Version 001 EXAM 7 PRACTICE PROBLEMS chemistry (78712) 1

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1 Version 001 EXAM 7 PRACTICE PROBLEMS chemistry (78712) 1 This print-out should have 72 questions. Multiple-choice questions may continue on the next column or page find all choices before answering points For the reaction 2 2 (g)+o 2 (g) 2 2 O(l) find the value for the work done at 300 K kj kj kj correct MW C 3 O = g (fusion) = (freezing) q = 25.6 g 1mol 3.16 kj = kj g mol points A 0.2 gram sample of a candy bar is combusted in a bomb calorimeter, increasing the temperature of the 2000 g of water from C to C. What is U in kj/g? Ignore any heat loss or gain by the calorimeter itself kj/g kj T = 300 K n = 3 mol R = J mol K kj/g kj/g kj/g correct w = P V = nrt = ( 3 mol) = J ( ) J (300 K) mol K points The enthalpy of fusion of methanol (C 3 O) is 3.16 kj/mol. ow much heat would be absorbed or released upon freezing 25.6 grams of methanol? kj absorbed kj absorbed kj released correct kj released kj absorbed kj released kj/g kj/g None points For the combustion reaction of ethylene (C 2 4 ) C O 2 2CO O assume all reactants and products are gases, and calculate the rxn 0 using bond energies kj/mol kj/mol kj/mol kj/mol correct kj/mol kj/mol

2 Version 001 EXAM 7 PRACTICE PROBLEMS chemistry (78712) kj/mol 0 rxn = BE reactants BE products = [ (C C)+4 (C ) +3 (O O) ] [ 4 (C O)+4 ( O) ] [( = 602 kj ) ( kj ) mol mol ( kj )] mol [ ( kj ) ( kj )] mol mol = 1300 kj mol points Methyl tert-butyl ether or MTBE is an octane booster for gasoline. The combustion of grams of MTBE (C 5 12 O(l), g/mol) is carried out in a bomb calorimeter. The calorimeter s hardware has a heat capacity of kj/ C and is filled with exactly L of water. The initial temperature was C. After the combustion, the temperature was C. Analyze this calorimeter data and determine the molar internal energy of combustion ( U) for this octane booster kj/mol correct kj/mol kj/mol kj/mol kj/mol kj/mol kj/mol T = = C cal = [2022(4.184) ]/1000 = kj/ C q cal = 10(3.513) = kj moles = /88.15 = mol U = kj / mol = kj/mol points Calculate the standard reaction enthalpy for the oxidation of nitric oxide to nitrogen dioxide 2NO(g)+O 2 (g) 2NO 2 (g) given N 2 (g)+o 2 2NO(g) = kj mol 1 2NO 2 (g) N 2 (g)+2o 2 (g) = 66.4 kj mol kj mol kj mol kj mol 1 correct kj mol kj mol 1 We need to reverse the reactions and add them: 2NO(g) N 2 (g) +O 2 (g) = kj/mol NO 2 (g) +2O 2 (g) 2NO 2 (g) = kj/mol 2NO(g)+2O 2 (g) O 2 (g)+2no 2 (g) 2NO(g)+O 2 (g) 2NO 2 (g) = kj/mol points You have a 12 oz. can (355 ml) of beer. You testthetemperatureandseethatitreads0 C. Now this isn t just any beer; this is Guinness and you ve heard that Guinness is best at room temperature (20 C). If the specific heat

3 Version 001 EXAM 7 PRACTICE PROBLEMS chemistry (78712) 3 of Guinness is J/g C, how much heat should you add in order to raise the temperature? The density of Guinness is 1.2 g/ml. X kj J J kj correct T (K) Z Y (J) points A student runs a reaction in a closed system. In the course of the reaction, 64.7 kj of heat is released to the surroundings and 14.3 kj of work is done on the system. What is the change in internal energy ( U) of the reaction? kj kj kj kj correct kJ The change in internal energy is given by the formula: U = q + w. In this reaction, q is kj and w is 14.3 kj. The answer is kj points Consider the plot below for three different samples of pure water. Based on the plot, which answer choice below is a correct statement regarding the three samples of pure water? 1. All three samples have the same heat capacity. 2. Sample Z has the greatest heat capacity. 3.SampleXhasthesmallestmass. correct 4. All three samples have different specific heat capacities. 5. Sample Y would require the least heat to raise its temperature by 1 K. It is given that the samples are all pure water, which ensures that all three samples have the same specific heat capacity. Since the sampleshavedifferentchangesintforagiven input of energy, we can infer that all three samples have different heat capacities. Since sample X experiences the largest increase in T for a given amount of energy, it must be the least massive sample. Sample Z would actually have a heat capacity intermediate between that of X and Y. Sample Y requries the most heat to raise its temperature by any given amount points What is the total heat flow when 12 grams of ice at -40 C are heated to become water at 25 C?

