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AP® Chemistry 2009 Free-Response Questions The College Board 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 5,600 schools, colleges, universities and other educational organizations. Each year, the College Board serves seven million students and their parents, 23,000 high schools and 3,800 colleges through major programs and services in college readiness, 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. © 2009 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 E (V)∞ -+-2F (g)2 e Æ 2 F 2.87 3+-+Co+ e Æ 2Co 1.82 3+-Au+ 3e Æ Au(s) 1.50 --Cl+2 (g)2 e Æ 2 Cl 1.36 +-O++2 (g)4 H4 e Æ 2 H2O(l) 1.23 -+-2Br (l)2 e Æ 2 Br 1.07 2+-2 Hg+ 2e Æ 2+Hg2 0.92 2+-Hg+ 2e Æ Hg(l) 0.85 +-Ag + e Æ Ag(s) 0.80 2+-Hg+22 e Æ 2 Hg(l) 0.79 3+-Fe+ e Æ 2+Fe 0.77 --I+2 (s)2 e Æ 2 I 0.53 +-Cu + e Æ Cu(s) 0.52 2+-Cu+ 2e Æ Cu(s) 0.34 2+-Cu+ e Æ +Cu 0.15 4+-Sn+ 2e Æ 2+Sn 0.15 +-S(s) + 2 H + 2 e Æ H2S(g) 0.14 +-2 H + 2 e Æ H2(g) 0.00 2+-Pb+ 2e Æ Pb(s) – 0.13 2+-Sn+ 2e Æ Sn(s) – 0.14 2+-Ni+ 2e Æ Ni(s) – 0.25 2+-Co+ 2e Æ Co(s) – 0.28 2+-Cd+ 2e Æ Cd(s) – 0.40 3+-Cr+ e Æ 2+Cr – 0.41 2+-Fe+ 2e Æ Fe(s) – 0.44 3+-Cr+ 3e Æ Cr(s) – 0.74 2+-Zn+ 2e Æ Zn(s) – 0.76 --2 H+2O(l)2 e Æ H2(g) + 2OH– 0.83 2+-Mn+ 2e Æ Mn(s) – 1.18 3+-Al+ 3e Æ Al(s) – 1.66 2+-Be+ 2e Æ Be(s) – 1.70 2+-Mg+ 2e Æ Mg(s) – 2.37 +-Na + e Æ Na(s) – 2.71 2+-Ca+ 2e Æ Ca(s) – 2.87 2+-Sr+ 2e Æ Sr(s) – 2.89 2+-Ba+ 2e Æ Ba(s) – 2.90 +-Rb + e Æ Rb(s) – 2.92 +-K + e Æ K(s) – 2.92 +-Cs + e Æ Cs(s) – 2.92 +-Li + e Æ Li(s) – 3.05 -3- GO ON TO THE NEXT PAGE. ADVANCED PLACEMENT CHEMISTRY EQUATIONS AND CONSTANTS ATOMIC STRUCTURE E = energyu = velocityv = frequencyn = principal quantum numberE = hvc = lv lh= wavelengthm = mass l =p = mumu p = momentum18--2.178 ¥ 10 E =joulen281-nSpeed of light, c = 3.0 ¥ 10 m s34-EQUILIBRIUM Planck’s constant, h = 6.63 ¥ 10 J s+-[H ][A ]23-1-K =Boltzmann’s constant, k = 1.38 ¥ 10 J Ka[HA] 231--+Avogadro’s number [OH ][HB ]= 6.022 ¥ 10 molK =b[B]19-Electron charge, 1.60e = -2 ¥ 10 coulomb-+14-K= [OH ][H ] = 1.0 ¥ 10@ 25 Cw-1 electron volt per a1tom = 96.5 kJ m lo= K ¥ Kab+- pH = - log [H ], pOH = - log[OH ] Equilibrium Constants14 = pH + pOH -K (weak acid)[A ]apH = pK + loga[HA]K (weak base) b +K (water)[HB ]wpOH = pK +blog [B]K (gas pressure)ppK = - log KK = -Kaa , plogbbK (molar concentrations)cDn K = K (RT ),pcS = standard entropywhere mDn = oles product gas - moles reactant gasH = standard enthalpyTHERMOCHEMISTRY/KINETICS G = standard free energy DS = ÂS products -ÂS reactantsE = standard reduction potentialT = temperatureDH = ÂDH products -Â DH reactantsffn = molesDG = ÂDG products -ÂDG reactantsff m = mass q = heatDG = DH - T DSc = specific heat capacity= -RT ln K = 2.30-3 RT log K C = molar heat capacity at constant pressurep= -n EE = activation energyak = rate constantDG = DG + RT lnQ = DG + 2.303RT logQA = frequency factorq = mcDT DHC =Faraday’s constant , = 96,500 coulombs per mole pDTof electrons 1-1ln[A] - ln= -ktt[A] -0Gas constant, R = 8.31 J molK 111-1--= kt= 0.0821 L atm mol K[A]t [A]01-1-= 62.4 L torr mol K 1-1-- aE1= 8.31 volt coulomb mol K ln k =( )+lnA RT -4- GO ON TO THE NEXT PAGE. GASES, LIQUIDS, AND SOLUTIONS PV = nRTP = pressure2ÊV = volumen a ˆP +(V - nb) = nRTÁ=2 ˜ËTtemperatureV¯ n = number of moles moles AP = P¥ X ,where X =AtotalAA=total molesDdensityP= P + P + P + ...m = masstotalABCmu = velocityn = M K = C + 273 u= root-mean-square speedPVP Vrms1 12 2=TTKE = kinetic energy12mr = rate of effusionD = VM = molar mass3kT3RTp = osmotic pressureu==rmsmMi = van’t Hoff factor12KE per molecule =muK= molal freezing-point depression constant2f=3Kmolal boiling-point elevation constantbKE per mole =RT2A = absorbance rMa = molar absorptivity12=rM=21bpath lengthmolarity, M = moles solute per liter solutionc = concentrationmolality = moles solute per kilogram solventQ = reaction quotientD T = iK ¥ molality=ffIcurrent (amperes)DT = iK ¥ molalityq = charge (coulombs)bbp = iMRTt = time (seconds)A = abcE = standard