ap08_chemistry_form_b_frq - AP® Chemistry 2008...

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Unformatted text preview: AP® Chemistry 2008 Free-Response Questions Form B The College Board: Connecting Students to College Success 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 5,000 schools, colleges, universities, and other educational organizations. Each year, the College Board serves seven 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. © 2008 The College Board. All rights reserved. College Board, Advanced Placement Program, AP, AP Central, SAT, and the acorn logo are registered trademarks of the College Board. PSAT/NMSQT is a registered trademark of the College Board and National Merit Scholarship Corporation. Permission to use copyrighted College Board materials may be requested online at: www.collegeboard.com/inquiry/cbpermit.html. Visit the College Board on the Web: www.collegeboard.com. AP Central is the official online home for the AP Program: apcentral.collegeboard.com. 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. -2- GO ON TO THE NEXT PAGE. STANDARD REDUCTION POTENTIALS IN AQUEOUS SOLUTION AT 25 C Half-reaction F2 ( g) Co 3+ 3+ E (V) 2F Co 2 2e e 3e + Au Au(s) 2 Cl 4e 2 H 2 O(l ) 2 Br Hg22+ Hg(l ) Ag(s) 2 Hg(l ) Fe 2+ 2I Cu(s) Cu(s) Cu+ Sn 2+ 2e H 2S(g ) H2 (g) Pb(s) Sn(s) Ni(s) Co(s) Cd(s) Cr 2+ Fe(s) Cr(s) Zn(s) H 2 ( g ) + 2 OH Mn(s) Al(s) Be(s) Mg(s) Na(s) Ca(s) Sr(s) Cl2 (g ) 2 e O2 (g) 4 H Br2 (l ) 2 e 2 Hg Ag Fe + 2+ 2+ 2e 2e e 2e e e 2e e 2e 2e 2e 2e 2e 2e 2e e 2e 3e 2e 2e 3e 2e 2e e 2e 2e 2e Hg2+ Hg2 3+ I 2 (s) 2 e Cu Cu Cu + 2+ 2+ Sn 4+ 2H Pb Sn Ni + S(s) 2 H + 2+ 2+ 2+ 2+ 2+ Co Cd Cr Cr 3+ Fe 2+ 3+ 2+ Zn 2 H 2 O(l ) 2 e Mn Al Be 2+ 3+ 2+ 2+ Mg Na Ca Sr Ba + 2+ 2+ 2+ + Ba(s) Rb(s) K ( s) Cs(s) Li(s) Rb K Cs Li e e e e + + + 2.87 1.82 1.50 1.36 1.23 1.07 0.92 0.85 0.80 0.79 0.77 0.53 0.52 0.34 0.15 0.15 0.14 0.00 – 0.13 – 0.14 – 0.25 – 0.28 – 0.40 – 0.41 – 0.44 – 0.74 – 0.76 – 0.83 – 1.18 – 1.66 – 1.70 – 2.37 – 2.71 – 2.87 – 2.89 – 2.90 – 2.92 – 2.92 – 2.92 – 3.05 -3- GO ON TO THE NEXT PAGE. ADVANCED PLACEMENT CHEMISTRY EQUATIONS AND CONSTANTS ATOMIC STRUCTURE hv c lv h l= p mu mu 2.178 10 18 En joule n2 E E v l p energy frequency wavelength momentum u n m velocity principal quantum number mass Speed of light, c Planck’s constant, h Boltzmann’s constant, k Avogadro’s number 14 3.0 108 m s 34 23 1 EQUILIBRIUM [H ][A ] Ka [HA] [OH ][HB ] Kb [B] K w [OH ][H ] 1.0 10 K a Kb pH 14 pH pOH pK a Kp where D n log [H ], pOH pH pOH pK a pK b log log [A ] [HA] 6.63 10 1.38 6.022 1.602 10 Js JK 1 1 1023 mol 10 19 1 Electron charge, e @ 25 C coulomb 1 electron volt per atom 96.5 kJ mol log[OH ] Equilibrium Constants K a (weak acid) K b (weak base) K w (water) K p (gas pressure) K c (molar concentrations) S H G E T n m q c Cp Ea k A standard entropy standard enthalpy standard free energy standard reduction potential temperature moles mass heat specific heat capacity molar heat capacity at constant pressure activation energy rate constant frequency factor 96,500 coulombs per mole of electrons 8.31 J mol 1 [HB ] [B] log K a , pK b log K b Dn K c ( RT ) , moles reactant gas moles product gas THERMOCHEMISTRY/KINETICS DS DH DG DG S products S reactants DHf products DGf products DH f reactants DGf reactants T DS DH 2.303 RT log K RT ln K nE DG q Cp ln A t DG RT ln Q mcDT DH DT ln A 0 1 A0 Ea 1 RT kt kt DG 2.303 RT log Q Faraday's constant, Gas constant, R K 1 1 1 A ln k 0.0821 L atm mol 62.4 L torr mol 1 K 1 1 t K ln A 8.31 volt coulomb mol 1 K 1 -4- GO ON TO THE NEXT PAGE. GASES, LIQUIDS, AND SOLUTIONS PV P n2 a (V V2 nb) PA nRT nRT Ptotal X A , where X A PB PC ... moles A total moles Ptotal n K PV1 1 T1 D urms KE per molecule KE per mole r1 r2 molarity, M molality DT f DTb p A PA m M P V T n D m u pressure volume temperature number of moles density mass velocity C 273 P2V2 T2 m V 3kT 3RT M m 12 mu 2 3 RT 2 M2 M1 