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Unformatted text preview: 2000 Advanced Placement Program® Free-Response Questions The materials included in these files are intended for use by AP® teachers for course and exam preparation in the classroom; permission for any other use must be sought from the Advanced Placement Program. Teachers may reproduce them, in whole or in part, in limited quantities, for face-to-face teaching purposes but may not mass distribute the materials, electronically or otherwise. These materials and any copies made of them may not be resold, and the copyright notices must be retained as they appear here. This permission does not apply to any third-party copyrights contained herein. These materials were produced by Educational Testing Service (ETS), which develops and administers the examinations of the Advanced Placement Program for the College Board. The College Board and Educational Testing Service (ETS) are dedicated to the principle of equal opportunity, and their programs, services, and employment policies are guided by that principle. The College Board is a national nonprofit membership association dedicated to preparing, inspiring, and connecting students to college and opportunity. Founded in 1900, the association is composed of more than 3,800 schools, colleges, universities, and other educational organizations. Each year, the College Board serves over three million students and their parents, 22,000 high schools, and 5,000 colleges, through major programs and services in college admission, guidance, assessment, financial aid, enrollment, and teaching and learning. Among its best-known programs are the SAT®, the PSAT/NMSQT®, the Advanced Placement Program® (AP®), and Pacesetter®. The College Board is committed to the principles of equity and excellence, and that commitment is embodied in all of its programs, services, activities, and concerns. Copyright © 2000 by College Entrance Examination Board and Educational Testing Service. All rights reserved. College Board, Advanced Placement Program, AP, and the acorn logo are registered trademarks of the College Entrance Examination Board. 22 8 9 27 28 29 30 -2- Ba Cs *La 57 Zr Hf 72 91.22 V Ta 73 92.91 Nb 41 50.94 Cr W 74 95.94 Mo 42 52.00 Mn Re 75 (98) Tc 43 54.938 †Actinide Series *Lanthanide Series Db (262) Rf †Ac 105 Pr 58 Ce Nd 60 (263) Sg 106 Pa Th U 92 Np 93 (145) Pm 61 (262) Bh 107 232.04 231.04 238.03 237.05 91 90 140.12 140.91 144.24 59 (261) 104 89 226.02 227.03 Ra Fr (223) 88 87 132.91 137.33 138.91 178.49 180.95 183.85 186.21 56 55 88.91 Y Sr 87.62 Rb 85.47 40 39 38 37 Ti 47.90 44.96 Ca 40.08 K 39.10 (244) Pu 94 150.4 Sm 62 (265) Hs 108 190.2 Os 76 101.1 Ru 44 55.85 Fe Ni Pd 46 58.69 Cu Ag 47 63.55 Zn Cd 48 65.39 S 16 30.974 33 28.09 32 26.98 31 Ar 18 Kr I Te Sb Sn Xe 54 In 53 50 49 52 72.59 69.72 51 83.80 Br 79.90 Se 78.96 As 74.92 Ge 36 35 35.453 39.948 Cl 17 Ga 34 32.06 P 15 Si 14 Al 13 19.00 16.00 Au 79 Hg 80 Tl 81 Gd 64 (269) § 110 Tb 65 (272) § 111 Dy 66 (277) § 112 207.2 Pb 82 208.98 Bi 83 Ho 67 Er 68 Tm 69 §Not yet named 195.08 196.97 200.59 204.38 Pt 78 Yb 70 (209) Po 84 Lu 71 (210) At 85 (243) Am 95 (247) Cm 96 (247) Bk 97 (251) Cf 98 (252) Es 99 (257) Fm 100 (258) Md 101 (259) No 102 (260) Lr 103 151.97 157.25 158.93 162.50 164.93 167.26 168.93 173.04 174.97 Eu 63 (266) Mt 109 192.2 Ir 77 (222) Rn 86 102.91 106.42 107.87 112.41 114.82 118.71 121.75 127.60 126.91 