ap07_sg_chemistry

AP® Chemistry 2007 Scoring Guidelines 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. © 2007 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. AP® CHEMISTRY 2007 SCORING GUIDELINES © 2007 The College Board. All rights reserved. Visit apcentral.collegeboard.com (for AP professionals) and www.collegeboard.com/apstudents (for students and parents). Question 1 HF(aq) + H2O( l ) ƨ H3O+(aq) + F −(aq) Ka = 7.2 ¥ 10− 4 Hydrofluoric acid, HF(aq) , dissociates in water as represented by the equation above. (a) Write the equilibrium-constant expression for the dissociation of HF(aq) in water. − = + 3[H O ][F ] [HF]aK One point is earned for the correct expression. (b) Calculate the molar concentration of H3O+ in a 0.40 M HF(aq) solution. − = + 3[H O ][F ] [HF]aK = ( )( ) 0.40 x x x− = 7.2 ¥ 10− 4 Assume x << 0.40, then x2 = (0.40)(7.2 ¥ 10− 4) x = [ H3O+ ] = 0.017 M One point is earned for the correct setup (or the setup consistent with part (a)). One point is earned for the correct concentration. HF(aq) reacts with NaOH(aq) according to the reaction represented below. HF(aq) + OH−(aq) → H2O(l) + F −(aq) A volume of 15 mL of 0.40 M NaOH(aq) is added to 25 mL of 0.40 M HF(aq) solution. Assume that volumes are additive. (c) Calculate the number of moles of HF(aq) remaining in the solution. mol HF(aq) = initial mol HF(aq) − mol NaOH(aq) added = (0.025 L)(0.40 mol L−1) − (0.015 L)(0.40 mol L−1) = 0.010 mol − 0.0060 mol = 0.004 mol One point is earned for determining the initial number of moles of HF and OH− . One point is earned for setting up and doing correct subtraction. (d) Calculate the molar concentration of F −(aq) in the solution. mol F −(aq) formed = mol NaOH(aq) added = 0.0060 mol F −(aq) − + 0.0060 mol F ( ) (0.015 0.025) L of solution aq = 0.15 M F −(aq) One point is earned for determining the number of moles of F −(aq). One point is earned for dividing the number of moles of F −(aq) by the correct total volume. AP® CHEMISTRY 2007 SCORING GUIDELINES © 2007 The College Board. All rights reserved. Visit apcentral.collegeboard.com (for AP professionals) and www.collegeboard.com/apstudents (for students and parents). Question 1 (continued) (e) Calculate the pH of the solution. [HF] = 0.004 mol HF 0.040 L = 0.10 M HF − = + 3[H O ][F ] [HF]aK ⇒ − × = + 3 [HF] [H O ] [F ] aK ⇒ −× 40.10 (7.2 10 ) 0.15 M M = 4.8 × 10−4 ⇒ pH = − log (4.8 × 10−4) = 3.32 OR pH = pKa + log −[F ] [HF] = − log (7.2 × 10−4) + log 0.15 0.10 M M = 3.14 + 0.18 = 3.32 One point is earned for indicating that the resulting solution is a buffer (e.g., by showing a ratio of [F −] to [HF] or moles of F − to HF ). One point is earned for the correct calculation of pH. AP® CHEMISTRY 2007 SCORING GUIDELINES © 2007 The College Board. All rights reserved. Visit apcentral.collegeboard.com (for AP professionals) and www.collegeboard.com/apstudents (for students and parents). Question 2 N2(g) + 3 F2(g) → 2 NF3(g) 298HΔ D = − 264 kJ mol−1; 298SΔ D = − 278 J K−1 mol−1 The following questions relate to the synthesis reaction represented by the chemical equation in the box above. (a) Calculate the value of the standard free energy change, DΔ 298G , for the reaction. DΔ 298G = DΔ 298H − DΔ 298T S = − 264 kJ mol−1 − (298 K)(−0.278 kJ mol−1 K−1) = −181 kJ mol−1 One point is earned for correct substitution. One point is earned for the value of DΔ 298G (including kJ or J). (b) Determine the temperature at which the equilibrium constant, Keq , for the reaction is equal to 1.00 . (Assume that ΔH° and ΔS° are independent of temperature.) When Keq = 1, then TGΔ D = −RT ln(1) = 0 If DΔ TG = 0 , then 0 = ΔH °− TΔS ° ⇒ T = D D Δ Δ 298 298 H S T = 1 1 1 264 kJ mol 0.278 kJ K mol - - - - - = 950. K One point is earned for indicating that if Keq = 1, then DΔ TG = 0. One point is earned for the answer (including the unit K). (c) Calculate the standard enthalpy change, ΔH°, that occurs when a 0.256 mol sample of NF3(g) is formed from N2(g) and F2(g) at 1.00 atm and 298 K. 0.256 mol NF3(g) × − 3 264 kJ 2.00 mol NF ( )g = −33.8 kJ One point is earned for multiplying DΔ 298H by the number of moles of NF3 formed. One point is earned for