AP Chemistry 1999 Free-Response Questions

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AP Chemistry 1999 Free-Response Questions The materials included in these iles are intended or non-commercial use y AP teachers or course and exam preparation; permission or any other use must e

1 AP Chemistry 1999 Free-Response Questions The materials included in these iles are intended or non-commercial use y AP teachers or course and exam preparation; permission or any other use must e sought rom the Advanced Placement Program. Teachers may reproduce them, in whole or in part, in limited quantities, or ace-to-ace teaching purposes ut may not mass distriute the materials, electronically or otherwise. These materials and any copies made o them may not e resold, and the copyright notices must e retained as they appear here. This permission does not apply to any third-party copyrights contained herein. These materials were produced y Educational Testing Service (ETS), which develops and administers the examinations o the Advanced Placement Program or the College Board. The College Board and Educational Testing Service (ETS) are dedicated to the principle o equal opportunity, and their programs, services, and employment policies are guided y that principle. The College Board is a national nonproit memership association dedicated to preparing, inspiring, and connecting students to college and opportunity. Founded in 1900, the association is composed o more than 3,900 schools, colleges, universities, and other educational organizations. Each year, the College Board serves over three million students and their parents,,000 high schools, and 3,500 colleges, through major programs and services in college admission, guidance, assessment, inancial aid, enrollment, and teaching and learning. Among its est-known programs are the SAT, the PSAT/NMSQT, the Advanced Placement Program (AP ), and Pacesetter. The College Board is committed to the principles o equity and excellence, and that commitment is emodied in all o its programs, services, activities, and concerns. Copyright 001 y College Entrance Examination Board. All rights reserved. College Board, Advanced Placement Program, AP, and the acorn logo are registered trademarks o the College Entrance Examination Board.

2 The College Board Advanced Placement Examination CHEMISTRY SECTION II MATERIAL IN THE FOLLOWING TABLE MAY BE USEFUL IN ANSWERING THE QUESTIONS IN THIS SECTION OF THE EXAMINATION. Copyright 1999 y College Entrance Examination Board and Educational Testing Service. All rights reserved.

3 STANDARD REDUCTION POTENTIALS IN AQUEOUS SOLUTION AT 5 C Li Cs R Ba Hal-reaction E ( V ) Æ Li( s) Æ Cs( s) -. 9 Æ ( s) -. 9 Æ R( s) e Æ Ba( s) Sr + e Æ Sr( s) Ca + e Æ Ca( s) Na Æ Na( s) Mg + e Æ Mg( s) Be + e Æ Be( s) Al + 3 e Æ Al( s) Mn + e Æ Mn( s) Zn + e Æ Zn( s) Cr + 3 e Æ Cr( s) Fe + e Æ Fe( s) Cr 3 + Æ Cr Cd + e Æ Cd( s) Tl Æ Tl( s) Co + e Æ Co( s) Ni + e Æ Ni( s) Sn + e- Æ Sn( s) P + e Æ P( s) Æ 4 + H + e Æ H ( g) S ( s) + H Sn + e Æ Sn Cu + Æ Cu + e Æ Cu( s) Cu Æ s - Æ Æ e Hg + Æ Hg( l) Æ H S ( g) 0.14 Cu Cu( s) 0. 5 I ( ) I Fe Ag Fe Ag( s) Hg + e Æ Hg( l) Hg Hg 0. 9 Br ( l) + e Æ Br O ( g) + 4 H + 4 e Æ H O( l) 1. 3 Cl ( g) + e Æ Cl 136. Au Co + - Æ e Æ Au( s) Æ Co F ( g) + e Æ F

4 ATOMIC STRUCTURE 1999 ADVANCED PLACEMENT CHEMISTRY EQUATIONS AND CONSTANTS DE hv c lv l h m u p m u E n n joule E v l p u n m energy requency wavelength momentum velocity principal quantum numer mass EQUILIBRIUM Speed o light, c m s a w [ H ] [ A ] [ HA] - + [ OH ] [ HB ] [ B] - 14 [ OH ] [ H ] C a ph - log [ H ], poh - log [ OH ] 14 ph + poh + log [ A ph p ] a [ HA] + log [ HB poh p ] [ B] p - log, p - log a a ( RT), p c Dn where Dn moles product gas - moles reactant gas THERMOCHEMISTRY DS S products - S reactants DH DH products - DH reactants DG DG products - DG reactants DG DH - TDS - RT ln RT log -n ^ E - + DG DG + RT ln Q DG RT log Q q mc DT C p Â Â Â DH DT Â Â Â -34 Planck s constant, h Boltzmann s constant, k Avogadro s numer Electron charge, e electron volt per atom kj mol Equilirium Constants a w p c C p (weak acid) (weak ase) (water) (gas pressure) (molar concentrations) J s -3-1 J S standard entropy H standard enthalpy G standard ree energy E standard reduction potential T temperature n moles m mass q heat c speciic heat capacity molecules mol coulom molar heat capacity at constant pressure 1 araday ^ 96, 500 couloms