4 Version 001 EXAM 7 PRACTICE PROBLEMS chemistry (78712) kj kj kj kj kj kj correct 12 g step 1 2 O(s) 40 C Step 1: Step 2: 12 g step 2 2 O(s) 0 C 12 g 2 O(l) 0 C step J g C (12 g) (0 40) C 334 J g (12 g) 12 g 2 O(l) 25 C = 1,003.2 J = 4,008 J J Step 3: g C (12 g) (25 0) C = 1,255.2 J Total = 1,003.2 J+4,008 J+1,255.2 J = 6,266.4 J = 6.27 kj points A CD player and its battery together do 500 kj of work, and the battery also releases 250 kj of energy as heat and the CD player releases 50 kj as heat due to friction from spinning. Whatisthechangeininternalenergyof the system, with the system regarded as the battery and CD player together? kj kj kj kj kj correct eat from the CD player is 50 kj. eat from the battery is 500 kj. Work from both together on the surroundings is 250 kj. This question is testing your ability to see what the system is, and then look at ONLY the energy flow for the system. ere the systemisthebatteryandthecdplayertogether. U = q +w = [ 50 kj+( 250 kj)]+( 500 kj) = 800 kj points 3 g of a hydrocarbon fuel is burned in a bomb calorimeter that contains 200 grams of water initially at C. After the combustion reaction,thetemperatureis27.00 C.owmuch heat is evolved per gram of fuel burned? The heat capacity of the calorimeter (hardware only) is 150 J/ C J/g J/g J/g J/g J/g J/g J/g J/g correct m fuel = 3 g m water = 200 g T = C C = 2.0 C Theamountofheatevolvedbythereaction is equal to the amount of heat gained by the water plus the amount of heat gained by the calorimeter. The specific heat of water is J/g C, so we have to multiply by grams and temperature change in order to obtain Joules: of water = (c 2O)(m water )( T)

5 Version 001 EXAM 7 PRACTICE PROBLEMS chemistry (78712) 5 = (4.184 J/g C)(200 g) (2.00 C) = 1673 J The heat capacity of the calorimeter is 150 J/ C. This is not per gram, so we just have to multiply by the temperature change to get Joules: of calorimeter = (c cal )( T) = (150 J/ C)(2.00 C) = 300 J of rxn = of water + of calorimeter = 1673 J+300 J = 1973 J This is the total amount of heat evolved by the combustion of the 3 g of fuel. To get the amount per gram of fuel burned, we divide of the reaction by the amount of fuel burned (3 grams): 1973 J 3 g = 657 J/g points Thespecificheatofwateris1.00 cal/g C,the heatofvaporizationofwater is540cal/g,and the heat of fusion of water is 80 cal/g. ow much heat would be required to convert 10 grams of ice at 0 C to 10 grams of water at 75 C? cal cal 1 cal Step 2: g C (10 g)(75 0) C = 750 cal Total = 800 cal+750 cal = 1550 cal = 1.55 kcal points 1 g of cake is combusted in a bomb calorimeter. The heat capacity of the calorimeter hardware is 12 calories K 1. The calorimeter contains 4 L of water; the specific heat capacity of water is 1 calorie g 1 K 1 and the density of water is 1 g ml 1. You detonate the cake and the temperature of the water increases by 1.2 K. Calculate the calories in the one-gram sample of cake, U calories correct calories calories calories calories calories 1calorie g 1 K ml 1g ml 1 1.2K points Reaction of tertiary butyl alcohol with hydrobromic acid produces tertiary butyl bromide by the following reaction. Use bond energies (provided in preamble) to estimate the change in enthalpy,, for this reaction cal O Br kcal correct kcal 10 g step 1 2 O(s) 0 C Step 1: 10 g step 2 2 O(l) 0 C 80 cal (10 g) = 800 cal g 10 g 2 O(l) 75 C + Br kj/mol kj/mol kj/mol + 2 O

6 Version 001 EXAM 7 PRACTICE PROBLEMS chemistry (78712) kj/mol kj/mol correct kj/mol In this reaction, you break two bonds in the reactants: a C-O bond and a -Br bond. You formnew bondsintheproducts: ac-brbond and a -O bond. The change in enthalpy is the energy in for breaking the bonds combined with the energy out gained from forming the new bonds. rxn = B.E. C O +B.E. Br B.E. C Br B.E. O rxn = rxn = 24kJmol points Estimate the heat released when 1-butene (C 3 C 2 C C 2 ) reacts with bromine to give C 3 C 2 CBrC 2 Br. Bond enthalpies are C : 412 kj/mol; C C : 348 kj/mol; C C : 612 kj/mol; C Br : 276 kj/mol; Br Br : 193 kj/mol kj/mol kj/mol kj/mol kj/mol kj/mol correct C C C C + Br Br C C Br = E break E make [ ] = (C C) +(Br Br) [ ] 2(C Br)+(C C) = 612 kj/mol+193 kj/mol [ ] 2(276 kj/mol)+348 kj/mol = 95 kj/mol, C C Br which means 95 kj/mol of heat was released points Which of the following is/are a reason that water is a desirable heat sink for use in calorimeters? I) Water s heat specific capacity is very precisely known. II) Water is readily available. III) Water has an unusually large specific heat capacity. 1. I only 2. II and III 3. I and II 4. I, II and III correct 5. II only 6. I and III 7. III only Water is a good heat sink for all of the reasons listed above. Moreover, its large heat capacity, liquid state and ready availability enable us to easily set up a calorimeter such that T is large enough that it can be easily measured and small enough that phase transition temperatures are not reached.

