ap08_chemistry_frq - AP® Chemistry 2008 Free-Response...

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Unformatted text preview: AP® Chemistry 2008 Free-Response Questions 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 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 booklet with the pink cover. Do NOT write your answers on the green insert. Answer Questions 1, 2, and 3. The Section II score weighting for each question is 20 percent. → C(s) + CO2(g) ← 2 CO(g) 1. Solid carbon and carbon dioxide gas at 1,160 K were placed in a rigid 2.00 L container, and the reaction represented above occurred. As the reaction proceeded, the total pressure in the container was monitored. When equilibrium was reached, there was still some C(s) remaining in the container. Results are recorded in the table below. Total Pressure of Gases in Container at 1,160 K (atm) 5.00 6.26 7.09 7.75 8.37 8.37 Time (hours) 0.0 2.0 4.0 6.0 8.0 10.0 (a) Write the expression for the equilibrium constant, Kp , for the reaction. (b) Calculate the number of moles of CO2(g) initially placed in the container. (Assume that the volume of the solid carbon is negligible.) © 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 (c) For the reaction mixture at equilibrium at 1,160 K, the partial pressure of the CO2(g) is 1.63 atm. Calculate (i) the partial pressure of CO(g) , and (ii) the value of the equilibrium constant, Kp . (d) If a suitable solid catalyst were placed in the reaction vessel, would the final total pressure of the gases at equilibrium be greater than, less than, or equal to the final total pressure of the gases at equilibrium without the catalyst? Justify your answer. (Assume that the volume of the solid catalyst is negligible.) In another experiment involving the same reaction, a rigid 2.00 L container initially contains 10.0 g of C(s), plus CO(g) and CO2(g), each at a partial pressure of 2.00 atm at 1,160 K. (e) Predict whether the partial pressure of CO2(g) will increase, decrease, or remain the same as this system approaches equilibrium. Justify your prediction with a calculation. © 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 2. Answer the following questions relating to gravimetric analysis. In the first of two experiments, a student is assigned the task of determining the number of moles of water in one mole of MgCl2 ⋅ n H2O. The student collects the data shown in the following table. Mass of empty container Initial mass of sample and container Mass of sample and container after first heating Mass of sample and container after second heating Mass of sample and container after third heating 22.347 g 25.825 g 23.982 g 23.976 g 23.977 g (a) Explain why the student can correctly conclude that the hydrate was heated a sufficient number of times in the experiment. (b) Use the data above to (i) calculate the total number of moles of water lost when the sample was heated, and (ii) determine the formula of the hydrated compound. (c) A different student heats the hydrate in an uncovered crucible, and some of the solid spatters out of the crucible. This spattering will have what effect on the calculated mass of the water lost by the hydrate? Justify your answer. In the second experiment, a student is given 2.94 g of a mixture containing anhydrous MgCl2 and KNO3 . To determine the percentage by mass of MgCl2 in the mixture, the student uses excess AgNO3(aq) to precipitate the chloride ion as AgCl(s). (d) Starting with the 2.94 g sample of the mixture dissolved in water, briefly describe the steps necessary to quantitatively determine the mass of the AgCl precipitate. (e) The student determines the mass of the AgCl precipitate to be 5.48 g. On the basis of this information, calculate each of the following. (i) The number of moles of MgCl2 in the original mixture (ii) The percent by mass of MgCl2 in the original mixture © 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 3. Answer the following questions related to chemical reactions involving nitrogen monoxide, NO(g). The reaction between solid copper and nitric acid to form copper(II) ion, nitrogen monoxide gas, and water is represented by the following equation. 3 Cu(s) + 2 NO3−(aq) + 8 H+(aq) → 3 Cu2+(aq) + 2 NO(g) + 4 H2O(l) E ° = + 0.62 V (a) Using the information above and in the table below, calculate the standard reduction potential, E °, for the reduction of NO3− in acidic solution. Half-Reaction Cu2+ (aq) + 2 e − → Cu(s) NO3− (aq) + 4 H+ (aq) + 3 e − → NO(g) + 2 H2O(l ) Standard Reduction Potential, E ° +0.34 V ? (b) Calculate the value of the standard free energy change, ΔG °, for the overall reaction between solid copper and nitric acid. (c) Predict whether the value of the standard entropy change, ΔS °, for the overall reaction is greater than 0, less than 0, or equal to 0. Justify your prediction. Nitrogen monoxide gas, a product of the reaction above, can react with oxygen to produce nitrogen dioxide gas, as represented below. 