131.29 Rh 45 58.93 Co Sc 26 20 19 25 24.30 22.99 24 Mg Na 23 12 Ne 20.179 F 11 10.811 12.011 14.007 O 9.012 6.941 N Be Li C 10 B 4.0026 6 3 5 1.0079 He 21 7 2 4 PERIODIC TABLE OF THE ELEMENTS H 1 DO NOT DETACH FROM BOOK. INFORMATION IN THE TABLE BELOW AND IN THE TABLES ON PAGES 3-5 MAY BE USEFUL IN ANSWERING THE QUESTIONS IN THIS SECTION OF THE EXAMINATION. GO ON TO THE NEXT PAGE. STANDARD REDUCTION POTENTIALS IN AQUEOUS SOLUTION AT 25°C E °( V ) Half-reaction Li + + e - + K + eRb + + e Ba 2 + + 2 e Sr 2 + + 2 e Ca 2 + + 2 e Na + + e Mg 2 + + 2 e Be 2 + + 2 e Al 3 + + 3 e Mn 2 + + 2 e Zn 2 + + 2 e Cr 3 + + 3 e Fe 2 + + 2 e Cr 3 + + e Cd 2 + + 2 e Tl + + e Co 2 + + 2 e Ni 2 + + 2 e Sn 2 + + 2 e Pb 2 + + 2 e 2 H + + 2 eS( s) + 2 H + + 2 e Sn 4 + + 2 e Cu 2 + + e Cu 2 + + 2 e Cu + + e I 2 ( s) + 2 e Fe 3 + + e Hg 2 2 + + 2 e Ag + + e Hg 2 + + 2 e 2 Hg 2 + + 2 e Br 2 (l ) + 2 e - “ “ “ “ “ “ “ “ “ “ “ “ “ “ “ “ “ “ “ “ “ “ “ “ “ “ “ “ “ “ “ “ “ “ “ Li( s) Cs( s) K ( s) Rb( s) Ba ( s) Sr( s) Ca ( s) Na ( s) Mg( s) Be( s) Al( s) Mn( s) Zn( s) Cr ( s) Fe( s) Cr 2 + Cd( s) Tl( s) Co( s) Ni( s) Sn( s) Pb( s) H 2 ( g) H 2S ( g) Sn 2 + Cu + Cu( s) Cu( s) 2 IFe 2 + 2 Hg(l ) Ag( s) Hg(l ) Hg 2 2 + 2 Br - -3.05 -2.92 -2.92 -2.92 -2.90 -2.89 -2.87 -2.71 -2.37 -1.70 -1.66 -118 . -0.76 -0.74 -0.44 -0.41 -0.40 -0.34 -0.28 -0.25 -0.14 -0.13 0.00 0.14 0.15 0.15 0.34 0.52 0.53 0.77 0.79 0.80 0.85 0.92 1.07 O 2 ( g) + 4 H + + 4 e Cl 2 ( g ) + 2 e Au 3 + + 3 e Co 3 + + e F2 ( g ) + 2 e - “ “ “ “ “ 2 H 2 O(l ) 2 Cl Au( s) Co 2 + 2 F- 1.23 1.36 1.50 1.82 2.87 Cs + e- + GO ON TO THE NEXT PAGE. -3- ADVANCED PLACEMENT CHEMISTRY EQUATIONS AND CONSTANTS ATOMIC STRUCTURE D E = hv c = lv = = = = u= n= m= E v l p h l = mu p = mu En = −2.178 × 10 −18 joule n2 EQUILIBRIUM Ka = Speed of light, c = 3.0 × 10 8 m s −1 [H + ] [A − ] [ HA ] Planck’s constant, h = 6.63 × 10 −34 J s [OH − ] [ HB + ] Kb = [ B] Boltzmann’s constant, k = 1.38 × 10 −23 J K −1 Avogadro’s number = 6.022 × 10 23 molecules mol −1 K w = [OH − ] [ H + ] = 1.0 × 10 −14 @ 25o C Electron charge, e = −1.602 × 10 −19 coulomb = Ka × K b pH = − log [ H + ], pOH = − log [OH − ] 14 = pH + pOH pH = pKa + log 1 electron volt per atom = 96.5 kJ mol −1 [A − ] [ HA ] Equilibrium Constants [HB + ] pOH = pKb + log [ B] pKa = − log Ka , pKb = − log Kb K p = Kc ( RT ) Dn energy frequency wavelength momentum velocity principal quantum number mass Ka Kb Kw Kp , where D n = moles product gas − moles reactant gas (weak acid) (weak base) (water) (gas pressure) Kc (molar concentrations) S o = standard entropy THERMOCHEMISTRY H o = standard enthalpy G o = standard free energy ∑ S o products −∑ S o reactants = ∑ DHfo products − ∑ DH fo reactants = ∑ DGfo products − ∑ DGfo reactants DS o = DH o DG o Eo T n m q c Cp DG o = DH o − TD S o = − RT ln K = −2.303 RT log K = −n Eo 1 faraday DG = DG o + RT ln Q = DG o + 2.303 RT log Q q = mcDT DH Cp = DT = = = = = = = standard reduction potential temperature moles mass heat specific heat capacity molar heat capacity at constant pressure = 96,500 coulombs GO ON TO THE NEXT PAGE. -4- GASES, LIQUIDS, AND SOLUTIONS PV = nRT  P + n a  (V - nb) = nRT  V 2 2 PA = Ptotal ™ X A , where X A = moles A total moles Ptotal = PA + PB + PC + . . . m n= M K = o C + 273 P1V1 PV = 22 T1 T2 m D= V 3kT 3 RT urms = = m M 1 KE per molecule = mu 2 2 3 KE per mole = RT 2 r1 M2 = r2 M1 = = = = = = u= P V T n D m pressure volume temperature number of