CHEMISTRY 123-07 Mdterm #2 answer key November 04, 2010 Statstcs: Average: 68 p (68%); Hghest: 91 p (91%); Lowest: 37 p (37%) Number of students performng at or above average: 58 (53%) Number of students performng below 55%: 19 (17%) PART I: MULTIPLE CHOICE (32 questons, each multple choce queston has a 2-pont value, 64 ponts total). 1. Pressure s A. defned as the mass that an object exerts when at rest B. measured n Newtons C. defned as the number of moles of substance dvded by the mass of the substance D. defned as the force per unt area E. measured n grams 2. Whch of the followng correctly descrbes the relatonshp between the pressure of a gas and the volume of a gas at constant temperature? A. as one ncreases the other ncreases B. unrelated C. drectly proportonal D. rreversbly proportonal E. nversely proportonal 3. For an deal gas, whch two varables are drectly proportonal to each other (f all other condtons reman constant)? 1. P and V 2. V and n 3. V and T A. 1 only B. 2 only C. 3 only D. 1 and 2 E. 2 and 3 4. A sample of N 2 gas occupes 2.40 L at 20 C. If the gas s n a contaner that can contract or expand at constant pressure, at what temperature wll the N 2 occupy 4.80 L? A. 10 C D. 313 C B. 40 C E. 685 C C. 146 C 5. If the pressure on a gas sample s trpled and the absolute temperature s quadrupled, by what factor wll the volume of the sample change? A. 12 D. 1/3 B. 4/3 E. 4 C. 3/4 6. At what temperature wll a fxed amount of gas wth a volume of 175 L at 15 C and 760 mmhg occupy a volume of 198 L at a pressure of 640 mm Hg? A. 274 C D. 1 C B. 214 C E. -59 C C. 114 C 7. You are holdng two balloons, an orange balloon and a blue balloon. The orange balloon s flled wth neon (Ne) gas and the blue balloon s flled wth argon (Ar) gas. The orange balloon has twce the volume of the blue balloon. Whch of the followng best represents the mass rato of Ne:Ar n the balloons? A. 1:1 D. 1:3 B. 1:2 E. 3:1 C. 2:1 8. Calculate the volume occuped by 35.2 g of methane gas (CH 4 ) at 25 C and 1.0 atm (R = 0.0821 L.atm/mol.K) A. 0.0186 L D. 49.2 L B. 4.5 L E. 53.7 C. 11.2 L
9. Calculate the densty, n g/l, of CO 2 gas at 27 C and 0.50 atm pressure. A. 0.89 g/l D. 46.0 g/l B. 1.12 g/l E. 2.17 kg/l C. 9.93 g/l 10. Whch of the followng gases wll have the greatest densty at the same specfed temperature and pressure? A. H 2 D. C 2 H 6 B. CClF 3 E. CF 4 C. CO 2 11. Two moles of chlorne gas at 20.0 C are heated to 350 C whle the volume s kept constant. The densty of the gas: A. ncreases. D. Not enough nformaton s gven to correctly B. decreases. answer the queston. C. remans the same. 12. You have two samples of the same gas n the same sze contaner, wth the same pressure. The gas n the frst contaner has a Kelvn temperature four tmes that of the gas n the other contaner. The rato of the number of moles of gas n the frst contaner compared to that n the second s: A. 1:1 B. 4:1 C. 1:4 D. 2:1 E. 1:2 13. How many lters of chlorne gas at 200 C and 0.500 atm can be produced by the reacton of 12.0 g of MnO 2 wth HCl as follows? MnO 2 (s) + 4HCl(aq) MnCl 2 (aq) + 2H 2 O(l) + Cl 2 (g) A. 10.7 L B. 3.09 L C. 4.53 L D. 0.138 L E. 0.093 L 14. If equal masses of O 2 (g) and HBr(g) are n separate contaners of equal volume and temperature, whch one of the followng statements s true? A. The pressure n the O 2 contaner s greater than that n the HBr contaner. B. There are more HBr molecules than O 2 molecules. C. The average velocty of the O 2 molecules s less than that of the HBr molecules. D. The average knetc energy of HBr molecules s greater than that of O 2 molecules. E. The pressures of both gases are the same. 15. Whch of the followng IS NOT a postulate of the knetc-molecular theory of gases? A. All gases consst of atoms and molecules movng randomly n all drectons at varous speeds. B. The average knetc energy of gas molecules ncreases as the temperature ncreases. C. The volume of a gas molecule s neglgble when compared to the space that separates t from other gas molecules. D. Gas molecules wll eventually stop colldng due to the energy lost n each collson. E. There are no attractve or repulsve forces between gas molecules. 16. Whch of the followng gas molecules have the hghest average knetc energy at 25 C? A. H 2 D. Cl 2 B. O 2 E. All the gases have the same average knetc energy C. N 2 17. Oxygen gas at 1.000 atm, placed n a contaner havng a pnhole openng n ts sde, leaks from the contaner 2.14 tmes faster than does and unknown gas at 1.000 atm, placed n ths same apparatus. Whch of the followng speces could be the unknown gas? A. Cl 2 B. SF 6 C. Kr D. UF 6 E. Xe 18. The van der Waals equaton, nrt = [P + a(n/v) 2 ] (V nb), ncorporates correctons to the deal gas law n order to account for the propertes of real gases. Whch of the followng propertes of a real gas s related to the b coeffcent n the van der Waals equaton?
