AP Chemistry 2009 Free-Response Questions Form B

Transcription

1 AP Chemistry 009 Free-Response Questions Form B 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, 3,000 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. 009 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: AP Central is the official online home for the AP Program: apcentral.collegeboard.com.

2 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. -- GO ON TO THE NEXT PAGE.

3 STANDARD REDUCTION POTENTIALS IN AQUEOUS SOLUTION AT 5 C Half-reaction F ( g) e - 3 Co e - Æ E (V) Æ F -.87 Co Au 3e - Æ Au( s ) 1.50 Cl ( g) e Æ Cl O ( g) 4H 4e Æ HO( ) Br ( l) e Hg e - Æ Hg e - Æ Hg( ) Ag e - Æ Ag( ) Hg e - l 1.3 Æ Br Hg 0.9 l 0.85 s 0.80 Æ Hg( l ) Fe e - Æ Fe 0.77 I ( s) e - Æ I Cu e - Æ Cu( s ) 0.5 Cu e - Æ Cu( s ) 0.34 Cu e - Æ Cu Sn e - Æ Sn 0.15 S( s) H e - HS() g 0.14 Æ Æ H ( g ) 0.00 Æ Pb( s ) 0.13 H e - Pb e - Sn e - Æ Sn( s ) 0.14 Ni e - Æ Ni( s ) 0.5 Co e - Æ Co( s ) 0.8 Cd e - Æ Cd( s ) Cr e - Æ Cr 0.41 Fe e - Æ Fe( s ) Cr 3e - Æ Cr( s ) 0.74 Zn e - Æ Zn( s ) 0.76 HO( l) e Æ - H ( g) OH 0.83 Mn e - Æ Mn( s ) Al 3e - Æ Al( s ) 1.66 Be e - Æ Be( s ) 1.70 Mg e - Æ Mg( s ).37 Na e - Æ Na( s ).71 Ca e - Æ Ca( s ).87 Sr e - Æ Sr( s ).89 Ba e - Æ Ba( s ).90 Rb e - Æ Rb( s ).9 K e - Æ K( s ).9 Cs e - Æ Cs( s ).9 Li e - Æ Li( s ) GO ON TO THE NEXT PAGE.

4 ADVANCED PLACEMENT CHEMISTRY EQUATIONS AND CONSTANTS ATOMIC STRUCTURE E hv c lv h l p mu mu En joule n EQUILIBRIUM K K K a b w [H ][A ] [HA] [OH ][HB ] [B] [OH ][H ] C K K a ph - log [H ], poh -log[oh ] 14 ph poh [A ] ph pka log [HA] [HB ] poh pkb log [B] pk - log K, pk -log K K K ( RT), b - a a b b p c - - Dn where Dn moles product gas - moles reactant gas THERMOCHEMISTRY/KINETICS DS S products - S reactants   DH DH products - DH reactants f DG DG products - DG reactants  f DG DH - TDS - RT ln K -.303RT log K -n E DG DG RT lnq DG.303RT logq q mcdt DH Cp DT ln[ A] t - ln[ A] 0 -kt [ A] [ A] kt t 0 1 ( ) -E ln k a ln A R T -    f f E energy u velocity v frequency n principal quantum number l wavelength m mass p momentum 8-1 Speed of light, c m s -34 Planck s constant, h J s -3-1 Boltzmann s constant, k J K 3-1 Avogadro s number mol -19 Electron charge, e coulomb 1 electron volt per atom 96.5 kjmol - Equilibrium Constants Ka (weak acid) Kb (weak base) Kw (water) K p (gas pressure) K (molar concentrations) a c S standard entropy H standard enthalpy G standard free energy E standard reduction potential T temperature n moles m mass q heat c specific heat capacity Cp molar heat capacity at constant pressure E activation energy k A rate constant frequency factor Faraday s constant, 96,500 coulombs per mole of electrons Gas constant, R 8.31 J mol K L atm mol K 6.4 L torr mol K 8.31 volt coulomb mol K 1-4- GO ON TO THE NEXT PAGE.