2 NO(g) + O2(g) → 2 NO2(g) © 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. -9- 2008 AP® CHEMISTRY FREE-RESPONSE QUESTIONS A rate study of the reaction yielded the data recorded in the table below. Initial Rate of Formation of NO2 (mol L−1 s−1) 8.52 × 10−2 2.56 × 10−1 7.67 × 10−1 Experiment 1 2 3 Initial Concentration of NO (mol L−1) 0.0200 0.0200 0.0600 Initial Concentration of O2 (mol L−1) 0.0300 0.0900 0.0300 (d) Determine the order of the reaction with respect to each of the following reactants. Give details of your reasoning, clearly explaining or showing how you arrived at your answers. (i) NO (ii) O2 (e) Write the expression for the rate law for the reaction as determined from the experimental data. (f) Determine the value of the rate constant for the reaction, clearly indicating the units. 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). -10- 2008 AP® CHEMISTRY FREE-RESPONSE QUESTIONS 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) Aqueous sodium hydroxide is added to a saturated solution of aluminum hydroxide, forming a complex ion. (i) Balanced equation: (ii) If the resulting mixture is acidified, would the concentration of the complex ion increase, decrease, or remain the same? Explain. ______________________________________________________________________________________ ______________________________________________________________________________________ ______________________________________________________________________________________ © 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 (b) Hydrogen chloride gas is oxidized by oxygen gas. (i) Balanced equation: (ii) If three moles of hydrogen chloride gas and three moles of oxygen gas react as completely as possible, which reactant, if any, is present in excess? Justify your answer. ______________________________________________________________________________________ ______________________________________________________________________________________ ______________________________________________________________________________________ (c) Solid potassium oxide is added to water. (i) Balanced equation: (ii) If a few drops of phenolphthalein are added to the resulting solution, what would be observed? Explain. ______________________________________________________________________________________ ______________________________________________________________________________________ ______________________________________________________________________________________ YOU MAY USE THE SPACE BELOW FOR SCRATCH WORK, BUT ONLY EQUATIONS THAT ARE WRITTEN IN THE ANSWER BOXES PROVIDED WILL BE GRADED. © 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 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. Using principles of atomic and molecular structure and the information in the table below, answer the following questions about atomic fluorine, oxygen, and xenon, as well as some of their compounds. Atom F O Xe First Ionization Energy (kJ mol−1) 1,681.0 1,313.9 ? (a) Write the equation for the ionization of atomic fluorine that requires 1,681.0 kJ mol−1. (b) Account for the fact that the first ionization energy of atomic fluorine is greater than that of atomic oxygen. (You must discuss both atoms in your response.) (c) Predict whether the first ionization energy of atomic xenon is greater than, less than, or equal to the first ionization energy of atomic fluorine. 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. -13- 2008 AP® CHEMISTRY FREE-RESPONSE QUESTIONS (d) Xenon can react with oxygen and fluorine to form compounds such as XeO3 and XeF4 . In the boxes provided, draw the complete Lewis electron-dot diagram for each of the molecules represented below. (e) On the basis of the Lewis electron-dot diagrams you drew for part (d), predict the following: (i) The geometric shape of the XeO3 molecule (ii) The hybridization of the valence orbitals of xenon in XeF4 (f) Predict whether the XeO3 molecule is polar or nonpolar. 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. -14- 2008 AP® CHEMISTRY FREE-RESPONSE QUESTIONS 6. Answer the following questions by using principles of molecular structure and intermolecular forces. (a) Structures of the pyridine molecule and the benzene molecule are shown below. Pyridine is soluble in water, whereas benzene is not soluble in water. Account for the difference in solubility. You must discuss both of the substances in your answer. (b) Structures of the dimethyl ether molecule and the ethanol molecule are shown below. The normal boiling point of dimethyl ether is 250 K, whereas the normal boiling point of ethanol is 351 K. Account for the difference in boiling points. You must discuss both of the substances in your answer. (c) SO2 melts at 201 K, whereas SiO2 melts at 1,883 K. Account for the difference in melting points. You must discuss both of the substances in your answer. (d) The normal boiling point of Cl2 (l) (238 K) is higher than the normal boiling point of HCl(l) (188 K). Account for the difference in normal boiling points based on the types of intermolecular forces in the substances. You must discuss both of the substances in your answer. 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). -15- ...
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