moles solute per liter solution moles solute per kilogram solvent iK f molality iK b molality iMRT abc urms KE r M p i Kf Kb A a b c Q I q t E K root-mean-square speed kinetic energy rate of effusion molar mass osmotic pressure van't Hoff factor molal freezing -point depression constant molal boiling -point elevation constant absorbance molar absorptivity path length concentration reaction quotient current (amperes) charge (coulombs) time (seconds) standard reduction potential equilibrium constant OXIDATION-REDUCTION; ELECTROCHEMISTRY Gas constant, R 8.31 J mol 1 K 1 1 0.0821 L atm mol Q I Ecell log K [C] c [D] d [A] [B] q t Ecell nE 0.0592 RT ln Q n Ecell 0.0592 log Q @ 25 C n a b K 1 1 , where a A bB cC dD 62.4 L torr mol Boltzmann's constant, k K f for H2 O K b for H2 O 1 atm STP Faraday's constant, 1.38 10 23 1 K 1 8.31 volt coulomb mol JK 1 1 1 K 1 1.86 K kg mol 0.512 K kg mol 760 mm Hg 760 torr 0.00 C and 1.0 atm 96,500 coulombs per mole of electrons -5- GO ON TO THE NEXT PAGE. 2008 AP® CHEMISTRY FREE-RESPONSE QUESTIONS (Form B) CHEMISTRY Section II (Total time—95 minutes) Part A Time— 55 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 goldenrod booklet. Do NOT write your answers on the lavender insert. Answer Questions 1, 2, and 3. The Section II score weighting for each question is 20 percent. 1. Answer the following questions regarding the decomposition of arsenic pentafluoride, AsF5(g) . (a) A 55.8 g sample of AsF5(g) is introduced into an evacuated 10.5 L container at 105°C. (i) What is the initial molar concentration of AsF5(g) in the container? (ii) What is the initial pressure, in atmospheres, of the AsF5(g) in the container? At 105°C, AsF5(g) decomposes into AsF3(g) and F2(g) according to the following chemical equation. AsF5(g) AsF3(g) + F2(g) (b) In terms of molar concentrations, write the equilibrium-constant expression for the decomposition of AsF5(g). (c) When equilibrium is established, 27.7 percent of the original number of moles of AsF5(g) has decomposed. (i) Calculate the molar concentration of AsF5(g) at equilibrium. (ii) Using molar concentrations, calculate the value of the equilibrium constant, Keq , at 105°C. (d) Calculate the mole fraction of F2(g) in the container at equilibrium. © 2008 The College Board. All rights reserved. Visit apcentral.collegeboard.com (for AP professionals) and www.collegeboard.com/apstudents (for students and parents). GO ON TO THE NEXT PAGE. -6- 2008 AP® CHEMISTRY FREE-RESPONSE QUESTIONS (Form B) A(g) + B(g) → C(g) + D(g) 2. For the gas-phase reaction represented above, the following experimental data were obtained. Initial [A] (mol L−1) 0.033 0.034 0.136 0.202 Initial [B] (mol L−1) 0.034 0.137 0.136 0.233 Initial Reaction Rate (mol L−1 s−1) 6.67 × 10− 4 1.08 × 10−2 1.07 × 10−2 ? Experiment 1 2 3 4 (a) Determine the order of the reaction with respect to reactant A . Justify your answer. (b) Determine the order of the reaction with respect to reactant B . Justify your answer. (c) Write the rate law for the overall reaction. (d) Determine the value of the rate constant, k , for the reaction. Include units with your answer. (e) Calculate the initial reaction rate for experiment 4. (f) The following mechanism has been proposed for the reaction. Step 1: Step 2: B+B → E+D E+A B+C slow fast equilibrium Provide two reasons why the mechanism is acceptable. (g) In the mechanism in part (f), is species E a catalyst, or is it an intermediate? Justify your answer. © 2008 The College Board. All rights reserved. Visit apcentral.collegeboard.com (for AP professionals) and www.collegeboard.com/apstudents (for students and parents). GO ON TO THE NEXT PAGE. -7- 2008 AP® CHEMISTRY FREE-RESPONSE QUESTIONS (Form B) 3. A 0.150 g sample of solid lead(II) nitrate is added to 125 mL of 0.100 M sodium iodide solution. Assume no change in volume of the solution. The chemical reaction that takes place is represented