recognizing that 2.00 mol of NF3 are produced for the reaction as it is written. One point is earned for the answer (including kJ or J). AP® CHEMISTRY 2007 SCORING GUIDELINES © 2007 The College Board. All rights reserved. Visit apcentral.collegeboard.com (for AP professionals) and www.collegeboard.com/apstudents (for students and parents). Question 2 (continued) The enthalpy change in a chemical reaction is the difference between energy absorbed in breaking bonds in the reactants and energy released by bond formation in the products. (d) How many bonds are formed when two molecules of NF3 are produced according to the equation in the box above? There are six N–F bonds formed. One point is earned for the correct answer. (e) Use both the information in the box above and the table of average bond enthalpies below to calculate the average enthalpy of the F - F bond. Bond Average Bond Enthalpy (kJ mol−1) N≡N 946 N–F 272 F –F ? 298HΔ D = Σ Ebonds broken − Σ Ebonds formed = −264 kJ mol−1 = [ BEN≡N + (3 × BEF-F) ] − (6 × BEN-F) = [946 kJ mol−1+ (3 × BEF-F) ] − 6(272 kJ mol−1) = −264 kJ mol−1 ⇒ 3 mol BEF-F = (− 264 − 946 + 1,632) kJ mol−1 ⇒ BEF-F = 141 kJ mol−1 One point is earned for the correct number of bonds in all three compounds multiplied by the average bond enthalpies. One point is earned for the answer (including kJ or J). Note: A total of one point is earned if an incorrect number of bonds is substituted in a correct equation and the answer is reasonable (i.e., positive). AP® CHEMISTRY 2007 SCORING GUIDELINES © 2007 The College Board. All rights reserved. Visit apcentral.collegeboard.com (for AP professionals) and www.collegeboard.com/apstudents (for students and parents). Question 3 An external direct-current power supply is connected to two platinum electrodes immersed in a beaker containing 1.0 M CuSO4(aq) at 25°C, as shown in the diagram above. As the cell operates, copper metal is deposited onto one electrode and O2(g) is produced at the other electrode. The two reduction half-reactions for the overall reaction that occurs in the cell are shown in the table below. Half-Reaction E°(V) O2(g) + 4 H+(aq) + 4 e− → 2 H2O(l) +1.23 Cu2+(aq) + 2 e− → Cu(s) +0.34 (a) On the diagram, indicate the direction of electron flow in the wire. The electron flow in the wire is from the right toward the left (counterclockwise). One point is earned for the correct direction. (b) Write a balanced net ionic equation for the electrolysis reaction that occurs in the cell. 2 H2O(l) + 2 Cu2+(aq) → 4 H+(aq) + 2 Cu(s) + O2(g) One point is earned for all three products. One point is earned for balancing the equation. (c) Predict the algebraic sign of ΔG° for the reaction. Justify your prediction. The sign of ΔG° would be positive because the reaction is NOT spontaneous. One point is earned for indicating that ΔG° is greater than zero and supplying a correct explanation. AP® CHEMISTRY 2007 SCORING GUIDELINES © 2007 The College Board. All rights reserved. Visit apcentral.collegeboard.com (for AP professionals) and www.collegeboard.com/apstudents (for students and parents). Question 3 (continued) (d) Calculate the value of ΔG° for the reaction. E ° = −1.23 V + 0.34 V = − 0.89 V = − 0.89 J C−1 ΔG° = − nF E ° = − 4 (96,500 C mol−1)(− 0.89 J C−1) = +340,000 J mol−1 = +340 kJ mol−1 One point is earned for calculating E°. One point is earned for calculating ΔG° (consistent with the calculated E°). An electric current of 1.50 amps passes through the cell for 40.0 minutes. (e) Calculate the mass, in grams, of the Cu(s) that is deposited on the electrode. q = (1.50 C s−1)(40.0 min) × 60 s1 minute = 3,600 C mass Cu = (3,600 C) × 1 mol96,500 C e− × 1 mol Cu 2 mol e- × 63.55 g Cu 1 mol Cu = 1.19 g Cu One point is earned for calculating q. One point is earned for calculating the mass of copper deposited. OR Two points are earned for calculating the mass of copper in one step. (f) Calculate the dry volume, in liters measured at 25°C and 1.16 atm, of the O2(g) that is produced. nO2 = (1.19 g Cu) × 1 mol Cu 63.55 g Cu × 2 1 mol O 2 mol Cu = 0.00936 mol O2 V = nRT P = − −1 1(0.00936 mol)(0.0821 L atm mol K )(298 K) 1.16 atm = 0.197 L