ap04_frq_chemistry_b

ap04_frq_chemistry_b - AP® Chemistry 2004 Free-Response...

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Unformatted text preview: AP® Chemistry 2004 Free-Response Questions Form B The materials included in these files are intended for noncommercial use by AP teachers for course and exam preparation; 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. This permission does not apply to any third-party copyrights contained herein. These materials and any copies made of them may not be resold, and the copyright notices must be retained as they appear here. The College Board is a not-for-profit membership association whose mission is to connect students to college success and opportunity. Founded in 1900, the association is composed of more than 4,500 schools, colleges, universities, and other educational organizations. Each year, the College Board serves over three million students and their parents, 23,000 high schools, and 3,500 colleges through major programs and services in college admissions, guidance, assessment, financial aid, enrollment, and teaching and learning. Among its best-known programs are the SAT®, the PSAT/NMSQT®, and the Advanced Placement Program® (AP®). The College Board is committed to the principles of excellence and equity, and that commitment is embodied in all of its programs, services, activities, and concerns. For further information, visit www.collegeboard.com Copyright © 2004 College Entrance Examination Board. All rights reserved. College Board, Advanced Placement Program, AP, AP Central, AP Vertical Teams, APCD, Pacesetter, Pre-AP, SAT, Student Search Service, and the acorn logo are registered trademarks of the College Entrance Examination Board. PSAT/NMSQT is a registered trademark jointly owned by the College Entrance Examination Board and the National Merit Scholarship Corporation. Educational Testing Service and ETS are registered trademarks of Educational Testing Service. Other products and services may be trademarks of their respective owners. For the College Board’s online home for AP professionals, visit AP Central at apcentral.collegeboard.com. 22 8 9 2 39 56 Ba 55 Cs *La 57 88.91 Hf 72 91.22 Zr 40 Ta 73 92.91 Nb 41 W 74 95.94 Mo 42 Re 75 (98) Tc 43 †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 Sr 87.62 Rb Y 38 85.47 37 (244) Pu 94 150.4 Sm 62 (265) Hs 108 190.2 Os 76 101.1 Ru 44 55.85 Ni Pd 46 58.69 Cu Ag 47 63.55 Zn Cd 48 65.39 Ar 18 51 Sb Sn In Te 52 54 Xe 53 Kr 83.80 Br 79.90 Se 78.96 As 74.92 36 35.453 39.948 Cl 17 35 16 34 15 50 14 49 13 19.00 16.00 I 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 Ge 54.938 Fe 72.59 52.00 Mn Ga 50.94 Cr 69.72 47.90 V Sc Ca 40.08 K 39.10 Ti 33 44.96 32.06 30.974 30 32 29 28.09 28 31 27 26.98 26 20 19 25 24.30 22.99 24 Mg Na 23 S P Si Al 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. STANDARD REDUCTION POTENTIALS IN AQUEOUS SOLUTION AT 25°C E °( V ) Half-reaction F2 ( g) + 2 e − 3+ Co + e− 3+ Au → O 2 ( g) + 4 H + +4 +2 + Fe 3+ Cu Cu + 4+ Sn 0.77 2I Cu( s) 0.52 Cu( s) 0.34 + Cu + 2+ 0.53 0.15 → +2 Sn → e− H 2 S ( g) 0.14 0.00 0.15 → Pb( s) −0.13 +2 e− → Sn( s) −0.14 +2 e− → Ni( s) −0.25 +2 e− → Co( s) −0.28 → Tl( s) −0.34 → Cd( s) −0.40 2+ e− + 2+ e− +2 3+ + e− 2+ 3+ 2+ Zn → Fe( s) −0.44 + 3 e− → Cr( s) −0.74 + 2 e− → Zn( s) −0.76 + 2 e− → Mn( s) −118 . +3 e− → Al( s) −1.66 +2 e− → Be( s) −1.70 +2 e− → Mg( s) −2.37 2+ + Na + → Na( s) −2.71 +2 → Ca( s) −2.87 +2 e− → Sr( s) −2.89 + 2 e− → Ba( s) −2.90 → Rb( s) −2.92 + e− → K( s ) −2.92 + e− → Cs( s) −2.92 + e− → Li( s) −3.05 2+ Rb + + + + e− e− 2+ 2+ Ba −0.41 2+ Mg Ca Cr + 2 e− 3+ Be → 2+ 2+ Mn Li − + 2 e− + Cs Fe H 2 ( g) Cd K 0.79 2+ → Co Sr 2 Hg(l ) + 2 e− 2+ Ni Al 0.80 2+ Sn Cr Ag( s) 2+ Pb Fe 0.85 + 2H Cr Hg(l ) → e− S( s) + 2 H Tl 0.92 → e− +2 Hg 2 2 + → e− +2 2+ 1.07 → + e− 2+ 2 → +2 e− I 2 (s) + 2 e − Cu 1.23 Br − → e− + e− + 1.36 → e− Hg 2 2 H 2 O(l ) 1.50 → 2+ + → 1.82 → 2 Hg 2 + + 2 e − 2+ 2 Cl − 2.87 → e− Br 2 (l ) + 2 Ag Au( s) → e− Co → e− Cl 2 ( g) + 2 Hg 2+ → e− +3 2 F− + e− 3 ADVANCED PLACEMENT CHEMISTRY EQUATIONS AND CONSTANTS Gas constant, R kt 8.31 J mol = ln A 4 1 K 1 0.0821 L atm mol 1 K 8.31 volt coulomb mol 1 1 - kt 