reduction potential K = equilibrium constant OXIDATION-REDUCTION; ELECTROCHEMISTRY 1-1- Gas constant, R = 8.31 J molK 1-1-= 0.0821 L atm mol K[C]c [D]d1-1-Q =, where Aa+ b B Æ c C + d D= 62.4 L torr mol K [A]a [B]b1-1-= 8.31 volt coulomb mol Kq23-1-=¥I =Boltzmann’s constant, k1.3810 J Kt 1-K for H=f2O1.86 K kg mol RT0.0592E= E-ln Q = E-logQ @ 25 C-cellcellcell1nnK for H=b2O0.512 K kg mol1 atm = 760 mm HgnElog K == 760 torr0.0592 STP = 0.00 C and 1.0 atmFaraday’s constant, = 96,500 coulombs per mole of electrons -5- GO ON TO THE NEXT PAGE. 2009 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. 1. Answer the following questions that relate to the chemistry of halogen oxoacids. (a) Use the information in the table below to answer part (a)(i). AcidK at 298 K aHOCl2.9 × 10-8 HOBr2.4 × 10-9 (i) Which of the two acids is stronger, HOCl or HOBr ? Justify your answer in terms of K . a (ii) Draw a complete Lewis electron-dot diagram for the acid that you identified in part (a)(i). (iii) Hypoiodous acid has the formula HOI. Predict whether HOI is a stronger acid or a weaker acid than the acid that you identified in part (a)(i). Justify your prediction in terms of chemical bonding. (b) Write the equation for the reaction that occurs between hypochlorous acid and water. (c) A 1.2 M NaOCl solution is prepared by dissolving solid NaOCl in distilled water at 298 K. The hydrolysis reaction OCl−(aq) + H O(l) Æ2¨ HOCl(aq) + OH−(aq) occurs. (i) Write the equilibrium-constant expression for the hydrolysis reaction that occurs between OCl -(aq) and H O(l) .2 (ii) Calculate the value of the equilibrium constant at 298 K for the hydrolysis reaction. (iii) Calculate the value of [OH -] in the 1.2 M NaOCl solution at 298 K . © 2009 The College Board. All rights reserved. Visit the College Board on the Web: www.collegeboard.com. -6- GO ON TO THE NEXT PAGE. 2009 AP® CHEMISTRY FREE-RESPONSE QUESTIONS (d) A buffer solution is prepared by dissolving some solid NaOCl in a solution of HOCl at 298 K . The pH of the buffer solution is determined to be 6.48. (i) Calculate the value of [H O+] in the buffer solution. 3 (ii) Indicate which of HOCl(aq) or OCl−(aq) is present at the higher concentration in the buffer solution. Support your answer with a calculation. 2. A student was assigned the task of determining the molar mass of an unknown gas. The student measured the mass of a sealed 843 mL rigid flask that contained dry air. The student then flushed the flask with the unknown gas, resealed it, and measured the mass again. Both the air and the unknown gas were at 23.0°C and 750. torr. The data for the experiment are shown in the table below. Volume of sealed flask843 mL Mass of sealed flask and dry air157.70 g Mass of sealed flask and unknown gas158.08 g (a) Calculate the mass, in grams, of the dry air that was in the sealed flask. (The density of dry air is 1.18 g L−1 at 23.0°C and 750. torr.) (b) Calculate the mass, in grams, of the sealed flask itself (i.e., if it had no air in it). (c) Calculate the mass, in grams, of the unknown gas that was added to the sealed flask. (d) Using the information above, calculate the value of the molar mass of the unknown gas. After the experiment was completed, the instructor informed the student that the unknown gas was carbon dioxide (44.0 g mol−1) . (e) Calculate the percent error in the value of the molar mass calculated in part (d). (f) For each of the following two possible occurrences, indicate whether it by itself could have been responsible for the error in the student’s experimental result. You need not include any calculations with your answer. For each of the possible occurrences, justify your answer. Occurrence 1: The flask was incompletely flushed with CO (g) , resulting in some dry air remaining 2in the flask. Occurrence 2: The temperature of the air was 23.0°C, but the temperature of the CO (g)2 was lower than the reported 23.0°C. (g) Describe the steps of a laboratory method that the student could use to verify that the volume of the rigid flask is 843 mL at 23.0°C. You need not include any calculations with your answer. © 2009 The College Board. All rights reserved. Visit the College Board on the Web: www.collegeboard.com. -7- GO ON TO THE NEXT PAGE. 2009 AP® CHEMISTRY FREE-RESPONSE QUESTIONS CH (g) + 2 Cl (g) → CH Cl (g) + 2 HCl(g) 4222 3. Methane gas reacts with chlorine gas to form dichloromethane and hydrogen chloride, as represented by the equation above. (a) A 25.0 g sample of methane gas is placed in a reaction vessel containing 2.58 mol of Cl (g). 