moles density mass velocity urms KE r M p i Kf root-mean-square speed kinetic energy rate of effusion molar mass osmotic pressure van’t Hoff factor molal freezing-point depression constant Kb Q I q t molarity, M = moles solute per liter solution molality = moles solute per kilogram solvent DT f = iK f ™ molality = = = = = = = = = = = = molal boiling-point elevation constant reaction quotient current (amperes) charge (coulombs) time (seconds) E o = standard reduction potential K = equilibrium constant DTb = iKb ™ molality p = nRT i V Gas constant, R = 8.31 J mol −1 K −1 = 0.0821 L atm mol −1 K −1 = 8.31 volt coulomb mol −1 K −1 OXIDATION-REDUCTION; ELECTROCHEMISTRY Boltzmann’s constant, k = 1.38 × 10 −23 J K −1 K f for H 2 O = 1.86 K kg mol −1 Q= [C ] [ D ] c d a b Kb for H 2 O = 0.512 K kg mol −1 , where a A + b B → c C + d D [ A ] [ B] q I= t RT 0.0592 o o E cell = E cell − ln Q = E cell − log Q @ 25o C n n log K = STP = 0.000 o C and 1.000 atm Faraday’s constant, = 96,500 coulombs per mole of electrons nE o 0.0592 GO ON TO THE NEXT PAGE. -5- 2000 AP® CHEMISTRY FREE-RESPONSE QUESTIONS CHEMISTRY—SECTION II (Total time—90 minutes) Part A Time—40 minutes YOU MAY USE YOUR CALCULATOR FOR PART A. CLEARLY SHOW THE METHOD USED AND STEPS INVOLVED IN ARRIVING AT YOUR ANSWERS. It is to your advantage to do this, because you may earn partial credit if you do and you will receive little or no credit if you do not. Attention should be paid to significant figures. Be sure to write all your answers to the questions on the lined pages following each question in this booklet. Answer Question 1 below. The Section II score weighting for this question is 20 percent. 2 H2S(g) → 2 H2(g) + S2(g) ← 1. When heated, hydrogen sulfide gas decomposes according to the equation above. A 3.40 g sample of H2S(g) is introduced into an evacuated rigid 1.25 L container. The sealed container is heated to 483 K, and 3.72 × 10−2 mol of S2(g) is present at equilibrium. (a) Write the expression for the equilibrium constant, Kc , for the decomposition reaction represented above. (b) Calculate the equilibrium concentration, in mol L-1, of the following gases in the container at 483 K. (i) H2(g) (ii) H2S(g) (c) Calculate the value of the equilibrium constant, Kc , for the decomposition reaction at 483 K. (d) Calculate the partial pressure of S2(g) in the container at equilibrium at 483 K. → (e) For the reaction H2(g) + 1 S2(g) ← H2S(g) at 483 K, calculate the value of the equilibrium constant, Kc . 2 Copyright © 2000 College Entrance Examination Board and Educational Testing Service. All rights reserved. AP is a registered trademark of the College Entrance Examination Board. GO ON TO THE NEXT PAGE. -6- 2000 AP® CHEMISTRY FREE-RESPONSE QUESTIONS Answer EITHER Question 2 below OR Question 3 printed on page 8. Only one of these two questions will be graded. If you start both questions, be sure to cross out the question you do not want graded. The Section II score weighting for the question you choose is 20 percent. 2. Answer the following questions that relate to electrochemical reactions. (a) Under standard conditions at 25°C, Zn(s) reacts with Co2+(aq) to produce Co(s). (i) Write the balanced equation for the oxidation half reaction. (ii) Write the balanced net-ionic equation for the overall reaction. (iii) Calculate the standard potential, E°, for the overall reaction at 25°C. (b) At 25°C, H2O2 decomposes according to the following equation. 