96,500 coulombs per mole of electrons K 1 - Faraday's constant, = = + j e = D= D - - Ea 1 RT 2.303 RT log Q - 1 A0 Ea activation energy k = rate constant A = frequency factor - 0 -= D D= D - t + ln A G standard reduction potential temperature moles mass heat specific heat capacity molar heat capacity at constant pressure = G RT ln Q mc T H T standard free energy - -= 1 A E D= D -= t 2.303 RT log K -= D ln A Gf reactants standard enthalpy - D Cp H f reactants + D D= n E T n m q c Cp standard entropy = D- RT ln K D ÂÂ- TS S reactants D DÂ = DÂ = Â= H G q = ln k Â- G G = = = = = = = = D H = S = Gf products 1 (weak acid) (weak base) (water) (gas pressure) = D G = Hf products coulomb - H -= , THERMOCHEMISTRY/KINETICS S products 96.5 kJ mol 19 1 Kc (molar concentrations) where n = moles product gas − moles reactant gas S ¥ = [HB + ] [ B] pKa = − log Ka , pKb = − log Kb K p = Kc ( RT ) 10 - Ka Kb Kw Kp pOH = pKb + log n 1 Equilibrium Constants [A − ] [ HA ] pH = pKa + log = - pH = − log [ H + ], pOH = − log [OH − ] 14 = pH + pOH JK 10 23 mol ¥ 1 electron volt per atom 1.602 Js - ¥ l = Ka × K b = Electron charge, e K w = [OH − ] [ H + ] = 1.0 × 10 −14 @ 25 C 6.022 10 34 23 ¥ Avogadro’s number 1.38 10 - u [OH − ] [ HB + ] [ B] Kb = 6.63 1 - - Boltzmann’s constant, k = = Planck’s constant, h 108 m s ¥ = EQUILIBRIUM [H + ] [A − ] Ka = [ HA ] 3.0 - u Speed of light, c = l= p n m ¥ hv c v h pm = m 2.178 10 18 En joule n2 velocity principal quantum number mass - = =l = = E energy frequency wavelength momentum = = =u E v ATOMIC STRUCTURE GASES, LIQUIDS, AND SOLUTIONS E K 5 23 JK 1 K 1 1 1.86 K kg mol 0.512 K kg mol = = STP Faraday' s constant, 10 = = = + = = - = - nE 0.0592 1 atm 1.38 = Æ 0.0592 log Q @ 25 C n 8.31 volt coulomb mol 1 1 - + K b for H 2 O E cell K - Boltzmann' s constant, k K f for H 2 O RT ln Q n 1 = dD q t E cell 0.0821 L atm mol ¥ cC 1 K - = = log K bB 1 - Ecell , where a A 8.31 J mol = I [ A ] a [ B] b = Q Gas constant, R - = =p =D =D = = OXIDATION-REDUCTION; ELECTROCHEMISTRY [ C ] c [ D] d standard reduction potential equilibrium constant 760 mm Hg 760 torr 0.000 C and 1.000 atm 96,500 coulombs per mole of electrons 1 - iKb molality MRT abc - moles solute per liter solution moles solute per kilogram solvent iK f molality - = A molal boiling-point elevation constant absorbance molar absorptivity path length concentration reaction quotient current (amperes) charge (coulombs) time (seconds) = = Tb Kb A a b c Q I q t - = molarity, M molality Tf Kf - = r1 r2 pressure volume temperature number of moles density mass velocity root-mean-square speed kinetic energy rate of effusion molar mass osmotic pressure van't Hoff factor molal freezing-point depression constant i ¥ ¥ = KE per mole urms KE r M u = KE per molecule C 273 P2V2 T2 m V 3kT 3 RT M m 1 2 m 2 3 RT 2 M2 M1 = = urms ... + = D PC = = = = = = = = = = = =p = = = = H G F K PV1 1 T1 + + n PB moles A total moles P V T n D m = = = PA m M Ptotal X A , where X A + Ptotal + = K J I PA nRT ¥ nb) = (V - V2 = n2 a = P nRT =u = = = = = = PV 2004 AP® CHEMISTRY FREE-RESPONSE QUESTIONS (Form B) 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 THE STEPS INVOLVED IN ARRIVING AT YOUR ANSWERS. It is to your advantage to do this, since you may obtain 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 the booklet with the goldenrod cover. Do NOT write your answers on the lavender insert. Answer Question 1 below. The Section II score weighting for this question is 20 percent. → N2(g) + 3 H2(g) ← 2 NH3(g) 1. For the reaction represented above, the value of the equilibrium constant, Kp , is 3.1 × 10 – 4 at 700. K. (a) Write the expression for the equilibrium constant, Kp , for the reaction. (b) Assume that the initial partial pressures of the gases are as follows: pN = 0.411 atm, pH = 0.903 atm, and pNH = 0.224 atm. 2 2 3 (i) Calculate the value of the reaction quotient, Q, at these initial conditions. (ii) Predict the direction in which the reaction will proceed at 700. K if the initial partial pressures are those given above. Justify your answer. (c) Calculate the value of the equilibrium constant, Kc , given that the value of Kp for the reaction at 700. K is 3.1 × 10 – 