A. Real gases consst of molecules or atoms that have volume. B. The average speed of the molecules of a real gas ncreases wth temperature. C. There are attractve forces between atoms or molecules of a real gas. D. The rate of effuson of a gas s nversely proportonal to the square root of the molecular weght of the gas. E. None of these. 19. Whch of the followng s not an example of knetc energy? A. The moton of a molecule B. The moton of a golf ball C. The vbraton of an object 20. An exothermc reacton causes the surroundngs to A. warm up. B. become acdc. C. expand. D. A loosely held brck on the top of a buldng E. The moton of electrons through a wre D. decrease ts temperature. E. release CO 2. 21. Whch statement s true? A. A postve change n enthalpy occurs wth endothermc processes. B. A postve change n enthalpy occurs wth exothermc processes. C. A postve change n enthalpy occurs when work s done on the surroundngs. D. A postve change n enthalpy occurs when work s done on the system. E. A postve change n enthalpy occurs when a process s endothermc and does work on the surroundngs. 22. For a partcular process q = 20 kj and w = 15 kj. Whch of the followng statements s true? A. Heat flows from the system to the surroundngs. B. The system does work on the surroundngs. C. E = 35 kj D. All of the above are true. E. None of the above are true. 23. Two metals of equal mass wth dfferent heat capactes are subjected to the same amount of heat. Whch undergoes the smallest change n temperature? A. The metal wth the hgher heat capacty. B. The metal wth the lower heat capacty. C. Both undergo the same change n temperature. D. You need to know the ntal temperatures of the metals. E. You need to know whch metals you have. 24. How many degrees of temperature rse wll occur when a 25.0 g block of alumnum absorbs 10.0 kj of heat? The specfc heat of Al s 0.900 J/g C. A. 0.44 C D. 360 C B. 22.5 C E. 444 C C. 225 C 25. To whch one of these reactons occurrng at 25 C does the symbol [HNO 3 (l)] refer? A. H(g) + N(g) + O 3 (g) HNO 3 (l) D. HNO 3 (l) H(g) + N(g) + 3O(g) B. (1/2)H 2 (g) + (1/2)N 2 (g) + (3/2)O 2 (g) HNO 3 (l) E. H 2 (g) + N 2 (g) + O 3 (g) HNO 3 (l) C. HNO 3 (l) (1/2)H 2 (g) + (1/2)N 2 (g) + (3/2)O 2 (g) 26. When 0.560 g of Na(s) reacts wth excess F 2 (g) to form NaF(s), 13.8 kj of heat s evolved at standard-state condtons. What s the standard enthalpy of formaton ( ) of NaF(s)? A. 24.8 kj/mol D. -7.8 kj/mol B. 570 kj/mol E. -570 kj/mol C. -24.8 kj/mol
27. Glycne, C 2 H 5 O 2 N, s mportant for bologcal energy. The combuston reacton of glycne s gven by the equaton: 4C 2 H 5 O 2 N(s) + 9O 2 (g) 8CO 2 (g) + 10H 2 O(l) + 2N 2 (g) = -3857 kj. Gven that [CO 2 (g)] = -393.5 kj/mol and [H 2 O(l)] = -285.8 kj/mol, calculate the enthalpy of formaton of glycne. A. -537.2 kj/mol B. -268.2 kj/mol C. 2,149 kj/mol D. -3,178 kj/mol E. -964 kj/mol 28. Gven H 2 (g) + (1/2)O 2 (g) H 2 O(l), H = -286 kj/mol, determne the standard enthalpy change for the reacton A. H = -286 kj/mol B. H = +286 kj/mol C. H = -572 kj/mol 2H 2 O(l) 2H 2 (g) + O 2 (g) D. H = +572 kj/mol E. H = -143 kj/mol 29. Pentaborane B 5 H 9 (s) burns vgorously n O 2 to gve B 2 O 3 (s) and H 2 O(l). Calculate for the combuston of 1 mol of B 5 H 9. [B 2 O 3 (s)] = -1,273.5 kj/mol [B 5 H 9 (s)] = 73.2 kj/mol [H 2 O(l)] = -285.8 kj/mol A. -1273.5 kj/mol B. -4,543 kj/mol C. -18,170 kj/mol D. -9,086 kj/mol E. -8,448 kj/mol 30. Accordng to the frst law of thermodynamcs: A. Energy s nether lost nor ganed n any energy transformatons. B. Perpetual moton s possble. C. Energy s conserved n qualty but not n quantty. D. Energy s beng created as tme passes. We have more energy n the unverse now than when tme began. 