2 H2O2(aq) → 2 H2O(l) + O2(g) E° = 0.55 V (i) Determine the value of the standard free energy change, ∆G° , for the reaction at 25°C. (ii) Determine the value of the equilibrium constant, Keq , for the reaction at 25°C. (iii) The standard reduction potential, E°, for the half reaction O2(g) + 4 H+(aq) + 4 e− → 2 H2O(l) has a value of 1.23 V. Using this information in addition to the information given above, determine the value of the standard reduction potential, E°, for the half reaction below. O2(g) + 2 H+(aq) + 2 e- → H2O2(aq) (c) In an electrolytic cell, Cu(s) is produced by the electrolysis of CuSO4(aq). Calculate the maximum mass of Cu(s) that can be deposited by a direct current of 100. amperes passed through 5.00 L of 2.00 M CuSO4(aq) for a period of 1.00 hour. Copyright © 2000 College Entrance Examination Board and Educational Testing Service. All rights reserved. AP is a registered trademark of the College Entrance Examination Board. GO ON TO THE NEXT PAGE. -7- 2000 AP® CHEMISTRY FREE-RESPONSE QUESTIONS 3. Answer the following questions about BeC2O4(s) and its hydrate. (a) Calculate the mass percent of carbon in the hydrated form of the solid that has the formula BeC2O4 3 H2O (b) When heated to 220.•C, BeC2O4 3 H2O(s) dehydrates completely as represented below. BeC2O4 3 H2O(s) “ BeC2O4(s) + 3 H2O(g) If 3.21 g of BeC2O4 3 H2O(s) is heated to 220.•C, calculate (i) the mass of BeC2O4(s) formed, and, (ii) the volume of the H2O(g) released, measured at 220.•C and 735 mm Hg. (c) A 0.345 g sample of anhydrous BeC2O4 , which contains an inert impurity, was dissolved in sufficient water to produce 100. mL of solution. A 20.0 mL portion of the solution was titrated with KMnO4(aq). The balanced equation for the reaction that occurred is as follows. 16 H+(aq) + 2 MnO4-(aq) + 5 C2O42-(aq) → 2 Mn2+(aq) + 10 CO2(g) + 8 H2O(l). The volume of 0.0150 M KMnO4(aq) required to reach the equivalence point was 17.80 mL. (i) Identify the reducing agent in the titration reaction. (ii) For the titration at the equivalence point, calculate the number of moles of each of the following that reacted. • MnO4-(aq) • C2O42-(aq) (iii) Calculate the total number of moles of C2O42-(aq) that were present in the 100. mL of prepared solution. (iv) Calculate the mass percent of BeC2O4(s) in the impure 0.345 g sample. Copyright © 2000 College Entrance Examination Board and Educational Testing Service. All rights reserved. AP is a registered trademark of the College Entrance Examination Board. GO ON TO THE NEXT PAGE. -8- 2000 AP® CHEMISTRY FREE-RESPONSE QUESTIONS CHEMISTRY Part B Time—50 minutes NO CALCULATORS MAY BE USED FOR PART B. Answer Question 4 below. The Section II score weighting for this question is 15 percent. 4. Write the formulas to show the reactants and the products for any FIVE of the laboratory situations described below. Answers to more than five choices will not be graded. In all cases, a reaction occurs. Assume that solutions are aqueous unless otherwise indicated. Represent substances in solution as ions if the substances are extensively ionized. Omit formulas for any ions or molecules that are unchanged by the reaction. You need not balance the equations. Example: A strip of magnesium is added to a solution of silver nitrate. (a) A small piece of calcium metal is added to hot distilled water. (b) Butanol is burned in air. (c) Excess concentrated ammonia