One point is earned for calculating the number of moles of O2. One point is earned for calculating V (consistent with previous calculations). AP® CHEMISTRY 2007 SCORING GUIDELINES © 2007 The College Board. All rights reserved. Visit apcentral.collegeboard.com (for AP professionals) and www.collegeboard.com/apstudents (for students and parents). Question 4 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) A solution of sodium hydroxide is added to a solution of lead(II) nitrate. (i) Balanced equation: 2 OH− + Pb2+ → Pb(OH)2 One point is earned for the correct reactants. Two points are earned for the correct product. One point is earned for balancing the equation for mass and charge. (ii) If 1.0 L volumes of 1.0 M solutions of sodium hydroxide and lead(II) nitrate are mixed together, how many moles of product(s) will be produced? Assume the reaction goes to completion. A total of 0.5 mol of Pb(OH)2 will be produced. One point is earned for the correct number of moles. (b) Excess nitric acid is added to solid calcium carbonate. (i) Balanced equation: 2 H+ + CaCO3 → Ca2+ + H2O + CO2 One point is earned for the correct reactants. Two points are earned for all three of the correct products; one point is earned for any one or two of the three. One point is earned for balancing the equation for mass and charge. (ii) Briefly explain why statues made of marble (calcium carbonate) displayed outdoors in urban areas are deteriorating. The H+ ions in acid rain react with the marble statues and the soluble compounds of Ca that are formed wash away. One point is earned for a correct answer involving acid precipitation. AP® CHEMISTRY 2007 SCORING GUIDELINES © 2007 The College Board. All rights reserved. Visit apcentral.collegeboard.com (for AP professionals) and www.collegeboard.com/apstudents (for students and parents). Question 4 (continued) (c) A solution containing silver(I) ion (an oxidizing agent) is mixed with a solution containing iron(II) ion (a reducing agent). (i) Balanced equation: Ag+ + Fe2+ → Ag + Fe3+ One point is earned for the correct reactants. One point is earned for each of the two correct products. One point is earned for balancing the equation for mass and charge. (ii) If the contents of the reaction mixture described above are filtered, what substance(s), if any, would remain on the filter paper? The precipitated solid silver will remain on the filter paper. One point is earned for the correct substance. AP® CHEMISTRY 2007 SCORING GUIDELINES © 2007 The College Board. All rights reserved. Visit apcentral.collegeboard.com (for AP professionals) and www.collegeboard.com/apstudents (for students and parents). Question 5 5 Fe2+(aq) + MnO4−(aq) + 8 H+(aq) → 5 Fe3+(aq) + Mn2+(aq) + 4 H2O(l) The mass percent of iron in a soluble iron(II) compound is measured using a titration based on the balanced equation above. (a) What is the oxidation number of manganese in the permanganate ion, MnO4−(aq) ? +7 One point is earned for the correct oxidation number. (b) Identify the reducing agent in the reaction represented above. Fe2+(aq) One point is earned for the correct iron ion. The mass of a sample of the iron(II) compound is carefully measured before the sample is dissolved in distilled water. The resulting solution is acidified with H2SO4(aq) . The solution is then titrated with MnO4−(aq) until the end point is reached. (c) Describe the color change that occurs in the flask when the end point of the titration has been reached. Explain why the color of the solution changes at the end point. The solution in the flask changes from colorless to faint purple-pink at the endpoint of the titration. At the endpoint there is no Fe2+(aq) left in the flask to reduce the colored permanganate ion, so when a small amount of permanganate ion is added after the endpoint, the unreacted permanganate ion present in the solution colors the solution faint purple/pink. One point is earned for stating that a faint pink color appears (unless indication of acid-base reaction). One point is earned for a correct explanation involving excess MnO4− after all Fe2+ has reacted. (d) Let the variables g, M, and V be defined as