Chem. 1B Midterm 1 Version B February 3, 2017

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1 First initial of last name Chem. 1B Midterm 1 Version B February 3, 2017 Name: Print Neatly. You will lose 1 point if I cannot read your name or perm number. Perm Number: All work must be shown on the exam for partial credit. Points will be taken off for incorrect or missing units. Calculators are allowed. Cell phones may not be used as calculators. On fundamental and challenge problems you must show your work in order to receive credit for the problem. If your cell phone goes off during the exam, you will have your exam removed from you. Fundamentals (of 36 possible) Problem 1 (of 16 possible) Problem 2 (of 18 possible) Multiple Choice (of 30 possible) Midterm Total (of 100 possible) 1

2 Fundamental Questions Each of these fundamental chemistry questions is worth 6 points. You must show work to get credit. Little to no partial credit will be awarded. Make sure to include the correct units on your answers. 1) 6 pts Consider the following exothermic reaction at 25C and 1 atm. 2Na(s) + 2H 2O(l) 2NaOH(aq) + H 2(g) Circle the correct answer for each of the following quantities: w, ΔH, ΔE, ΔS, and ΔG. w<0 w>0 w=0 ΔH<0 ΔH>0 ΔH=0 ΔE<0 ΔE>0 ΔE=0 ΔS<0 ΔS>0 ΔS=0 ΔG<0 ΔG>0 ΔG=0 2) 6 pts Consider the following equilibrium: 2NO 2(g) N 2O 4(g) ΔG =-5.4 k (at 881 C) Now suppose a reaction vessel is filled with atm of dinitrogen tetroxide (N 2O 4) at 881. C. Answer the following questions about this system: Under these conditions, will the pressure of N 2O 4 tend to rise of fall? Is it possible to revese this tendency by adding NO 2? In other words, if you said the pressure of N 2O 4 will tend to rise, can that be changed to a tendency to fall by adding NO 2? Similarly, if you said the pressure of N 2O 4 will tend to fall, can that be changed to a tendency to rise by adding NO 2? If you said the tendency can be reversed in the second question, calculate the minimum presure of NO 2 needed to reverse it. Round your answer to 2 significant digits Rise Fall Yes No G = G + RTln(Q) The tendency will reverse when ΔG=0 G = RTln ( P N 2 O 4 2 ) P NO2 T = = 1,154K 5.4 k = ( k )(1,154K) ln (0.594 P2 ) NO 2 atm P NO2 = 0.58 atm Therefore, when the pressure of NO 2 is above atm the pressure of N 2O 4 will fall 3) 6 pts Determine the mass of CO 2 produced by burning enough C 3H 8 to produce k of heat. C 3H 8(g) + 5O 2(g) 3CO 2(g) + 4H 2O(g) ΔH rxn=-2,217 k k ( 3 CO 2 2,217 k ) ( g CO 2 1 CO 2 ) = 5.96 g CO 2 2

3 4) 6 pts Calculate the standard enthalpy of formation of CCl 4(g) using the thermochemical equations. CCl 4(g) + 4HCl(g) CH 4(g) + 4Cl 2(g) ΔH 1 = k ½H 2(g) + ½Cl 2(g) HCl(g) ΔH 2 = k C(graphite) + 2H 2(g) CH 4(g) ΔH 3 = k Standard enthalpy of formation reaction C(graphite) + 2Cl 2(g) CCl 4(g) ΔH f CH 4(g) + 4Cl 2(g) CCl 4(g) + 4HCl(g) -ΔH 1 = k 4HCl(g) 2H 2(g) + 2Cl 2(g) -4ΔH 2 = k C(graphite) + 2H 2(g) CH 4(g) ΔH 3 = k C(graphite) + 2Cl 2(g) CCl 4(g) ΔH f = k k k = k 5) 6 pts What is the heat capacity of the metal if 60.0 g of metal at 95.0 C is put into a coffee cup calorimeter that contains 75.0 g of water at 25.0 C. The final temperature of the system is 35.0 C The heat capacity of the calorimeter is 10.0 C q metal = (q water + q cal ) q metal = m metal C metal (T f T i(metal) ) q metal = (60.0g)C metal ( ) = (3,600 g )C metal q water = m water C water (T f T i(water) ) q water = (75.0 g) (4.184 q cal = C cal (T f T i(water) ) ) ( ) = 3,135 q cal = (10.0 ) ( ) = 100 (3,600 g )C metal = (3, ) C metal = ) 6 pts What is the maximum amount of work that can be obtained when Ar(g) in a volume of 1.00 L at 25 C is allowed to expand to 2.00 L? The maximum work that can be done by a system occurs when the process is carried out reversibly. w rev = nrtln ( V f V i ) w rev = 172 = (0.100 ) ( L ) (298K)ln ( L ) 3