solution is added to a solution of nickel(II) sulfate. (d) A solution of copper(II) chloride is added to a solution of sodium sulfide. (e) A solution of tin(II) nitrate is added to a solution of silver nitrate. (f) Excess hydrobromic acid solution is added to a solution of potassium hydrogen carbonate. (g) Powdered strontium oxide is added to distilled water. (h) Carbon monoxide gas is passed over hot iron(III) oxide. Copyright © 2000 College Entrance Examination Board and Educational Testing Service. All rights reserved. AP is a registered trademark of the College Entrance Examination Board. GO ON TO THE NEXT PAGE. -9- 2000 AP® CHEMISTRY FREE-RESPONSE QUESTIONS Your responses to the rest of the questions in this part of the examination will be graded on the basis of the accuracy and relevance of the information cited. Explanations should be clear and well organized. Examples and equations may be included in your responses where appropriate. Specific answers are preferable to broad, diffuse responses. Answer BOTH Question 5 below AND Question 6 printed on page 11. Both of these questions will be graded. The Section II score weighting for these questions is 30 percent (15 percent each). 5. The molar mass of an unknown solid, which is nonvolatile and a nonelectrolyte, is to be determined by the freezing-point depression method. The pure solvent used in the experiment freezes at 10°C and has a known molal freezing-point depression constant, Kf . Assume that the following materials are also available. œ test tubes œ beaker œ stirrer œ stopwatch œ pipet œ graph paper œ thermometer œ hot-water bath œ balance œ ice (a) Using the two sets of axes provided below, sketch cooling curves for (i) the pure solvent and for (ii) the solution as each is cooled from 20°C to 0.0°C. (b) Information from these graphs may be used to determine the molar mass of the unknown solid. (i) Describe the measurements that must be made to determine the molar mass of the unknown solid by this method. (ii) Show the setup(s) for the calculation(s) that must be performed to determine the molar mass of the unknown solid from the experimental data. (iii) Explain how the difference(s) between the two graphs in part (a) can be used to obtain information needed to calculate the molar mass of the unknown solid. (c) Suppose that during the experiment a significant but unknown amount of solvent evaporates from the test tube. What effect would this have on the calculated value of the molar mass of the solid (i.e., too large, too small, or no effect)? Justify your answer. (d) Show the setup for the calculation of the percentage error in a student’s result if the student obtains a value of 126 g mol−1 for the molar mass of the solid when the actual value is 120. g mol−1. Copyright © 2000 College Entrance Examination Board and Educational Testing Service. All rights reserved. AP is a registered trademark of the College Entrance Examination Board. GO ON TO THE NEXT PAGE. -10- 2000 AP® CHEMISTRY FREE-RESPONSE QUESTIONS O3(g) + NO(g) → O2(g) + NO2(g) 6. Consider the reaction represented above. (a) Referring to the data in the table below, calculate the standard enthalpy change, ∆ H•, for the reaction at 25°C. Be sure to show your work. O3(g) Standard enthalpy of formation, D H fo , at 25°C NO(g) NO2(g) 143 90. 