7 Version 001 EXAM 7 PRACTICE PROBLEMS chemistry (78712) points Consider a thermodynamic system that is simultaneously releasing heat and doing work. The internal energy of this system will: 1. Decrease correct 2. Increase, decrease, or stay the same depending onthemagnitudes ofheat andwork 3. Stay exactly the same. 4. Increase The change in internal energy is equal to the sum of the heat absorbed by the system and work done on the system based on the equation: U = q + w. In this case, q and w are both negative. Therefore the internal energy will be decreasing regardless of the magnitudes of heat and work points Which of the following statements is/are true? I) For a given process, must be zero when external pressure is zero. II) For a given process, U and must have different values. III) For a given process, U sys and U surr must have the same magnitude. 1. I, II 2. I, II, III 3. I only 4. III only correct 5. II, III 6. II only 7. I, III is heat transferred at constant pressure. For a given process U and do not always have different values; for example, they can both be zero for a given process. Because U univ must be zero according to the 1st law, U of a system and its surroundings not only can but must have the same magnitude and opposite signs points If you drop a piece of potassium metal into water you get the following exothermic reaction: 2K(s)+2 2 O(l) 2KO(aq)+ 2 (g) Whatarethevaluesofq andwforthisreaction, at constant temperature and pressure? 1. Both are positive. 2. q is negative and w is positive. 3. q is positive and w is negative. 4. Both are negative. correct points The formation of chemical bonds from separated atoms 1. is never spontaneous. 2. increases entropy. 3. may be either endothermic or exothermic. 4. is always exothermic. correct 5. is always endothermic. The energies to break bonds are positive (require energy input, positive ) and energies to form bonds are negative (release energy, negative ). Which of points O 2 (g), O 2 (l), 2 (g), 2 (l), 2 O(g), 2 O(l)

8 Version 001 EXAM 7 PRACTICE PROBLEMS chemistry (78712) 8 have a standard enthalpy of formation equal to zero? 1. O 2 (g), O 2 (l), 2 (g), 2 (l), 2 O(g), 2 O(l) 2. O 2 (g), 2 (g), 2 O(g) 3. O 2 (g), O 2 (l), 2 (g), 2 (l) 4. O 2 (g), 2 (g) correct 5. All of them, but only at absolute zero Molecules in their native state at STP have a standard enthalpy of formation of zero points When 1 molof methane is burned at constant pressure, 890 kj/mol of energy is released as heat. If a 3.64 g sample of methane is burned atconstant pressure, whatwillbethevalueof? (int: Convert the grams of methane to moles. Also make sure your answer has the correct sign for an exothermic process.) Correct answer: kj. C 4 : c = 890 kj/mol =? m = 3.64 g ( ) mol C4 n C4 = (3.64 gc 4 ) 16 g C 4 = mol C points Calculate the quantity of energy required to change 3.00 mol of liquid water at 100 C to steam at 100 C. The molar heat of vaporization of water is 40.6 kj/mol kj kj correct 3. None of these kj kj n = 3.0 mol 40.6 kj mol 3 mol = kj points Calculate the heat of formation for 2.6 mol of sulfur dioxide (SO 2 ) from its elements, sulfur and oxygen. Use the balanced chemical equation and the following information. S(s)+ 3 2 O 2(g) SO 3 (g) c 0 = kj/mol 2SO 2 (g)+o 2 (g) 2SO 3 (g) 0 = kj/mol Correct answer: kj. f of SO 2 from S and O =? The balanced equation is = ( mol)( 890 kj/mol) = kj S(s)+O 2 (g) SO 2 (g)

9 Version 001 EXAM 7 PRACTICE PROBLEMS chemistry (78712) 9 Which change(s) are endothermic? Reaction 0 f (kj/mol) S(s)+ 3 2 O 2(g) SO 3 (g) ] [2SO 3 (g) 2SO 2 (g)+o (198.2) = S(s)+O 2 (g) SO 2 (g) For 2.6 mol sulfur dioxide, the energy required is ( kj/mol)(2.6 mol) = kj points? heat capacity is the amount of heat required to raise the temperature of one? of an object by 1 C. It is an? property. 1. Specific; gram; extensive 2. Molar; gram; intensive 3. Specific; mole; extensive 4. Molar; gram; extensive 5. Specific; gram; intensive correct 6. Molar; mole; extensive None 027 (part 1 of 3) 10.0 points Consider the following chemical and physical changes: A. 2 O(l) 2 O(g) B. 2 O(l) 2 O(s) C (g)+ 1 2 F 2(g) F(g) = kj/mol D. C 3 8 (g)+5o 2 (g) 3CO 2 (g)+4 2 O(g) E. N 2 (g)+o 2 (g) 2NO(g) = kj/mol 1. C and D only 2. B and E only 3. C only 4. A, D, and E only 5. A, C, and D only 6. A and E only correct None 028 (part 2 of 3) 10.0 points For which change(s) would = U? 1. B and E only 2. C and D only 3. A and B only 4. A and D only 5. B, C, and E only correct None 029 (part 3 of 3) 10.0 points For which change(s) would rxn = f of the product? 1. A, B, and C only 2. C only correct 3. A and C only 4. A, B, C, and E only 5. C and E only None points