5 GASES, LIQUIDS, AND SOLUTIONS 1999 F HG I J PV nrt n a P + ( V - n) nrt V X X P P + P + P +... n PV T u P D E per molecule E per mole molarity, r r M DT DT P A total A A total A B C rms 1 m M C + 73 P V T m V 3kT 3RT m M 1 m u 3 RTn M M molality moles solute per kilogram solvent p i molality i molality nrt V 1 i, where moles solute per liter solution moles A total moles OXIDATION-REDUCTION; ELECTROCHEMISTRY c d [ C] [ D] Q, where a A + B Æ c C + d D a [ A] [ B] I q t Ecell RT E - ln Q E cell cell - log 5 C n^ n log ne u P pressure V volume T temperature n numer o moles D density m mass u velocity rms E r M p i Q I q t E root-mean-square speed kinetic energy rate o eusion molar mass osmotic pressure van' t Ho actor molal reezing-point depression constant molal oiling-point elevation constant reaction quotient current (amperes) charge (couloms) time (seconds) standard reduction potential equilirium constant Gas constant, R J mol Faraday's constant, ^ L atm mol Boltzmann's constant, k or H O kg mol J or H O kg mol volt coulom mol STP C and atm , 500 couloms per mole o electrons

6 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, ecause you may earn partial credit i you do and you will receive little or no credit i you do not. Attention should e paid to signiicant igures. Be sure to write all your answers to the questions on the lined pages ollowing each question in the ooklet with the pink cover. Do NOT write your answers on the green insert. Answer Question 1 elow. The Section II score weighting or this question is 0 percent. NH 3 (aq) + H O(l) Æ NH 4 + (aq) + OH - (aq) 1. In aqueous solution, ammonia reacts as represented aove. In M NH 3 (aq) at 5 C, the hydroxide ion concentration, [OH - ], is M. In answering the ollowing, assume that temperature is constant at 5 C and that volumes are additive. (a) Write the equilirium-constant expression or the reaction represented aove. () Determine the ph o M NH 3 (aq). (c) Determine the value o the ase ionization constant,, or NH 3 (aq). (d) Determine the percent ionization o NH 3 in M NH 3 (aq). (e) In an experiment, a 0.0 ml sample o M NH 3 (aq) was placed in a lask and titrated to the equivalence point and eyond using M HCl(aq). (i) Determine the volume o M HCl(aq) that was added to reach the equivalence point. (ii) Determine the ph o the solution in the lask ater a total o 15.0 ml o M HCl(aq) was added. (iii) Determine the ph o the solution in the lask ater a total o 40.0 ml o M HCl(aq) was added.

7 Answer EITHER Question elow OR Question 3 printed on the next page. Only one o these two questions will e graded. I you start oth questions, e sure to cross out the question you do not want graded. The Section II score weighting or the question you choose is 0 percent.. Answer the ollowing questions regarding light and its interactions with molecules, atoms, and ions. (a) The longest wavelength o light with enough energy to reak the Cl Cl ond in Cl (g) is 495 nm. (i) Calculate the requency, in s 1, o the light. (ii) Calculate the energy, in J, o a photon o the light. (iii) Calculate the minimum energy, in kj mol 1, o the Cl Cl ond. () A certain line in the spectrum o atomic hydrogen is associated with the electronic transition in the H atom rom the sixth energy level (n 6) to the second energy level (n ). (i) Indicate whether the H atom emits energy or whether it asors energy during the transition. Justiy your answer. (ii) Calculate the wavelength, in nm, o the radiation associated with the spectral line. (iii) Account or the oservation that the amount o energy associated with the same electronic transition (n 6 to n ) in the He + ion is greater than that associated with the corresponding transition in the H atom.

8 NO(g) + Br (g) NOBr(g) 3. A rate study o the reaction represented aove was conducted at 5 C. The data that were otained are shown in the tale elow. Experiment Initial [NO] (mol L 1 ) Initial [Br ] (mol L 1 ) Initial Rate o Appearance o NOBr (mol L -1 s -1 ) (a) Calculate the initial rate o disappearance o Br (g) in experiment 1. () Determine the order o the reaction with respect to each reactant, Br (g) and NO(g). In each case, explain your reasoning. (c) For the reaction, (i) write the rate law that is consistent with the data, and (ii) calculate the value o the speciic rate constant, k, and speciy units. (d) The ollowing mechanism was proposed or the reaction: Br (g) + NO(g) NOBr (g) NOBr (g) + NO(g) NOBr(g) slow ast Is this mechanism consistent with the given experimental oservations? Justiy your answer. STOP DO NOT GO ON TO PART B UNTIL YOU ARE TOLD TO DO SO.