4. (d) The value of Kp for the reaction represented below is 8.3 × 10 –3 at 700. K. → NH3(g) + H2S(g) ← NH4HS(g) Calculate the value of Kp at 700. K for each of the reactions represented below. → (i) NH4HS(g) ← NH3(g) + H2S(g) → (ii) 2 H2S(g) + N2(g) + 3 H2(g) ← 2 NH4HS(g) Copyright © 2004 by College Entrance Examination Board. All rights reserved. Visit apcentral.collegeboard.com (for AP professionals) and www.collegeboard.com/apstudents (for AP students and parents). GO ON TO THE NEXT PAGE. 6 2004 AP® CHEMISTRY FREE-RESPONSE QUESTIONS (Form B) 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 related to hydrocarbons. (a) Determine the empirical formula of a hydrocarbon that contains 85.7 percent carbon by mass. (b) The density of the hydrocarbon in part (a) is 2.0 g L–1 at 50°C and 0.948 atm. (i) Calculate the molar mass of the hydrocarbon. (ii) Determine the molecular formula of the hydrocarbon. (c) Two flasks are connected by a stopcock as shown below. The 5.0 L flask contains CH4 at a pressure of 3.0 atm, and the 1.0 L flask contains C2H6 at a pressure of 0.55 atm. Calculate the total pressure of the system after the stopcock is opened. Assume that the temperature remains constant. (d) Octane, C8H18(l), has a density of 0.703 g mL–1 at 20°C. A 255 mL sample of C8H18(l) measured at 20°C reacts completely with excess oxygen as represented by the equation below. 2 C8H18(l) + 25 O2(g) → 16 CO2(g) + 18 H2O(g) Calculate the total number of moles of gaseous products formed. Copyright © 2004 by College Entrance Examination Board. All rights reserved. Visit apcentral.collegeboard.com (for AP professionals) and www.collegeboard.com/apstudents (for AP students and parents). GO ON TO THE NEXT PAGE. 7 2004 AP® CHEMISTRY FREE-RESPONSE QUESTIONS (Form B) 2 H2O2(aq) → 2 H2O(l) + O2(g) 3. Hydrogen peroxide decomposes according to the equation above. (a) An aqueous solution of H2O2 that is 6.00 percent H2O2 by mass has a density of 1.03 g mL–1. Calculate each of the following. (i) The original number of moles of H2O2 in a 125 mL sample of the 6.00 percent H2O2 solution (ii) The number of moles of O2(g) that are produced when all of the H2O2 in the 125 mL sample decomposes (b) The graphs below show results from a study of the decomposition of H2O2 . (i) Write the rate law for the reaction. Justify your answer. (ii) Determine the half-life of the reaction. (iii) Calculate the value of the rate constant, k . Include appropriate units in your answer. (iv) Determine [H2O2] after 2,000 minutes elapse from the time the reaction began. Copyright © 2004 by College Entrance Examination Board. All rights reserved. Visit apcentral.collegeboard.com (for AP professionals) and www.collegeboard.com/apstudents (for AP students and parents). 8 2004 AP® CHEMISTRY FREE-RESPONSE QUESTIONS (Form B) 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) Cadmium metal is placed in a solution of tin(II) chloride. (b) Magnesium pellets are placed in 1.0 M hydrochloric acid. (c) Sulfur in its standard state is burned in air. (d) Solutions of silver nitrate and sodium chloride are combined. (e) Solid iron(II) sulfite is heated strongly. (f) Powdered barium oxide is mixed with water. (g) Excess saturated sodium fluoride solution is added to a solution of aluminum sulfate. (h) Solid potassium carbonate is added to 1.0 M sulfuric acid. Copyright © 2004 by College Entrance Examination Board. All rights reserved. Visit apcentral.collegeboard.com (for AP professionals) and www.collegeboard.com/apstudents (for AP students and parents). GO ON TO THE NEXT PAGE. 9 2004 AP® CHEMISTRY FREE-RESPONSE QUESTIONS (Form B) 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. An experiment is performed to determine the molar mass of an unknown solid monoprotic acid, HA, by titration with a standardized NaOH solution. (a) What measurement(s) must be made to determine the number of moles of NaOH used in the titration? (b) Write a mathematical expression that can be used to determine the number of moles of NaOH used to reach the endpoint of the titration. (c) How can the number of moles of HA consumed in the titration be determined? (d) In addition to the measurement(s) made in