Chem. 1B Midterm 1 Version A February 9, 2018

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1 First initial of last name Chem. 1B Midterm 1 Version A February 9, 2018 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 14 possible) Problem 2 (of 20 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 If the heat capacity of a calorimeter is 44.7 k and the temperature of the water in the calorimeter rises 25.3 C when 20.0 of a substance is combusted. What is ΔE com ( k ) of the substance? E com = q com + w com Volume is constant therefore w com=0 E com = q com = q cal = C cal T E com = (44.7 k ) (25.3) = 1,130 k E com = 1130 k 20.0 = 56.5 k 2) 6 pts Determine the sign of ΔS for the following 2Cl 3(s) 2Cl(s) + 2Cl 2(g) ΔS = + If ΔH of this reaction is positive, is the reaction endothermic or exothermic? endothermic Predict the conditions (high temperature, low temperature, all temperatures, or no temperatures) under which the reaction CO 2(s) CO 2(g) is spontaneous. You must show your work to get credit. ΔH = + ΔS = + ΔG=ΔH-TΔS Therefore, at high temperatures the reaction is spontaneous. 3) 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) ) g ) ( ) = 3,135 q cal = (10.0 ) ( ) = 100 (3,600 g )C metal = (3, ) C metal = g 2

3 4) 6 pts Consider the following equilibrium: 2NOCl(g) 2NO(g) + Cl 2(g) ΔG = 41.k Now suppose a reaction vessel is filled with 2.47 atm of nitrosyl chloride (NOCl) and atm of chlorine (Cl 2) at 986. C. Answer the following questions about this system: Under these conditions, will the pressure of Cl 2 tend to rise or fall? Is it possible to revese this tendency by adding NO? In other words, if you said the pressure of Cl 2 will tend to rise, can that be changed to a tendency to fall by adding NO? Similarly, if you said the pressure of Cl 2 will tend to fall, can that be changed to a tendency to rise by adding NO? If you said the tendency can be reversed in the second question, calculate the minimum presure of NO 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 NO 2 P Cl2 2 ) P NOCl atm T = = 1,259 k 41. k = ( ) (1,259)ln (P NO 2 (0.960) (2.47) 2 ) P NO = 0.36 atm Therefore, when the pressure of NO is above 0.36 atm, the pressure of Cl 2 will fall 5) 6 pts Calculate the heat consumed by the reaction when 1 of CO reacts at constant pressure. H 2(g) + 2CO(g) H 2O 2(l) + 2C(s) ΔH = 33.3 k 1 CO ( 33.3 k 2 CO ) = 16.6 k 6) 6 pts Calculate the boiling point of dinitrogen tetroxide (N 2O 4). Round your answer to the nearest degree. N 2O 4(l) N 2O 4(g) At the boiling point, the reaction above will be at equilibrium. Therefore, ΔG=0. ΔH and ΔS are fairly temperature independent. Therefore, calculate these quantities at 25 C and assume they are the same value at the boiling point. H rxn = H f (prod) H f (react) H rxn = H f (N 2 O 4 (g)) H f (N 2 O 4 (l)) = 10 k k k ( 20 ) = 30 S rxn = S prod S react S rxn = S N2 O 4 (g) S N2 O 4 (l) = = 95 G = H T S H G T = = 30. k 0 k S k = 315 = 43 C

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. 1) 14 pts 350. ml of 7.0 M H 2CO 3 and 300. ml of 15.0 M NaOH are mixed in a constant pressure calorimeter with heat capacity of 54. The initial temperature of both solutions are C and the final temperature of the system is C. What is ΔH rxn in k. Assume that the heat capacity of the solution is and the g density of the solution is 1.00 g. ml Reaction of Interest H 2CO 3(aq) + 2NaOH(aq) Na 2CO 3(aq) + 2H 2O(l) H rxn = q rxn = (q sol + q cal ) q sol = m sol C sol T = (650. g) (4.184 g ) ( ) = 435 q cal = C cal T = (54 ) ( ) = 8.6 H rxn = ( ) = 443 Change to k This is a limiting reagent problem H 2CO 3 NaOH (L.R.) Na 2CO 3 H 2O I() 0.350L(7.0M) 0.300L(15.0M) 0 N/A = 2.45 = 4.50 F() = N/A x = 0 x = 2.45 H rxn = = 197 =