10 Version 001 EXAM 7 PRACTICE PROBLEMS chemistry (78712) 10 The standard enthalpy of formation of Br 2 (l) is 1. negative. 2. zero. correct 3. positive. Since bromineisaliquidat25 Cand1atm pressure, it is already in its standard state, so the standard enthalpy of formation is zero points Consider the combustion reaction below. 2C 3 O(g)+3O 2 (g) 2CO 2 (g)+4 2 O(l) If this reaction took place in a closed, rigid container, work would be (positive/negative/zero) and heat would be (positive/negative/zero). 1. positive, zero 2. positive, negative 3. zero, positive 4. negative, positive 5. negative, zero 6. zero, negative correct Despite the fact that the reaction results in a net decrease in moles of gas, which would normally result in positive work (work done on the system), the closed, rigid container precludes a change in volume and results in zero work. Like other combustion reactions, heat is released, and so heat would be negative points Which of the reactions below is a formation reaction? 1. 2Fe(s)+3O(g) Fe 2 O 3 (s) 2. B 2 (s)+2i 2 (l)+cl 2 (g) 2BI 2 Cl(g) 3. C diamond (s)+ 1 2 O 2(g) CO(g) 4. N 2 (g)+2 2 (g)+ 1 2 O 2(g) N 2 4 O(g) correct A formation reaction produces exactly one mole of one product from elements in their standard states points Energy in the amount of 455 J is added to a 67.0 g sample of water at a temperature of 7.00 C. What will be the final temperature of the water? Correct answer: C. q = 455 J T i = 7 C m = 67 g T f =? S 2O = 4.18 J/g K S 2O = q m T q T = ms 2O 455 J = (67 g)(4.18 J/g K) = K = C T = T f T i T f = T +T i = C+7 C = C

11 Version 001 EXAM 7 PRACTICE PROBLEMS chemistry (78712) points A system did 150 kj of work and its internal energy increased by 60 kj. ow much energy did the system gain or lose as heat? 1. The system gained 60 kj of energy as heat. 2. The system gained 90 kj of energy as heat. 3.Thesystemlost210kJofenergyasheat. 4. The system lost90 kjof energy as heat. 5. The system gained 210 kj of energy as heat. correct points An important reaction that takes place in the atmosphere is NO 2 (g) NO(g)+O(g) which is brought about by sunlight. Calculate the standard enthalpy of the reaction from the following information reaction (kj) O 2 (g) 2O(g) NO(g)+O 3 (g) NO 2 (g)+o 2 (g) O 2(g) O 3 (g) kj correct kj kj kj kj kj kj Using ess Law we add the reverse (flip) of reaction2; the reverse (flip) of reaction3; and one half of reaction 1: NO 2 (g)+o 2 (g) NO(g)+O 3 (g)+200 O 3 (g) 3 2 O 2(g) O 2(g) O(g) NO 2 (g) NO(g)+O(g) points What is the value of work when an external pressure of2atmcompresses apistonfrom an initial volume of 11.2 liters to a final volume of 2 liters kj kj kj correct kj For expansion against a constant external pressure, w = P ext V = ( 2 atm)(2 L 11.2 L) ( J L 1 atm 1 ) = 1.86 kj points A bomb calorimeter with a heat capacity of 30 J/Ccontains1000 gofwaterwithaninitial temperature of 25 C. A 0.5 g sample of a candybarisplacedinabombcalorimeterand ignited, resulting in a new water temperature of 30 C. What is E for this reaction? kj/g correct 2. 0 kj/g