5 GASES, LIQUIDS, AND SOLUTIONS PV nrt Ê naˆ ÁP ( V - nb) nrt Ë V P P X where X Ptotal PA PB PC... m n M K C 73 PV PV A total A, A 1 1 T1 T moles A total moles m D V 3kT 3RT urms m M 1 KE per molecule mu 3 KE per mole RT r1 M r M1 molarity, M moles solute per liter solution molality moles solute per kilogram solvent DTf ikf molality DTb ikb molality p imrt A abc P V T n D m u pressure volume temperature number of moles density mass velocity urms root-mean-square speed KE kinetic energy r rate of effusion M molar mass p osmotic pressure i van t Hoff factor K f molal freezing-point depression constant Kb molal boiling-point elevation constant A absorbance a molar absorptivity b path length c concentration Q reaction quotient I current (amperes) q charge (coulombs) t time (seconds) E K standard reduction potential equilibrium constant OXIDATION-REDUCTION; ELECTROCHEMISTRY c d Q [C] [D], where A B C D a b [A] [B] a b Æ c d q I t RT Ecell E cell - lnq E cell - log C n n ne log K Gas constant, R 8.31 J mol K L atm mol K 6.4 L torr mol K 8.31 volt coulomb mol K -3-1 Boltzmann s constant, k J K -1 K for H O 1.86 K kg mol K f b for HO 0.51 K kg mol 1atm 760 mm Hg 760 torr -1 STP 0.00 C and 1.0 atm Faraday s constant, 96,500 coulombs per mole of electrons -5- GO ON TO THE NEXT PAGE.

6 009 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 this booklet. Do NOT write your answers on the lavender insert. Answer Questions 1,, and 3. The Section II score weighting for each question is 0 percent. 1. A pure g sample of the weak base ethylamine, C H 5 NH, is dissolved in enough distilled water to make 500. ml of solution. (a) Calculate the molar concentration of the C H 5 NH in the solution. The aqueous ethylamine reacts with water according to the equation below. C H 5 NH (aq) H O(l) Æ C H 5 NH 3 (aq) OH - (aq) (b) Write the equilibrium-constant expression for the reaction between C H 5 NH (aq) and water. (c) Of C H 5 NH (aq) and C H 5 NH 3 (aq), which is present in the solution at the higher concentration at equilibrium? Justify your answer. (d) A different solution is made by mixing 500. ml of M C H 5 NH with 500. ml of 0.00 M HCl. Assume that volumes are additive. The ph of the resulting solution is found to be (i) Calculate the concentration of OH - (aq) in the solution. (ii) Write the net-ionic equation that represents the reaction that occurs when the C H 5 NH solution is mixed with the HCl solution. (iii) Calculate the molar concentration of the C H 5 NH 3 (aq) that is formed in the reaction. (iv) Calculate the value of K b for C H 5 NH. 009 The College Board. All rights reserved. -6- GO ON TO THE NEXT PAGE.

7 009 AP CHEMISTRY FREE-RESPONSE QUESTIONS (Form B) S O 3 - (aq) H ææææ SO 3 - (aq) S(s). A student performed an experiment to investigate the decomposition of sodium thiosulfate, Na S O 3, in acidic solution, as represented by the equation above. In each trial the student mixed a different concentration of sodium thiosulfate with hydrochloric acid at constant temperature and determined the rate of disappearance of S O 3 (aq). Data from five trials are given below in the table on the left and are plotted in the graph on the right. (a) Identify the independent variable in the experiment. (b) Determine the order of the reaction with respect to S O 3 -. Justify your answer by using the information above. (c) Determine the value of the rate constant, k, for the reaction. Include units in your answer. Show how you arrived at your answer. (d) In another trial the student mixed 0.10 M Na S O 3 with hydrochloric acid. Calculate the amount of time it would take for the concentration of S O - 3 to drop to 0.00 M. (e) On the graph above, sketch the line that shows the results that would be expected if the student repeated the five trials at a temperature lower than that during the first set of trials. 009 The College Board. All rights reserved. -7- GO ON TO THE NEXT PAGE.