31. A calormeter A. s equal to the molar enthalpy of a reacton. B. s a detng ad. C. s an ndcator of a spontaneous reacton. D. s a devce used to measure the transfer of heat energy. E. s useful n measurng the amount of heat released by endothermc reactons. 32. For whch of these reactons wll the dfference between H and E be the greatest? A. 2H 2 O 2 (l) 2H 2 O(l) + O 2 (g) D. 2C 2 H 6 (g) + 7O 2 (g) 4CO 2 (g) + 6H 2 O(l) B. CaCO 3 (s) CaO(s) +CO 2 (g) E. 4NH 3 (g) + 5O 2 (g) 4NO(g) + 6H 2 O(g) C. NO(g) + O 3 (g) NO 2 (g) + O 2 (g) PART II: SHORT ANSWER (Each short answer queston has a 1-pont value, 8 ponts total) 33. The gas law, whch relates pressure and volume for a sample of gas at constant temperature, s known as the Boyle s law. 34. Accordng to knetc-molecular theory, gas partcles undergo elastc collsons. 35. The nternal energy of a system s the sum of the knetc and potental energes of all the partcles n the system. 36. Accordng to the Frst Law of Thermodynamcs, the total energy of the unverse s constant. 37. Heat nvolves the transfer of energy between two objects due to a temperature dfference. 38. The quantty of energy requred to ncrease the temperature of one gram of a sample by 1 C s called specfc heat. 39. In an endothermc reacton, heat s transferred from the surroundngs to the system. 40. Enthalpy change s equal to heat transfer at constant pressure.
PART III: CONCEPTS 41. (3 pts) For a sample of gas at constant pressure, what s T n the table below? We use Charles law to arrve at: V 1 /V 2 = T 1 /T 2 The fnal temperature T 2 = (V 2 x T 1 )/V 1 = 182 K 42. (5 pts) Below s gven a process that results n a change of both volume and temperature. a. Has any work been done? If so, s the work postve or negatve wth respect to the system? There s a change n volume so work was done. Snce the fnal volume s greater than the ntal, the system has expanded aganst the surroundngs and w system < 0 (.e. the system does work on the surroundngs) b. Has there been an enthalpy change? If so, s the reacton exothermc or endothermc? There s a change of temperature, so heat transfer occurred as well. Snce T fnal > T ntal, q > 0. Hence the process s endothermc. 43. (4 pts) A reacton s carred out n a cylnder, ftted wth a movable pston (see below). The startng volume s 5.00 L. At constant pressure of 1 atm the changes n enthalpy and energy are: H = -35.0 kj and E = -34.8 kj. Does the volume ncrease, decrease or reman the same? Soluton: We know that E = H + w = H - p V p V = H - E p V < 0 But the pressure s constant and p > 0. Therefore V < 0. V = V f V < 0 V f < V.e. the volume decreased 44. (4 pts) Consder the followng numbered processes: 1. A 2B H 1 2. B C + D H 2 3. E 2D H 3 H for the process A 2C + E s: A. H 1 + H 2 + H 3 B. H 1 + H 2 C. H 1 + H 2 H 3 D. H 1 + 2 H 2 H 3 E. H 1 + 2 H 2 + H 3 Soluton: In order to arrve at the equaton A 2C + E, we need to retan (1), multply (2) by a factor of 2, and reverse (3). Thus, H = H 1 + 2 H 2 H 3, whch corresponds to answer (D).