12 Version 001 EXAM 7 PRACTICE PROBLEMS chemistry (78712) 12 ( kj )] kj/g mol [ ( kj/g kj ) ( kj )] mol mol kj/g = 1300 kj mol kj/g kj/g points For the combustion reaction of ethylene (C 2 4 ) C O 2 2CO O assume all reactants and products are gases, and calculate the rxn 0 using bond energies from the list below. C : 413 kj/mol; O : 463 kj/mol; O O : 146 kj/mol; O O : 498 kj/mol. C C : 346 kj/mol; C C : 602 kj/mol. C O : 358 kj/mol; C O : 799 kj/mol kj/mol kj/mol kj/mol points 2.26 g of liquid water at 23.5 C was completely converted to ice at 0 C. ow much heat was (absorbed/released) by the system during this process? J; absorbed J; absorbed J; released J; absorbed J; released correct J; released for 1 gram (cooling + freezing): 23.5(4.184) = J/g scale up to 2.26 g : (2.26) = J = 977 J released kj/mol correct kj/mol kj/mol kj/mol 0 rxn = BE reactants BE products = [ (C C)+4 (C ) +3 (O O) ] [ 4 (C O)+4 ( O) ] [( = 602 kj ) ( kj ) mol mol points Which of the following reactions is an enthalpy of formation reaction? N 2(l) (g) N 3 (g) 2. NaO(aq)+Cl(g) 2 O(l)+NaCl(aq) 3. 2Fe(s)+ 3 2 O 2(g) Fe 2 O 3 (s) correct 4. C 4 (g) C graphite +2 2 (g) Formation reactions describe production of exactly one mole of one product from stoichiometric quantities of elements in their standard states.

13 Version 001 EXAM 7 PRACTICE PROBLEMS chemistry (78712) points Consider a system where 2.50 L of ideal gas expands to 6.25 L against a constant external pressure of 330 torr. Calculate the work (w) for this system J J J J J J correct Convert torr to atm, and then convert answer in L atm to joules. The answer will be negative due to expansion of the gas. w = P V = (330/760)(3.75L) w = L atm J/(L atm) = 165 J We need to reverse and double the second reaction as well as triple the third reaction and then add all three reactions points Calculate the standard reaction enthalpy ( rxn ) for the final stage in the production of nitric acid, when nitrogen dioxide dissolves in and reacts with water: 3NO 2 (g)+ 2 O(l) 2NO 3 (aq)+no(g) kj kj kj kj kj kj correct points Calculate the standard reaction enthalpy for the reaction C 2 5 O(l)+3O 2 (g) 2CO 2 (g)+3 2 O(l) Values for f from external table are in order (from reaction) +33, -286, -207, and +90 given ( ) C 2 5 O(l) 2C graphite (s)+3 2 (g)+1/2o 2 (g) rxn = n j products ( ) = 228 kj mol 1 n j CO 2 (g) C graphite (s)+o 2 (g) [ reactants ] = 394 kj mol 1 = 2 f, NO 3(aq) + f, O(g) [ ] 2 (g)+1/2o 2 (g) 2 O(l) = 286 kj mol 1 3 f, NO 2(g) + f, 2O(l) [ ] [ ] kj mol 1 = 2( 207)+90 3(33)+( 286) = 137 kj kj mol ,418 kj mol 1 correct kj mol kj mol points The molar heat capacity of C 6 6 (l) is 136 J/mol C and of C 6 6 (g) is 81.6 J/mol C. The molar heat of fusion for benzene is 9.92 kj/mol and its molar heat of vaporization is

14 Version 001 EXAM 7 PRACTICE PROBLEMS chemistry (78712) kj/mol. The melting point of benzene is 5.5 C, its boiling point is 80.1 C, and its molecular weight 78.0 g/mol. ow much heat would be required to convert 234 g of solid benzene (C 6 6 (s)) at 5.5 C into benzene vapor (C 6 6 (g)) at C? kj kj correct kj kj kj m benzene = 234 g T 2 = C C 6 6 (s) 5.5 C C 6 6 (l) 80.1 C Step 1 : Step 2 : 234g mol 78.0g = 3mol step 1 C 6 6 (l) 5.5 C step 3 C 6 6 (g) 80.1 C step 2 T 1 = 5.5 C step 4 C 6 6 (g) C 9.92 kj 3mol = 29.76kJ mol 136 J mol C (3 mol) ( ) C = J = kJ 30.8 kj Step 3 : (3mol) = 92.4kJ mol 81.6 J Step 4 : (3 mol) mol C ( ) C = J = kJ Total = kj kj kj kj = kj points Calculate the standard reaction enthalpy for the reaction NO 2 (g) NO(g)+O(g) given kj/mol for the standard enthalpy of formation of ozone and O 2 (g) 2O(g) = kj/mol NO(g)+O 3 (g) NO 2 (g)+o 2 (g) = 200 kj/mol Remember the definition of the standard enthalpy of formation of a substance kj/mol correct kj/mol kj/mol kj/mol kj/mol O 2 (g) 2O(g) = kj/mol NO(g)+O 3 (g) NO 2 (g)+o 2 (g) = 200 kj/mol The standard formation of ozone is 3 2 O 2(g) O 3 (g) = kj/mol We calculate the rxn using ess Law: To combine the reactions and get the desired reaction, reverse the second and third equations and add half of the first one: NO 2 (g)+o 2 (g) NO(g)+O 3 (g) = +200 kj/mol O 3 (g) 3 2 O 2(g) = kj/mol 1 2 O 2(g) O(g) = 1 (498.4 kj/mol) 2 NO 2 (g) NO(g)+O(g) rxn = kj/mol