4 Challenge Problems Each of the following short answer questions are worth the noted points. Partial credit will be given. You must show your work to get credit. Make sure to include proper units on your answer. 1a) 8 pts In a coffee cup calorimeter, 1.60 g of NH 4NO 3 is mixed with 75.0 g of water at an initial temperature of C. After dissolution of the salt, the final temperature of the calorimeter contents is C. Assuming the solution has a heat capacity of 4.18 g and assuming no heat loss to the calorimeter, calculate the enthalpy change for the solution of NH 4NO 3 in units of k The question wants you to calculate ΔH rxn. Reaction of interest is: NH 4NO 3(s) NH 4+ (aq) + NO 3- (aq) You need to calculate ΔH rxn. Since, it is a coffee cup calorimeter, pressure is constant and ΔH rxn=q rxn. The question also tells you that q cal=0. The heat from the reaction will go into the solution therefore q rxn=-q sol and H q H rxn = m sol C sol T sol m sol = 1.60 g g = 76.5 g T sol = = 1.66 H rxn = (76.6 g) (4.18 Calculate ΔH per g ) ( 1.66 ) = g NH 4 NO 3 ( 1 NH 4NO 3 ) = NH g NH 4 NO 4 NO 3 3 Since there is only one e of NH 4NO 3 in the equation, divide by H rxn = 532 = k = 26.6 rxn sol. 1b) 8 pts In the same calorimeter 200. g of ice (T i(ice) = 0 C) was added to 440.g water (T i(water)=80.0 C). When the system reached thermal equilibrium the temperature of the system was 30.1 C. What is ΔH fus in k H 2 O(l) 4.18 q H2 O = q ice q ice = q fus + m ice C H2 O T 0 Tf q fus = n ice H fus m water C H2 O T Ti(water) T f = n ice H fus + m ice C H2 O T 0 Tf H fus = m waterc H2 O T Ti(water) T f m ice C H2 O T 0 Tf n ice Calculate the es of ice 200. g H 2 O ( 1 H 2O ) = g H 2 O H fus = (440. g) (4.18 ) ( ) (200. g) (4.18 ) ( ) 11.1 = 6.00 k 4