by the following equation. Pb(NO3)2(s) + 2 NaI(aq) → PbI2(s) + 2 NaNO3(aq) (a) List an appropriate observation that provides evidence of a chemical reaction between the two compounds. (b) Calculate the number of moles of each reactant. (c) Identify the limiting reactant. Show calculations to support your identification. (d) Calculate the molar concentration of NO3−(aq) in the mixture after the reaction is complete. (e) Circle the diagram below that best represents the results after the mixture reacts as completely as possible. Explain the reasoning used in making your choice. © 2008 The College Board. All rights reserved. Visit apcentral.collegeboard.com (for AP professionals) and www.collegeboard.com/apstudents (for students and parents). GO ON TO THE NEXT PAGE. -8- 2008 AP® CHEMISTRY FREE-RESPONSE QUESTIONS (Form B) STOP If you finish before time is called, you may check your work on this part only. Do not turn to the other part of the test until you are told to do so. © 2008 The College Board. All rights reserved. Visit apcentral.collegeboard.com (for AP professionals) and www.collegeboard.com/apstudents (for students and parents). -9- 2008 AP® CHEMISTRY FREE-RESPONSE QUESTIONS (Form B) CHEMISTRY Part B Time— 40 minutes NO CALCULATORS MAY BE USED FOR PART B. Answer Question 4 below. The Section II score weighting for this question is 10 percent. 4. For each of the following three reactions, in part (i) write a balanced equation for the reaction and in part (ii) answer the question about the reaction. In part (i), coefficients should be in terms of lowest whole numbers. Assume that solutions are aqueous unless otherwise indicated. Represent substances in solutions as ions if the substances are extensively ionized. Omit formulas for any ions or molecules that are unchanged by the reaction. You may use the empty space at the bottom of the next page for scratch work, but only equations that are written in the answer boxes provided will be graded. (a) Chlorine gas, an oxidizing agent, is bubbled into a solution of potassium bromide at 25°C. (i) Balanced equation: (ii) Predict the sign of ΔS° for the reaction at 25°C. Justify your prediction. ______________________________________________________________________________________ ______________________________________________________________________________________ ______________________________________________________________________________________ © 2008 The College Board. All rights reserved. Visit apcentral.collegeboard.com (for AP professionals) and www.collegeboard.com/apstudents (for students and parents). GO ON TO THE NEXT PAGE. -10- 2008 AP® CHEMISTRY FREE-RESPONSE QUESTIONS (Form B) (b) Solid strontium hydroxide is added to a solution of nitric acid. (i) Balanced equation: (ii) How many moles of strontium hydroxide would react completely with 500. mL of 0.40 M nitric acid? ______________________________________________________________________________________ ______________________________________________________________________________________ ______________________________________________________________________________________ (c) A solution of barium chloride is added drop by drop to a solution of sodium carbonate, causing a precipitate to form. (i) Balanced equation: (ii) What happens to the pH of the sodium carbonate solution as the barium chloride is added to it? ______________________________________________________________________________________ ______________________________________________________________________________________ ______________________________________________________________________________________ © 2008 The College Board. All rights reserved. Visit apcentral.collegeboard.com (for AP professionals) and www.collegeboard.com/apstudents (for students and parents). GO ON TO THE NEXT PAGE. -11- 2008 AP® CHEMISTRY FREE-RESPONSE QUESTIONS (Form B) Answer Question 5 and Question 6. The Section II score weighting for these questions is 15 percent each. Your responses to these questions 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. 