follows: g = the mass, in grams, of the sample of the iron(II) compound M = the molarity of the MnO4−(aq) used as the titrant V = the volume, in liters, of MnO4−(aq) added to reach the end point In terms of these variables, the number of moles of MnO4−(aq) added to reach the end point of the titration is expressed as M × V. Using the variables defined above, the molar mass of iron (55.85 g mol−1), and the coefficients in the balanced chemical equation, write the expression for each of the following quantities. AP® CHEMISTRY 2007 SCORING GUIDELINES © 2007 The College Board. All rights reserved. Visit apcentral.collegeboard.com (for AP professionals) and www.collegeboard.com/apstudents (for students and parents). Question 5 (continued) (i) The number of moles of iron in the sample mol Fe2+ = 5 × M × V OR mol Fe2+ = − 2+ 4 5 mol Fe 1 mol MnO × M × V One point is earned for either expression. (ii) The mass of iron in the sample, in grams mass Fe = 5 × M × V × 55.85 g mol−1 OR mass Fe = mol Fe2+ × 55.85 g mol−1 One point is earned for the answer in part (d)(i) multiplied by 55.85. (iii) The mass percent of iron in the compound mass % Fe = × × ×5 55.85M V g × 100 OR mass % Fe = mass Fe g × 100 One point is earned for the answer in part (d)(ii) divided by g. One point is earned for converting to percent. (e) What effect will adding too much titrant have on the experimentally determined value of the mass percent of iron in the compound? Justify your answer. The experimentally determined mass percent of iron in the compound will be too large. V is too large ⇒ expression in (d)(iii) above is too large One point is earned for stating that the mass percent is too large, with justification. AP® CHEMISTRY 2007 SCORING GUIDELINES © 2007 The College Board. All rights reserved. Visit apcentral.collegeboard.com (for AP professionals) and www.collegeboard.com/apstudents (for students and parents). Question 6 Answer the following questions, which pertain to binary compounds. (a) In the box provided below, draw a complete Lewis electron-dot diagram for the IF3 molecule. One point is earned for a correct Lewis diagram (can be done with dots or lines). (b) On the basis of the Lewis electron-dot diagram that you drew in part (a), predict the molecular geometry of the IF3 molecule. T-shaped One point is earned for the molecular geometry consistent with the Lewis diagram in part (a). (c) In the SO2 molecule, both of the bonds between sulfur and oxygen have the same length. Explain this observation, supporting your explanation by drawing in the box below a Lewis electron-dot diagram (or diagrams) for the SO2 molecule. One point is earned for a correct diagram (can be done with dots or lines). One point is earned for some indication or discussion of resonance (but the point is not earned for a description of resonance as a dynamic process). OR The bonds are the same length because they are both double bonds. One point is earned for a correct diagram (can be done with dots or lines). One point is earned for stating that both bonds are double bonds. AP® CHEMISTRY 2007 SCORING GUIDELINES © 2007 The College Board. All rights reserved. Visit apcentral.collegeboard.com (for AP professionals) and www.collegeboard.com/apstudents (for students and parents). Question 6 (continued) (d) On the basis of your Lewis electron-dot diagram(s) in part (c), identify the hybridization of the sulfur atom in the SO2 molecule. sp2 One point is earned for hybridization consistent with part (c). The reaction between SO2(g) and O2(g) to form SO3(g) is represented below. 2 SO2(g) + O2(g) →← 2 SO3(g) The reaction is exothermic. The reaction is slow at 25°C; however, a catalyst will cause the reaction to proceed faster. (e) Using the axes provided below, draw the complete potential-energy diagram for both the catalyzed and uncatalyzed reactions. Clearly label the curve that represents the catalyzed reaction. One point is earned for an uncatalyzed reaction curve that must show that Ea > 0 and ΔH < 0. One point is earned for a catalyzed reaction curve that must show Ea < uncatalyzed Ea , must be clearly labeled, and must begin and end at the same energ