5 2a) 8 pts Using thermodynamic data from your constant sheet, calculate ΔG at 25 C for the process 2SO 2(g) + O 2(g) 2SO 3(g) where all gases are at 1.00 atm pressure. Also calculate ΔG at 25 C for this same reaction but with all gases at 10.0 atm pressure. Compound ΔGf ( k SO2(g) -300 O2(g) 0 SO3(g) -371 G rxn = G f (prod) G f (reac) G rxn = n SO3 G f (SO 3 ) n SO2 G f (SO 2 ) n O2 G f (O 2 ) G rxn = (2) ( 371 k ) (2) ( 300. k k k ) (1) (0 ) = 142 G rxn = G rxn + RTln(Q) = G rxn + RTln ( P 2 SO 3 P 2 ) SO2 P O2 G rxn = 142 k + ( k ) (298. K)ln ( ( )(10.0) ) = 148 k 2b) 10 pts What is ΔG rxn (in k ) when the temperature is 87 C and the pressures of all the gases are 10.0 atm. Assume that ΔH and ΔS are temperature independent. We need to find ΔG rxn G rxn = G rxn + RTln(Q) First find ΔG rxn at 87 C. The tables on the constant sheet list ΔG f at 25 C, therefore, you need to convert to 87 C. Since ΔH and ΔS are temperature independent you can use ΔG=ΔH-TΔS to change the temperature of ΔG rxn H rxn = H f (prod) H f (reac) H rxn = n SO3 H f (SO 3 ) n SO2 H f (SO 2 ) n O2 H f (O 2 ) H rxn = (2) ( 396 k k k k ) (2) ( 297 ) (1) (0 ) = 198 S rxn = S prod S reac S rxn = n SO3 S SO3 n SO2 S SO2 n O2 S O2 S rxn = (2) (257 ) (2)(248 Determine ΔG rxn at 87 C ) (1) (205 ) = 187 G rxn = H rxn T S rxn = 198 k (360. K)( k ) = 131 k Determine ΔG rxn when the gases are at 10.0 atm G rxn = G rxn + RTln(Q) = G rxn + RTln ( P 2 SO 3 P 2 ) SO2 P O2 G rxn = 131 k + ( k ) (360. K)ln ( ( )(10.0) ) = 138 k 5

6 Multiple Choice Questions On the ParScore form you need to fill in your answers, perm number, test version, and name. Failure to do any of these things will result in the loss of 1 point. Your perm number is placed and bubbled in under the ID number. Do not skip boxes or put in a hyphen; unused boxes should be left blank. Bubble in your test version (B) under the test form. Note: Your ParScore form will not be returned to you, therefore, for your records, you may want to mark your answers on this sheet. Each multiple choice question is worth 5 points. 1. What is ΔS for 88.0 g of CO2 undergoing the following reaction at constant pressure? CO2(s, 150.K) CO2(g, 195. K) Helpful Information: Tsub = 195K, ΔHsub = 25.2, CCO2(s) = 1.07 A) 283 B) 233 C) 154 D) 24.9 E) None of the above 2. For which of the following reaction(s) is the enthalpy change for the reaction not equal to ΔHf of the product? I. 2H(g) H2(g) II. H2(g) + O2(g) H2O2(l) III. H2O(l) + O(g) H2O2(l) A) II only B) II and III C) I only D) III only E) I and III 3. A g sample of methanol, CH3OH, was combusted in the presence of excess oxygen in a bomb calorimeter. The temperature of the water increased from C to C. The heat capacity of the calorimeter is 19,390 in. A) B) 1,512 C) D) 716 E) None of the above. Calculate ΔE for the reaction 6

7 4. For the vaporization of a liquid at a given pressure, A) ΔG is positive at all temperatures. B) ΔG is negative at low temperatures but positive at high temperatures (and zero at some temperature). C) ΔG is negative at all temperatures. D) ΔG is positive at low temperatures but negative at high temperatures (and zero at some temperature). Consider a process carried out on 1.00 of a monatomic ideal gas by the following two different pathways. The first pathway is A (3.00 atm, 20.0 L) to C (1.00 atm, 20.0 L) to D (1.00 atm, 50.0 L); and the second pathway is A (3.00 atm, 20.0 L) to B (3.00 atm, 50.0 L) to D (1.00 atm, 50.0 L). In each case, the gas is taken from state A to state D. 5. Calculate ΔHACD. A) 175 L atm B) 25 L atm C) 175 L atm D) 25 L atm E) None of the above 6. Which statement is true of a process in which 1 of a gas is expanded from state A to state B? A) It is not possible to have more than one path for a change of state. B) The final volume of the gas will depend on the path taken. C) When the gas expands from state A to state B, the surroundings are doing work on the system. D) The amount of work done in the process must be the same, regardless of the path. E) The amount of heat released in the process will depend on the path taken. A, E, E, D, D, E 7

Chem. 1B Midterm 1 Version A February 3, 2017

First initial of last name Chem. 1B Midterm 1 Version A February 3, 2017 Name: Print Neatly. You will lose 1 point if I cannot read your name or perm number. Perm Number: All work must be shown on the