5. The identity of an unknown solid is to be determined. The compound is one of the seven salts in the following table. Al(NO3)3. 9H2O NaCl BaCl2. 2H2O BaSO4 CaCO3 Ni(NO3)2. 6H2O CuSO4. 5H2O Use the results of the following observations or laboratory tests to explain how each compound in the table may be eliminated or confirmed. The tests are done in sequence from (a) through (e). (a) The unknown compound is white. In the table below, cross out the two compounds that can be eliminated using this observation. Be sure to cross out these same two compounds in the tables in parts (b), (c), and (d). Al(NO3)3. 9H2O NaCl BaCl2. 2H2O BaSO4 CaCO3 Ni(NO3)2. 6H2O CuSO4. 5H2O (b) When the unknown compound is added to water, it dissolves readily. In the table below, cross out the two compounds that can be eliminated using this test. Be sure to cross out these same two compounds in the tables in parts (c) and (d). Al(NO3)3. 9H2O NaCl BaCl2. 2H2O BaSO4 CaCO3 Ni(NO3)2. 6H2O CuSO4. 5H2O (c) When AgNO3(aq) is added to an aqueous solution of the unknown compound, a white precipitate forms. In the table below, cross out each compound that can be eliminated using this test. Be sure to cross out the same compound(s) in the table in part (d). Al(NO3)3. 9H2O NaCl BaCl2. 2H2O BaSO4 CaCO3 Ni(NO3)2. 6H2O CuSO4. 5H2O © 2008 The College Board. All rights reserved. Visit apcentral.collegeboard.com (for AP professionals) and www.collegeboard.com/apstudents (for students and parents). GO ON TO THE NEXT PAGE. -12- 2008 AP® CHEMISTRY FREE-RESPONSE QUESTIONS (Form B) (d) When the unknown compound is carefully heated, it loses mass. In the table below, cross out each compound that can be eliminated using this test. Al(NO3)3. 9H2O NaCl BaCl2. 2H2O BaSO4 CaCO3 Ni(NO3)2. 6H2O CuSO4. 5H2O (e) Describe a test that can be used to confirm the identity of the unknown compound identified in part (d). Limit your confirmation test to a reaction between an aqueous solution of the unknown compound and an aqueous solution of one of the other soluble salts listed in the tables. Describe the expected results of the test; include the formula(s) of any product(s). © 2008 The College Board. All rights reserved. Visit apcentral.collegeboard.com (for AP professionals) and www.collegeboard.com/apstudents (for students and parents). GO ON TO THE NEXT PAGE. -13- 2008 AP® CHEMISTRY FREE-RESPONSE QUESTIONS (Form B) 6. Use principles of thermodynamics to answer the following questions. (a) The gas N2O4 decomposes to form the gas NO2 according to the equation below. (i) Predict the sign of ΔH° for the reaction. Justify your answer. (ii) Predict the sign of ΔS° for the reaction. Justify your answer. (b) One of the diagrams below best represents the relationship between ΔG° and temperature for the reaction given in part (a). Assume that ΔH° and ΔS° are independent of temperature. Draw a circle around the correct graph. Explain why you chose that graph in terms of the relationship Δ G° = Δ H° – T Δ S ° . (c) A reaction mixture of N2O4 and NO2 is at equilibrium. Heat is added to the mixture while the mixture is maintained at constant pressure. (i) Explain why the concentration of N2O4 decreases. (ii) The value of Keq at 25°C is 5.0 × 10− 3 . Will the value of Keq at 100°C be greater than, less than, or equal to this value? (d) Using the value of Keq at 25°C given in part (c)(ii), predict whether the value of ΔH° is expected to be greater than, less than, or equal to the value of TΔS° . Explain. STOP END OF EXAM © 2008 The College Board. All rights reserved. Visit apcentral.collegeboard.com (for AP professionals) and www.collegeboard.com/apstudents (for students and parents). -14- ...
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