15 Version 001 EXAM 7 PRACTICE PROBLEMS chemistry (78712) points A coffee cup calorimeter measures the heat at constant? whereas a bomb calorimeter measures the heat at constant? 1. pressure (q p = ); volume (q v = U) correct 2. pressure (q p = U); volume (q v = ) 3. volume (q v = ); pressure (q p = U) 4. volume (q v = U); pressure (q p = ) None points You have two liquids of identical mass, and both with initial temperatures of 15 C. One is ethanol, C 2 5 O, with a specific heat of 2.46 J/g C and the other is benzene, C 6 6, with a specific heat of 1.74 J/g C. If both liquids absorb the same amount of heat, which one will have the highest final temperature? Assume that neither liquid reaches its boiling point. 1. Cannot tell without more information given. 2. ethanol 3. Bothliquids will have the same final temperature. 4. benzene correct Temperature rise ( T) is inversely proportional to the heat capacity. T = q mc s Therefore, because benzene has a smaller heat capacity, C s, it will have the larger temperature rise points 1-bromo-isobutane will undergo and elimination reaction to yield isobutene and hydrogen bromide as shown in the reaction below. Use bond energies (provided in preamble) to estimatethe change in enthalpy,, for this gas phase reaction. 3 C C C 2 Br 3 C kj/mol kj/mol kj/mol kj/mol kj/mol correct kj/mol 3 C C C 2 + Br 3 C In this reaction, you break three bonds in the reactants: a C-C, a C-, and a C-Br bond. You form new bonds in the products: a -Br bond and a C=C bond. The change in enthalpy is the energy in for breaking the bonds combined with the energy out gained from forming the new bonds. rxn = rxn = +76kJmol points Which is true, considering the first law of thermodynamics? 1. U = q w, where heat and work can both be positive for the same process 2. U = q + w, where heat and work can never both be positive for the same process 3. U = q w, where heat and work can never both be positive for the same process 4. U = q + w, where heat and work can both be positive for the same process. cor-

16 Version 001 EXAM 7 PRACTICE PROBLEMS chemistry (78712) 16 rect none points For which of the following chemical equations would rxn = f? 1. O 2 (g) + 2 (g) 2 O 2 (l) correct 2. C(s, graphite) O 2(g) + 2 (g) CO 2 (g) + 2 O(g) 3. CO(g) O 2(g) CO 2 (g) 4. N 2 (l) + 3 F 2 (g) 2 NF 3 (l) For O 2 (g) + 2 (g) 2 O 2 (l), f of O 2 (g) and 2 (g) are 0. Therefore, rxn = f ( 2O 2 (l)) points The combustion of methane gas (C 4 ) forms CO 2 (g)+ 2 O(l). Calculate the heat produced by burning 1.98 mol of the methane gas. Use these 0 f data to help: C 4 (g)= kj/mol CO 2 (g)= kJ/mol 2 O(l)= kJ/mol Correct answer: kj. n C4 = 1.98 mol C 4 (g)+2o 2 CO 2 (g)+ 2 O(l) 0 for combustion of C 4 =? Reaction f 0 (kj/mol) C 4 (g) C(s)+2 2 (g) 74.9 C(s)+O 2 (g) CO 2 (g) (g)+o O(l) 2( 285.8) = C 4 (g)+2o 2 CO 2 (g)+ 2 (l) = (890.2 kj/mol)(1.98mol) = kJ points Calculate the standard reaction enthalpy for the reaction. C 4 (g)+ 2 O(g) CO(g)+3 2 (g) given 2 2 (g)+co(g) C 3 O(l) = kj mol 1 2C 4 (g)+o 2 (g) 2C 3 O(l) = kj mol (g)+o 2 (g) 2 2 O(g) = kj mol kj mol kj mol 1 correct kj mol kj mol kj mol 1 We need to reverse the first reaction, halve the second, halve and reverse the third and add the results: C 3 O(l) 2 2 (g)+co(g) = kj/mol C 4 (g)+ 0.5O 2 (g) C 3 O(l) = kj/mol 2 O(g) 2 (g)+ 0.5O 2 (g) = kj/mol C 4 (g)+ 2 O(g) CO(g)+3 2 (g) = kj/mol

