temperature in the tank? Thermal energy leaves the metal and enters the water until both are at the same final temperature Tf. At constant pressure the heat transferred is just the change in enthalpy. Furthermore, since the tank is insulated, no thermal energy leaves the tank. Thus Hmetal + Hwater = 0 i.e., ( mc P!T) metal + ( mc P!T) water = 0 (350)(0.45)(Tf 1000) + (10 000)(4.18)(Tf 300) = Tf Tf = Tf = Tf = = K = C C 9. One mole of nitrogen gas at 250 K occupies a volume of 10.0 L in the frictionless piston/cylinder arrangement shown. The mass of the piston may be assumed to be negligible. The cross- sectional area of the piston face is 300 cm 2, and the pressure of the external atmosphere is 100 kpa. When a 500 kg weight is placed on the piston, the piston lowers, compressing the gas. The process may be assumed to be isothermal, and nitrogen may be assumed to behave as an ideal gas. The acceleration due to gravity is 9.80 m s 2. P atm = 100 kpa N 2 gas 1.0 mol 250 K 10.0 L restraining pin (a) How much work is done on the gas? (b) What is the minimum work required to carry out this compression? Page 8 of 11

9 (a) W = Pext Vsyst The external pressure is the sum of the atmospheric pressure plus the pressure resulting from the 500 kg weight being placed on the piston. The initial pressure of the gas is P 1 = nrt = (1.0)(8.3145)(250) = Pa V The compression will stop when the final gas pressure P2 is in mechanical equilibrium with the external pressure causing the compression. Therefore, P2 Isothermal process: = P atm + P weight = P atm + mg (500)(9.8) = A = = Pa P1V1 = P2V2 Therefore, V 2 = P 1V 1 ( )(0.0100) = P = m 3 For the gas, V = V2 V1 = = m 3 Therefore, W = Pext V = P2 V = ( )( ) = J The positive sign indicates a work input J (b) The minimum work for the isothermal compression of an ideal gas is " W =! nrt ln $ V 2 % ' " =! (1.00)(8.3145)(250) ln % # V 1 & # & = J A reversible compression always requires less work than a real compression J Page 9 of 11

10 10. Problem 8 17 (SOLUTION GIVEN IN MIDTERM PREPARATION SOLUTIONS) A closed system undergoes a cycle consisting of two processes. During the first process, which starts at an initial system pressure of 1.00 atm and system temperature of 300 K, 60 kj of heat is transferred from the system while 25 kj of work is done on the system. At the end of the first process, the system pressure is 0.50 atm. During the second process, 15 kj of work is done by the system. The temperature and pressure of the surroundings are constant throughout at 300 K and one atm, respectively. (a) Determine Qsyst for the second process. (b) Determine Qsyst for the cycle. (c) Determine Wsyst for the cycle. (d) Determine Usyst for the cycle. (e) Determine Hsyst for the cycle. For a closed system going through a cycle, Ucycle = (Q + W) = 0 (a) Let the first process be process A and the second process be process B. Ucycle = 0 = (Q + W) QA + QB + WA + WB = ( 60) + QB + (+ 25) + ( 15) = 0 Therefore, QB = = + 50 kj + 50 kj (b) Qcycle = Qnet = QA + QB = ( 60) + (50) = 10 kj 10 kj (c) Wcycle = Wnet = WA + WB = (+ 25) + ( 15) = + 10 kj + 10 kj (d) For a cyclic process, the system is restored to its original state, so that all state properties of the system have no net change. Therefore, Usyst = 0. 0 (e) Similarly, Hsyst = 0. 0 Page 10 of 11

11 3. Part A: Calculate the pressure in MPa using the following gas laws: (a) the ideal gas law (b) the van der Waals equation (c) the virial equation Part B: Assuming the pressure calculated using the virial equation is the true value, determine the % error in the pressure by using the ideal gas law and the van der Waals equation. For neon, a = Pa m 6 mol - 2, b = x 10-6 m 3 mol - 1. For neon at 25 o C, values for B(T) and C(T) are cm 3 mol - 1 and 221 cm 6 mol - 2, respectively. Page 11 of 11