part (a), what other measurement(s) must be made to determine the molar mass of the acid, HA ? (e) Write the mathematical expression that is used to determine the molar mass of HA. (f) The following diagram represents the setup for the titration. In the appropriate boxes below, list the chemical(s) needed to perform the titration. (g) Explain what effect each of the following would have on the calculated molar mass of HA. Justify your answers. (i) The original solid acid, HA, was not completely dry at the beginning of the experiment. (ii) The procedure called for 25 mL of H2O in the Erlenmeyer flask, but a student used 35 mL of H2O. Copyright © 2004 by College Entrance Examination Board. All rights reserved. Visit apcentral.collegeboard.com (for AP professionals) and www.collegeboard.com/apstudents (for AP students and parents). GO ON TO THE NEXT PAGE. 10 2004 AP® CHEMISTRY FREE-RESPONSE QUESTIONS (Form B) 6. The following questions refer to the electrochemical cell shown in the diagram above. (a) Write a balanced net ionic equation for the spontaneous reaction that takes place in the cell. (b) Calculate the standard cell potential, E°, for the reaction in part (a). (c) In the diagram above, (i) label the anode and the cathode on the dotted lines provided, and (ii) indicate, in the boxes below the half-cells, the concentration of AgNO3 and the concentration of Zn(NO3)2 that are needed to generate E° . (d) How will the cell potential be affected if KI is added to the silver half-cell? Justify your answer. Copyright © 2004 by College Entrance Examination Board. All rights reserved. Visit apcentral.collegeboard.com (for AP professionals) and www.collegeboard.com/apstudents (for AP students and parents). GO ON TO THE NEXT PAGE. 11 2004 AP® CHEMISTRY FREE-RESPONSE QUESTIONS (Form B) 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. → N2(g) + 2 H2(g) ← N2H4(g) ∆ H 298 = +95.4 kJ mol–1 ; ∆ S 298 = −176 J K–1 mol–1 7. Answer the following questions about the reaction represented above using principles of thermodynamics . (a) On the basis of the thermodynamic data given above, compare the sum of the bond strengths of the reactants to the sum of the bond strengths of the product. Justify your answer. (b) Does the entropy change of the reaction favor the reactants or the product? Justify your answer. (c) For the reaction under the conditions specified, which is favored, the reactants or the product? Justify your answer. (d) Explain how to determine the value of the equilibrium constant, Keq , for the reaction. (Do not do any calculations.) (e) Predict whether the value of Keq for the reaction is greater than 1, equal to 1, or less than 1. Justify your answer. Copyright © 2004 by College Entrance Examination Board. All rights reserved. Visit apcentral.collegeboard.com (for AP professionals) and www.collegeboard.com/apstudents (for AP students and parents). GO ON TO THE NEXT PAGE. 12 2004 AP® CHEMISTRY FREE-RESPONSE QUESTIONS (Form B) 8. The gas-phase conversion reaction between the geometric isomers cis-2-butene and trans-2-butene is represented by the equation above. The value of the equilibrium constant, Keq , for the reaction is 3.2 at 298 K and 1.0 atm. (a) In a mixture of the isomers at equilibrium at 298 K and 1.0 atm, which is present at a higher concentration, cis-2-butene or trans-2-butene? Justify your answer. (b) If 1.00 mol of pure cis-2-butene and 1.0 mol of pure trans-2-butene were introduced into an evacuated container at 298 K, in which direction (to the right or to the left) would the reaction proceed to establish equilibrium? Justify your answer. (c) Given that Keq for the reaction at 400 K has the value 1.3, predict whether the reaction is endothermic or exothermic. Justify your answer. (d) There are other structural isomers of cis-2-butene and trans-2-butene. Draw one of these isomers, including all atoms, and give its IUPAC name. END OF EXAMINATION Copyright © 2004 by College Entrance Examination Board. All rights reserved. Visit apcentral.collegeboard.com (for AP professionals) and www.collegeboard.com/apstudents (for AP students and parents). 13 ...
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