17 Version 001 EXAM 7 PRACTICE PROBLEMS chemistry (78712) points A system absorbs 237 J of heat while it performs 435 J of work. What is the change in the internal energy of the system? J J J correct J U = q +w = 237 J+ 435 J = 198 J points Calculate the enthalpy change that occurs when 1.00 kg of acetone condenses at its boiling point (329.4 K). The standard enthalpy of vaporization of acetone is 29.1 kj mol kj kj kj kj correct kj Acetone is C 3 COC 3 MM = g/mol n = (1000 g)(1 mol acetone) g/mol = mol Condensation is the opposite of vaporization: = q = n cond = n( vap ) = ( mol)( 29.1 kj/mol) = kj points For which of the following reactions at room temperature (25 C) would there be 5.0 kj of work done on the system? 1. N 2 2 (g)+c 3 O(g) C 2 O(g)+N 2 (g)+2 2 (g) 2. C 2 O(g)+N 2 (g)+2 2 (g) N 2 2 (g)+c 3 O(g) correct O(l)+O 2 (g) 2 2 O 2 (l) 4.C 4 (g)+2o 2 (g) CO 2 (g)+2 2 O(g) O 2 (l) 2 2 O(l)+O 2 (g) 6.CO 2 (g)+2 2 O(g) C 4 (g)+2o 2 (g) At room temperature (298 K), the product of the gas constant (R = J mol 1 K 1 ) and T is very close to 2.5 kj mol 1. Based on 5.0 kj = n gas ( 2.5 kj mol 1 ), the reaction for which n gas = 2 will be the correct answer points The value of for the reaction C 3 8 (g)+5o 2 (g) 3CO 2 (g)+4 2 O(l) is 2220 kj/mol rxn. ow much heat is given off when 33.0 g of propane gas (C 3 8 ) is burned at constant pressure? kj correct kj kj kj kj kj

18 Version 001 EXAM 7 PRACTICE PROBLEMS chemistry (78712) kj = 2220 kj/mol m C3 8 = 33.0 g ( 2220 kj )( 1 mol rxn ) q = mol rxn 1 mol C 3 8 ( 1 mol C3 ) 8 (33 g C 3 8 ) 44 g C 3 8 = 1665 kj or 1665 kj released points The two reactions shown below are both endothermic. For which reaction is < U? N 2 (g) + O 2 (g) 2NO(g) 2NO(g) + O 2 (g) 2NO 2 (g) 1. 2NO(g) + O 2 (g) 2NO 2 (g) correct 2. Neither reaction has < U. 3. Both reactions have < U. 4. N 2 (g) + O 2 (g) 2NO(g) points Consider the following reaction 2 (g)+co 2 (g) 2 O(g)+CO(g). f for CO 2 (g) is 22.5 kj/mol; f for CO(g) is 6.3 kj/mol; f for 2 O(g) is 13.8 kj/mol. 1. of the reaction is negative. 2. of the reaction is zero. 3. of the reaction is positive. correct Reactants: f CO2(g) = 22.5 kj/mol Products: f CO(g) = 6.3 kj/mol f 2O(g) = 13.8 kj/mol rxn 0 = nfprod 0 nfrct 0 ( = 13.8 kj ) kj 6.3 mol mol ( 22.5 kj ) mol = 2.4 kj mol is positive points Consider the following specific heats: copper, J/g C; lead, J/g C; water, 4.18 J/g C;glass,0.502J/g C.Whichsubstance, once warmed, would be more likely to maintain its heat and keep you warm through a long football game on a cold night? 1. water correct 2. glass 3. copper 4. lead Water has the highest specific heat of the substances listed, so it has the capacity to emit the largest quantity of heat with minimal temperature loss; the emitted heat keeps you warm. The substance continues to warm you until itstemperature isat or below your body temperature points A block of aluminum at 25 C and 1 atm is heated until it is a liquid at 700 C. It is then cooled back down until it is back in the initial state of being a solid at 25 C and 1 atm. For this entire process (heating and cooling) is positive 2. less than U 3. zero correct

19 Version 001 EXAM 7 PRACTICE PROBLEMS chemistry (78712) greater than U 5. negative Since the initial and final states of the system are identical, and since the value of a state function such as is dependent only on the state of the system, we can conclude that is zero points Which statement about internal energy is true? 1. The internal energy of a system is equal to w at constant volume. 2. The internal energy of a system is constant at constant volume. 3. The internal energy of a system is equal to w at constant pressure. 4. The internal energy of a system is equal to q at constant volume. correct 5. The internal energy of a system is equal to q at constant pressure. 6. The internal energy of a system is constant at constant pressure points When g of graphite is burned completely in a bomb calorimeter (heat capacity = kj/ C), containing 3000 g of water, a temperature rise of 0.21 C is observed. What is E for the combustion of graphite? The specific heat of liquid water is J/g C. 1. E = kj/mol 2. E = 40.1 kj/mol 3. E = 285. kj/mol 4. E = 3.34 kj/mol 5. E = kj/mol correct m graphite = g T = 0.21 C C = kj/ C m water = 3000 g S water = J/g C The amount of heat responsible for the temperature increase for 3000 g of water is ( J ) ( 1 kj ) q = (3000 g)(0.21 g C C) 1000 J = kj The amount of heat responsible for the warming of the calorimeter is ( q = kj ) (0.21 C) = kj C The amount of heat released for this reaction is kj kj = kj The reaction was exothermic and there were 0.1 g of graphite, so ( kj )( 12 g ) = 401 kj 0.1 g mol mol points When a given reaction was run at a constant pressure of 1 atm, the system absorbed 5 kj of heat and the gases were consumed, causing the volume to decrease from 3.5 L to 1.5 L. What are and U, respectively? kj, +0.2 kj kj, -4.8 kj kj, +5.2 kj correct kj, +5 kj kj, +4.8 kj kj, +0.2 kj kj, -5.2 kj