5 2) 20 pts A sample of ice weighing g initially at C is heated to C at constant pressure of 1.00 atm. Calculate q, w, ΔE, ΔH, and ΔS for this process. The ar heat capacities (C P) for solid, liquid, and gaseous water ( , and 36.4, respectively) are assumed to be temperature independent. The enthalpies of fusion and vaporization are 6.01 k and 40.7, respectively. Assume ideal gas behavior. k Break down the change into steps. ice (-30.0 C=243) ice(0.0 C=273) water(0.0 C=273) water(100.0 C=373) steam(100.0 C=373) steam(140.0 C=413) Calculate the es of H2O g H 2 O ( 1 H 2O ) H g H2O 2O Calculate q q 1 = nc T = (1.00 )(37.5 )( ) = 1,130 = 1.13 k q 2 = n H fus = (1.00 ) (6.01 k ) = 6.01 k q 3 = nc T = (1.00 ) (75.3 q 4 = n H vap = (1.00 ) (40.7 k ) = 40.7 k q 5 = nc P T = (1.00 ) (36.4 ) ( ) = 7,530 = 7.53 k ) ( L) = 1,460 = 1.46 k Calculate qtot: q tot = q 1 + q 2 + q 3 = 1.13 k k k k k = 56.8 k Calculate w. The only time the volume will change is when the liquid turns to a gas and when the gas is heated. Therefore w1, w2, and w3 = 0 w 4 = P ex V Assume that the initial volume is 0 because the volume occupied by the gas is >> (much greater) than the volume occupied by the liquid Calculate the final volume (use the ideal gas law) V f = nrt L atm (1.000 )( = )(373.2) = 30.6 L P 1.00 atm w 4 = P ext (V f V i ) = (1.00 atm)(30.6 L 0 L) = 30.6 L atm( ) 1 L atm = Calculate w5, the work that goes into expanding the gas. In the gas phase we can assume that it behaves like an ideal gas therefore, w 5 = P ext V = nr T = (1.000 ) ( ) ( ) = 333 w tot = w 4 + w 5 = 3.10 k k = 3.43 k Calculate ΔE E = q + w = 56.8 k k = 53.4 k Calculate ΔH H = q = 56.8 k Since the system is at constant pressure ΔH=q Calculate ΔS (need to break it up into steps): S 1 = ncln ( T 2 ) = (1.000 ) (37.5 ) ln (273.2 T ) = 4.36 S 2 = n H k 6.01 = (1.000 ) = k = 22.0 T S 3 = ncln ( T 2 ) = (1.000 )(74.3 )ln (373.2 ) = 23.5 T S 4 = n H k 40.7 = (1.000 ) = k = 109 T S 5 = nc P ln ( T 2 ) = (1.000 )(36.4 )ln (413.2 ) = 3.71 T S tot = S 1 + S 2 + S 3 + S 4 + S 5 = = 162 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 (A) 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. A machine employs the isothermal expansion of 1 of an ideal gas from 4.50 L to 15.0 L. At 25 C, the machine performs 3.00 k of work. What percent of the maximum possible work is the machine producing? a. 0.60% b. This amount of work cannot be correct. c. 4.50% d. 50.3% e. 15.0% 2. Consider a process carried out on 2.00 es of monatomic ideal gas by the following pathway atm, 4.00 L 3.00 atm, 4.00 L 3.00 atm, 2.00 L. What is E for this process a b c d e. None of the above 3. The heat combustion of acetylene, C2H2(g), at 25 C, is At this temperature, ΔH f values for CO2(g) and H2O(l) are 393 and 286 acetylene. a. 2376, respectively. Calculate ΔH f for b. 625 c. 625 d. 227 e. None of above 4. What is ΔG for SO2(g) + 1 O2(g) SO3(g) given the following information? 2 2S(s) +3O2 2SO3(g) ΔG = -742 a b c d. -71 S(s) + O2 SO2(g) ΔG = e. None of the above 6

7 5. In a system composed of nitrogen gas in a cylinder fitted with a piston, when 2.00 k of energy is transferred into the system as heat at an external pressure of 2.00 atm, the nitrogen gas expands form 2.00 L to 5.00 L against this constant pressure. What is ΔE for the process? a k b. 0 c k d k e. None of the above 6. The reaction 2H2O(g) 2H2(g) + O2(g) has a positive value of ΔG. Which of the following statements must be true? a. The reaction will not occur. That is, when H2O(g) is introduced into a flask, no O2 or H2 will form even over a long period of time. b. The equilibrium lies far to the right. c. The reaction is slow. d. The reaction is exothermic. e. None of these is true. B,B,D,D,C,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