2 (i) Identify the limiting reactant when the methane and chlorine gases are combined. Justify your answer with a calculation. (ii) Calculate the total number of moles of CH Cl (g)22 in the container after the limiting reactant has been totally consumed. Initiating most reactions involving chlorine gas involves breaking the Cl–Cl bond, which has a bond energy of 242 kJ mol-1. (b) Calculate the amount of energy, in joules, needed to break a single Cl – Cl bond. (c) Calculate the longest wavelength of light, in meters, that can supply the energy per photon necessary to break the Cl – Cl bond. The following mechanism has been proposed for the reaction of methane gas with chlorine gas. All species are in the gas phase. Step 1 Cl Æ2 ¨ 2 Cl fast equilibrium Step 2 CH + Cl → CH + HCl slow 43Step 3 CH + Cl → CH Cl + Cl fast 323Step 4 CH Cl + Cl → CH Cl + H fast 322Step 5 H + Cl → HCl fast (d) In the mechanism, is CH Cl3 a catalyst, or is it an intermediate? Justify your answer. (e) Identify the order of the reaction with respect to each of the following according to the mechanism. In each case, justify your answer. (i) CH (g) 4 (ii) Cl (g) 2 S T O P 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. © 2009 The College Board. All rights reserved. Visit the College Board on the Web: www.collegeboard.com. -8- 2009 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, write a balanced equation in part (i) and answer the question in part (ii). 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) A sample of solid iron(III) oxide is reduced completely with solid carbon. (i) Balanced equation: (ii) What is the oxidation number of carbon before the reaction, and what is the oxidation number of carbon after the reaction is complete? ______________________________________________________________________________________ ______________________________________________________________________________________ ______________________________________________________________________________________ © 2009 The College Board. All rights reserved. Visit the College Board on the Web: www.collegeboard.com. -9- GO ON TO THE NEXT PAGE. 2009 AP® CHEMISTRY FREE-RESPONSE QUESTIONS (b) Equal volumes of equimolar solutions of ammonia and hydrochloric acid are combined. (i) Balanced equation: (ii) Indicate whether the resulting solution is acidic, basic, or neutral. Explain. ______________________________________________________________________________________ ______________________________________________________________________________________ ______________________________________________________________________________________ (c) Solid mercury(II) oxide decomposes as it is heated in an open test tube in a fume hood. (i) Balanced equation: (ii) After the reaction is complete, is the mass of the material in the test tube greater than, less than, or equal to the mass of the original sample? Explain. ______________________________________________________________________________________ ______________________________________________________________________________________ ______________________________________________________________________________________ YOU MAY USE THE SPACE BELOW FOR SCRATCH WORK, BUT ONLY EQUATIONS THAT ARE WRITTEN IN THE ANSWER BOXES PROVIDED WILL BE GRADED. © 2009 The College Board. All rights reserved. Visit the College Board on the Web: www.collegeboard.com. -10- GO ON TO THE NEXT PAGE.