20 Version 001 EXAM 7 PRACTICE PROBLEMS chemistry (78712) kj, +5 kj m SiO2 = 3.00 g = kj/mol kj, -5 kj none points Juan freezes a bottle of water to ice (500.mL) inpreparationforaroadtrip. owmuchheat can be absorbed by that ice before it is fully melted? kj kj correct 3. 0 kj kj kj kj kj Density of water is 1 g/ml. So 1 g = 1 ml for water. q=(500g)x(334 J/g)= J = 167 kj points ow much heat is absorbed in the complete reaction of 3.00 grams of SiO 2 with excess carbon in the reaction below? SiO 2 (g)+3c(s) SiC(s)+2CO(g) for the reaction is kj/mol rxn kj correct kj kj kj kj ( kj )( 1 mol rxn ) q = mol rxn 1 mol SiO 2 ( 1 mol SiO2 ) (3 g SiO 2 ) 60 g SiO 2 = 31.2 kj points Using bond energies, estimate the enthalpy change for the reaction between hydrogen peroxide ( 2 O 2 ) and carbon disulfide (CS 2 ) to produce carbon dioxide (CO 2 ) and hydrogen disulfide ( 2 S 2 ) according to the balanced equation: 2 O 2 +CS 2 CO S kj/mol kj/mol kj/mol kj/mol kj/mol correct kj/mol rxn = ΣBE broken ΣBE formed Total bonds broken are 2 hydrogen oxygen single bonds (463 kj/mol), 1 oxygen oxygen single bond (146 kj/mol) and 2 carbon sulfur double bonds (577 kj/mol). Total bonds formed are 2 hydrogen sulfur single bonds (347 kj/mol), 1 sulfur sulfur single bond (226 kj/mol) and 2 carbon oxygen double bonds (799 kj/mol). rxn = 292 kj/mol points The following reaction occurs during the production of metallic iron:

21 Version 001 EXAM 7 PRACTICE PROBLEMS chemistry (78712) 21 2 Fe 2 O 3 (s)+3 C(graphite) 4 Fe(s)+3 CO 2 (g) Calculate for thisreactionat 25 C and 1 atm. f for CO 2 (g) = kj/mol, and f for Fe 2 O 3 (s) = kj/mol. 1. There is insufficient information to answer this question kj correct kj kj kj [ 2 ( 286 kj mol 1) + 2 ( 297 kj mol 1)] = 1,120 kj mol points Calculate the standard reaction enthalpy for the reaction of calcite with hydrochloric acid CaCO 3 (s)+2cl(aq) CaCl 2 (aq)+ 2 O(l)+CO 2 (g) The standard enthalpies of formation are: for CaCl 2 (aq) : kj/mol; for 2 O(l) : kj/mol; for CO 2 (g) : kj/mol; for CaCO 3 (s) : kj/mol; and for Cl(aq) : kj/mol kj/mol rxn = products reactants = ( )(3) ( 824.2)(2) = kj Note: The enthalpy of a pure element is always zero points Based on thermodynamic table data calculate rxn for 2 2 O(l)+2SO 2 (g) 2 2 S(g)+3O 2 (g) kj/mol kj/mol kj/mol correct kj/mol kj/mol kj/mol We use ess Law: kj mol kj mol kj mol 1 correct kj mol 1 rxn = f, products f, reactants = [ 2 ( 21 kj mol 1) +3(0) ] = n j,prod n j,reac = f, CaCl 2(aq) + f, 2O(l) + f, CO 2(g) [ f, CaCO 3(s) ( )] +2 f, Cl(aq) = kj/mol +( kj/mol) +( kj/mol) [ kj/mol ] + 2( kj/mol) = kj/mol.

22 Version 001 EXAM 7 PRACTICE PROBLEMS chemistry (78712) points For an exothermic reaction, the sum of bond energies for the reactants are (greater/lesser) than those of the products. 1. lesser correct kj/mol correct kj/mol None 2. greater For an exothermic reaction, the change in enthalpy is negative points Calculate the standard reaction enthalpy for the reaction 2 S(g)+2O 2 (g) SO 3 (g)+ 2 O(l) given 2 O(g) 2 O(l) = 11.0 kj mol 1 2 SO4(l) 2 S(g)+2O 2 (g) = kj mol 1 2 SO4(l) SO 3 (g)+ 2 O(g) = kj mol kj kj correct kj kj The overall reaction is sum of reaction 1, reaction 3 and the reverse of reaction points When 17.8 g sodium is treated with excess oxygen,160.2kjofheat isproduced. What is the rxn for the below reaction? 4Na(s) + O 2 (g) 2Na 2 O(s) kj/mol kj/mol kj/mol