PART IV: CALCULATION PROBLEMS (Show your work n ts entrety. Do not provde just a sngle number!). 45. (6 pts) When 21.45 g of KNO 3 was dssolved n water n a calormeter, the temperature fell from 25.00 o C to 14.14 o C. The heat capacty of the calormeter s 682 J/ o C. Wrte the complete thermochemcal equaton for the process: KNO 3 (s) K + (aq) + NO 3 - (aq) Soluton: It s based on the fact that q soln = - q cal q cal = (heat capacty) calormeter x T = C x T = 682 J/ o C (14.14 25.00 o C) = 682 x (- 10.86 o C) = - 7407 J Therefore q soln = + 7407 J Ths s the amount of heat absorbed by dssolvng 21.45 g x 1 mol/101.11 g = 0.2121 mol of KNO 3 Per mole: 7407 J/0.2121 mol KNO 3 = 34922 J/1 mol KNO 3 = 34.92 kj/1 mol KNO 3 KNO 3 (s) K + (aq) + NO 3 - (aq) H o = + 34.92 kj 46. (6 pts) Determne the enthalpy change for the process: C 2 H 4 (g) + 6 HCl (g) 2 CHCl 3 (g) + 4 H 2 (g) usng the nformaton gven below: 2 C (s) + 2 H 2 (g) C 2 H 4 (g) H = 52.3 kj H 2 (g) + Cl 2 (g) 2 HCl (g) H = -184.6 kj C (s) + ½ H 2 (g) + 3/2 Cl 2 (g) CHCl 3 (g) H = -103.1 kj Soluton: We begn by dentfyng reactants and products n the equaton n queston and also where they appear n the set of equatons. The followng changes/manpulatons are necessary: Equaton 1: Reverse: C 2 H 4 (g) 2 C (s) + 2 H 2 (g) H o = - 52.3 kj Equaton 2: Reverse and multply by 3: 6 HCl (g) 3 H 2 (g) + 3 Cl 2 (g) H o = +553.8 kj Equaton 3: Multply by 2: 2 C (s) + H 2 (g) + 3 Cl 2 (g) 2 CHCl 3 (g) H o = -206.2 kj Inspecton now shows that addng the three new equatons gves precsely the process under study. Therefore the summng of the enthalpy changes must gve us the enthalpy change for the above process: H o = -52.3 + 553.8 206.2 = + 295.3 kj 47. (3 pts) BONUS PROBLEM (In order to receve credt, you have to solve the problem completely!). A mxture was made form the followng: 25.0 g of water at 15.0 o C, 45.0 g of water at 50.0 o C, and 15.0 g of water at 37.0 o C. What s the fnal temperature of the mxture? Soluton: It s based on the fact that: q 1 + q 2 + q 3 = 0, due to the law of conservaton of energy c x m 1 x ( T) 1 + c x m 2 x ( T) 2 + c x m 3 x ( T) 3 = 0 m 1 x ( T) 1 + m 2 x ( T) 2 + m 3 x ( T) 3 = 0
m 1 x (T f T 1 ) + m 2 x (T f T 2 ) + m 3 x (T f T 3 ) = 0 (m 1 + m 2 + m 3 ) x T f = m 1 x T 1 + m 2 x T 2 + m 3 x T 3 T f = (m 1 x T 1 + m 2 x T 2 + m 3 x T 3 ) / (m 1 + m 2 + m 3 ) T f = (25.0 x 15.0 + 45.0 x 50.0 + 15.0 x 37.0) / (25.0 + 45.0 + 15.0) = 37.4 o C T f = 37.4 o C