9 CHEMISTRY Part B Time 50 minutes NO CALCULATORS MAY BE USED WITH PART B. Answer Question 4 elow. The Section II score weighting or this question is 15 percent. 4. Write the ormulas to show the reactants and the products or any FIVE o the laoratory situations descried elow. Answers to more than ive choices will not e graded. In all cases a reaction occurs. Assume that solutions are aqueous unless otherwise indicated. Represent sustances in solution as ions i the sustances are extensively ionized. Omit ormulas or any ions or molecules that are unchanged y the reaction. You need not alance the equations. Example: A strip o magnesium is added to a solution o silver nitrate. (a) Calcium oxide powder is added to distilled water. () Solid ammonium nitrate is heated to temperatures aove 300 C. (c) Liquid romine is shaken with a 0.5 M sodium iodide solution. (d) Solid lead(ii) caronate is added to a 0.5 M suluric acid solution. (e) A mixture o powdered iron(iii) oxide and powdered aluminum metal is heated strongly. () Methylamine gas is uled into distilled water. (g) Caron dioxide gas is passed over hot, solid sodium oxide. (h) A 0. M arium nitrate solution is added to an alkaline 0. M potassium chromate solution.

10 Your responses to the rest o the questions in this part o the examination will e graded on the asis o the accuracy and relevance o the inormation cited. Explanations should e clear and well organized. Examples and equations may e included in your responses where appropriate. Speciic answers are preerale to road, diuse responses. Answer BOTH Question 5 elow AND Question 6 printed on the next page. Both o these questions will e graded. The Section II score weighting or these questions is 30 percent (15 percent each). 5. A student perorms an experiment to determine the molar mass o an unknown gas. A small amount o the pure gas is released rom a pressurized container and collected in a graduated tue over water at room temperature, as shown in the diagram aove. The collection tue containing the gas is allowed to stand or several minutes, and its depth is adjusted until the water levels inside and outside the tue are the same. Assume that: the gas is not apprecialy solule in water the gas collected in the graduated tue and the water are in thermal equilirium a arometer, a thermometer, an analytical alance, and a tale o the equilirium vapor pressure o water at various temperatures are also availale. (a) Write the equation(s) needed to calculate the molar mass o the gas. () List the measurements that must e made in order to calculate the molar mass o the gas. (c) Explain the purpose o equalizing the water levels inside and outside the gas collection tue. (d) The student determines the molar mass o the gas to e 64 g mol -1. Write the expression (set-up) or calculating the percent error in the experimental value, assuming that the unknown gas is utane (molar mass 58 g mol 1 ). Calculations are not required. (e) I the student ails to use inormation rom the tale o the equilirium vapor pressures o water in the calculation, the calculated value or the molar mass o the unknown gas will e smaller than the actual value. Explain.

11 6. Answer the ollowing questions in terms o thermodynamic principles and concepts o kinetic molecular theory. (a) Consider the reaction represented elow, which is spontaneous at 98. CO (g) + NH 3 (g) CO(NH ) (s) + H O(l) DH kj (i) For the reaction, indicate whether the standard entropy change, DS 98, is positive, or negative, or zero. Justiy your answer. (ii) Which actor, the change in enthalpy, DH 98, or the change in entropy, DS 98, provides the principal driving orce or the reaction at 98? Explain. (iii) For the reaction, how is the value o the standard ree energy change, DG, aected y an increase in temperature? Explain. () Some reactions that are predicted y their sign o DG to e spontaneous at room temperature do not proceed at a measurale rate at room temperature. (i) Account or this apparent contradiction. (ii) A suitale catalyst increases the rate o such a reaction. What eect does the catalyst have on DG or the reaction? Explain.

12 Answer EITHER Question 7 OR Question 8 elow. Only one o these two questions will e graded. I you start oth questions, e sure to cross out the question you do not want graded. The Section II score weighting or the question you choose is 15 percent. 7. Answer the ollowing questions, which reer to the 100 ml samples o aqueous solutions at 5 C in the stoppered lasks shown aove. (a) Which solution has the lowest electrical conductivity? Explain. () Which solution has the lowest reezing point? Explain. (c) Aove which solution is the pressure o water vapor greatest? Explain. (d) Which solution has the highest ph? Explain. 8. Answer the ollowing questions using principles o chemical onding and molecular structure. (a) Consider the caron dioxide molecule, CO, and the caronate ion, CO 3. (i) Draw the complete Lewis electron-dot structure or each species. (ii) Account or the act that the caron-oxygen ond length in CO 3 - is greater than the caron-oxygen ond length in CO. () Consider the molecules CF 4 and SF 4. (i) Draw the complete Lewis electron-dot structure or each molecule. (ii) In terms o molecular geometry, account or the act that the CF 4 molecule is nonpolar, whereas the SF 4 molecule is polar. END OF EXAMINATION

AP Chemistry 2007 Free-Response Questions

AP Chemistry 2007 Free-Response Questions AP Chemistry 007 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