8 009 AP CHEMISTRY FREE-RESPONSE QUESTIONS (Form B) H O (aq) H O(l) O (g) 3. The mass of an aqueous solution of H O is g. The H O in the solution decomposes completely according to the reaction represented above. The O (g) produced is collected in an inverted graduated tube over water at 3.4 C and has a volume of 18.4 ml when the water levels inside and outside of the tube are the same. The atmospheric pressure in the lab is 76.6 torr, and the equilibrium vapor pressure of water at 3.4 C is 1.6 torr. (a) Calculate the partial pressure, in torr, of O (g) in the gas-collection tube. (b) Calculate the number of moles of O (g) produced in the reaction. (c) Calculate the mass, in grams, of H O that decomposed. (d) Calculate the percent of H O, by mass, in the original g aqueous sample. (e) Write the oxidation number of the oxygen atoms in H O and the oxidation number of the oxygen atoms in O in the appropriate cells in the table below. Substance Oxidation Number of Oxygen Atoms H O O (f) Write the balanced oxidation half-reaction for the reaction. 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. 009 The College Board. All rights reserved. -8-

9 009 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, 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 barium nitrate solution and a potassium fluoride solution are combined and a precipitate forms. (i) Balanced equation: (ii) If equimolar amounts of barium nitrate and potassium fluoride are combined, which reactant, if any, is the limiting reactant? Explain. 009 The College Board. All rights reserved. -9- GO ON TO THE NEXT PAGE.

10 009 AP CHEMISTRY FREE-RESPONSE QUESTIONS (Form B) (b) A piece of cadmium metal is oxidized by adding it to a solution of copper(ii) chloride. (i) Balanced equation: (ii) List two visible changes that would occur in the reaction container as the reaction is proceeding. (c) A hydrolysis reaction occurs when solid sodium sulfide is added to distilled water. (i) Balanced equation: (ii) Indicate whether the ph of the resulting solution is less than 7, equal to 7, or greater than 7. Explain. 009 The College Board. All rights reserved GO ON TO THE NEXT PAGE.

11 009 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. Answer the following questions about nitrogen, hydrogen, and ammonia. (a) In the boxes below, draw the complete Lewis electron-dot diagrams for N and NH 3. (b) Calculate the standard free-energy change, ΔG, that occurs when 1.0 g of H (g) reacts with excess N (g) at 98 K according to the reaction represented below. N (g) 3 H (g) Æ NH 3 (g) DG kj mol -1 (c) Given that ΔH 98 for the reaction is 9. kj mol -1, which is larger, the total bond dissociation energy of the reactants or the total bond dissociation energy of the products? Explain. (d) The value of the standard entropy change, ΔS 98, for the reaction is 199 J mol -1 K -1. Explain why the value of ΔS 98 is negative. (e) Assume that ΔH and ΔS for the reaction are independent of temperature. (i) Explain why there is a temperature above 98 K at which the algebraic sign of the value of ΔG changes. (ii) Theoretically, the best yields of ammonia should be achieved at low temperatures and high pressures. Explain. 009 The College Board. All rights reserved GO ON TO THE NEXT PAGE.

12 009 AP CHEMISTRY FREE-RESPONSE QUESTIONS (Form B) 6. Answer the following questions about electrochemical cells. It is observed that when silver metal is placed in aqueous thallium(i) fluoride, TlF, no reaction occurs. When the switch is closed in the cell represented above, the voltage reading is 1.14 V. (a) Write the reduction half-reaction that occurs in the cell. (b) Write the equation for the overall reaction that occurs in the cell. (c) Identify the anode in the cell. Justify your answer. (d) On the diagram above, use an arrow to clearly indicate the direction of electron flow as the cell operates. (e) Calculate the value of the standard reduction potential for the Tl /Tl half-reaction. The standard reduction potential, E, of the reaction Pt e Pt is 1.0 V. (f) Assume that electrodes of pure Pt, Ag, and Ni are available as well as 1.00 M solutions of their salts. Three different electrochemical cells can be constructed using these materials. Identify the two metals that when used to make an electrochemical cell would produce the cell with the largest voltage. Explain how you arrived at your answer. (g) Predict whether Pt metal will react when it is placed in 1.00 M AgNO 3 (aq). Justify your answer. STOP END OF EXAM 009 The College Board. All rights reserved. -1-