33 (kJ mol-1) (b) Make a qualitative prediction about the magnitude of the standard entropy change, ∆S°, for the reaction at 25°C. Justify your answer. (c) On the basis of your answers to parts (a) and (b), predict the sign of the standard free-energy change, ∆G°, for the reaction at 25°C. Explain your reasoning. (d) Use the information in the table below to write the rate-law expression for the reaction, and explain how you obtained your answer. Experiment Number Initial [O3] (mol L−1) Initial [NO] (mol L−1) 1 2 3 4 0.0010 0.0010 0.0020 0.0020 0.0010 0.0020 0.0010 0.0020 Initial Rate of Formation of NO2 (mol L−1 s−1) x 2x 2x 4x (e) The following three-step mechanism is proposed for the reaction. Identify the step that must be the slowest in order for this mechanism to be consistent with the rate-law expression derived in part (d). Explain. Step I: O3 + NO → O + NO3 Step II: O + O3 → 2 O2 Step III: NO3 + NO → 2 NO2 Copyright © 2000 College Entrance Examination Board and Educational Testing Service. All rights reserved. AP is a registered trademark of the College Entrance Examination Board. GO ON TO THE NEXT PAGE. -11- 2000 AP® CHEMISTRY FREE-RESPONSE QUESTIONS Answer EITHER Question 7 below OR Question 8 printed on page 13. Only one of these two questions will be graded. If you start both questions, be sure to cross out the question you do not want graded. The Section II score weighting for the question you choose is 15 percent. 7. Answer the following questions about the element selenium, Se (atomic number 34). (a) Samples of natural selenium contain six stable isotopes. In terms of atomic structure, explain what these isotopes have in common, and how they differ. (b) Write the complete electron configuration (e.g., 1s2 2s2. . . etc.) for a selenium atom in the ground state. Indicate the number of unpaired electrons in the ground-state atom, and explain your reasoning. (c) In terms of atomic structure, explain why the first ionization energy of selenium is (i) less than that of bromine (atomic number 35), and (ii) greater than that of tellurium (atomic number 52). (d) Selenium reacts with fluorine to form SeF4. Draw the complete Lewis electron-dot structure for SeF4 and sketch the molecular structure. Indicate whether the molecule is polar or nonpolar, and justify your answer. Copyright © 2000 College Entrance Examination Board and Educational Testing Service. All rights reserved. AP is a registered trademark of the College Entrance Examination Board. GO ON TO THE NEXT PAGE. -12- 2000 AP® CHEMISTRY FREE-RESPONSE QUESTIONS 8. A volume of 30.0 mL of 0.10 M NH3(aq) is titrated with 0.20 M HCl(aq). The value of the base-dissociation constant, Kb , for NH3 in water is 1.8 ™ 10-5 at 25•C. (a) Write the net-ionic equation for the reaction of NH3(aq) with HCl(aq). (b) Using the axes provided below, sketch the titration curve that results when a total of 40.0 mL of 0.20 M HCl(aq) is added dropwise to the 30.0 mL volume of 0.10 M NH3(aq). (c) From the table below, select the most appropriate indicator for the titration. Justify your choice. Indicator pKa Methyl Red 5.5 Bromothymol Blue 7.1 Phenolphthalein 8.7 (d) If equal volumes of 0.10 M NH3(aq) and 0.10 M NH4Cl(aq) are mixed, is the resulting solution acidic, neutral, or basic? Explain. END OF EXAMINATION Copyright © 2000 College Entrance Examination Board and Educational Testing Service. All rights reserved. AP is a registered trademark of the College Entrance Examination Board. -13- ...
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