AP Chemistry 2004 Scoring Guidelines

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1 AP Chemistry 2004 Scoring Guidelines The materials included in these iles are intended or noncommercial use by AP teachers or course and exam preparation; permission or any other use must be sought rom the Advanced Placement Program. Teachers may reproduce them, in whole or in part, in limited quantities, or ace-to-ace teaching purposes but may not mass distribute the materials, electronically or otherwise. This permission does not apply to any third-party copyrights contained herein. These materials and any copies made o them may not be resold, and the copyright notices must be retained as they appear here. The College Board is a not-or-proit membership association whose mission is to connect students to college success and opportunity. Founded in 1900, the association is composed o more than 4,500 schools, colleges, universities, and other educational organizations. Each year, the College Board serves over three million students and their parents, 23,000 high schools, and 3,500 colleges through major programs and services in college admissions, guidance, assessment, inancial 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 o excellence and equity, and that commitment is embodied in all o its programs, services, activities, and concerns. For urther inormation, visit Copyright 2004 College Entrance Examination Board. All rights reserved. College Board, Advanced Placement Program, AP, AP Central, AP Vertical Teams, APCD, Pacesetter, Pre-AP, SAT, Student Search Service, and the acorn logo are registered trademarks o the College Entrance Examination Board. PSAT/NMSQT is a registered trademark o the College Entrance Examination Board and National Merit Scholarship Corporation. Educational Testing Service and ETS are registered trademarks o Educational Testing Service. Other products and services may be trademarks o their respective owners. For the College Board s online home or AP proessionals, visit AP Central at apcentral.collegeboard.com.

2 Question 1 Answer the ollowing questions relating to the solubilities o two silver compounds, Ag 2 CrO 4 and Ag 3 PO 4. Silver chromate dissociates in water according to the equation shown below. Ag 2 CrO 4 (s) 2 Ag + (aq) + CrO 4 2 (aq) K sp = at 25 C (a) Write the equilibrium-constant expression or the dissolving o Ag 2 CrO 4 (s). K sp = [Ag + ] 2 [CrO 4 2 ] 1 point or correct expression (b) Calculate the concentration, in mol L 1, o Ag + (aq) in a saturated solution o Ag 2 CrO 4 at 25 C. Ag 2 CrO 4 (s) 2 Ag + (aq) + CrO 4 2 (aq) I 0 0 C +2x +x F 0 + 2x 0 + x 1 point or correct stoichiometry o Ag + (aq) and CrO 4 2 (aq) = [Ag + ] 2 [CrO 4 2 ] = [2x] 2 [x] = 4x = x = [CrO 2 4 ] 1 point or substituting and calculating [Ag + ] [Ag + ] = 2x = 2 ( M ) = M (c) Calculate the maximum mass, in grams, o Ag 2 CrO 4 that can dissolve in 100. ml o water at 25 C mole Ag CrO 1L g Ag 2CrO4 1L g 1moleAg CrO = g Ag 2CrO4 1L L = g Ag 2 CrO 4 1 point or molar mass o Ag 2 CrO 4 1 point or mass o Ag 2 CrO 4 in 100 ml Visit apcentral.collegeboard.com (or AP proessionals) and (or AP students and parents). 2

3 Question 1 (cont d.) (d) A mol sample o solid AgNO 3 is added to a 1.00 L saturated solution o Ag 2 CrO 4. Assuming no volume change, does [CrO 2 4 ] increase, decrease, or remain the same? Justiy your answer. The [CrO 4 2 ] will decrease. Adding [Ag + ] will make Q (nonequilibrium reaction quotient) greater than K. To re-establish equilibrium, the reaction goes rom right to let, decreasing the quotient to return to equilibrium. 1 point or correct prediction and explanation in terms o Q or LeChâtelier s principle In a saturated solution o Ag 3 PO 4 at 25 C, the concentration o Ag + (aq) is M. The equilibriumconstant expression or the dissolving o Ag 3 PO 4 (s) in water is shown below. K sp = [Ag + ] 3 [PO 4 3 ] (e) Write the balanced equation or the dissolving o Ag 3 PO 4 in water. Ag 3 PO 4 (s) 3 Ag + (aq) + PO 4 3 (aq) 1 point or correct, balanced chemical equation () Calculate the value o K sp or Ag 3 PO 4 at 25 C. [Ag + ] = M [PO 4 3 ] = M Ag + 1molPO 3molAg = M 1 point or correct [PO 4 3 ] K sp = [Ag + ] 3 [PO 4 3 ] = ( ) 3 ( ) = point or K sp (g) A 1.00 L sample o saturated Ag 3 PO 4 solution is allowed to evaporate at 25 C to a inal volume o 500. ml. What is [Ag + ] in the solution? Justiy your answer. [Ag + ] = M The [Ag + ] in a saturated solution o Ag 3 PO 4 is independent o the volume o the solution. 1 point or correct [Ag + ] and explanation Visit apcentral.collegeboard.com (or AP proessionals) and (or AP students and parents). 3

4 Question 2 2 Fe(s) O 2 (g) Fe 2 O 3 (s) H = 824 kj mol 1 Iron reacts with oxygen to produce iron(iii) oxide, as represented by the equation above. A 75.0 g sample o Fe(s) is mixed with 11.5 L o O 2 (g) at 2.66 atm and 298 K. (a) Calculate the number o moles o each o the ollowing beore the reaction begins. (i) Fe(s) n Fe = 75.0 g Fe 1molFe g Fe = 1.34 mol Fe 1 point or number o moles o Fe(s) (ii) O 2 (g) PV = nrt PV n O2 = RT = 2.66 atm 11.5 L L atm K mol K 1 point or number o moles o O 2 (g) n O2 = 1.25 mol O 2 (b) Identiy the limiting reactant when the mixture is heated to produce Fe 2 O 3 (s). Support your answer with calculations. 1.5 mol O2 n O2 reacting = 1.34 mol Fe 2 mol Fe = 1.01 mol O 2 There is 1.25 mol O 2 initially, so there is an excess o O 2, and Fe is the limiting reactant. 1 point or identiying limiting reactant OR 2 mol Fe n Fe reacting = 1.25 mol O mol O2 = 1.67 mol Fe There is 1.34 mol Fe initially, so there is not enough Fe, and Fe is the limiting reactant. 1 point or supporting calculation Visit apcentral.collegeboard.com (or AP proessionals) and (or AP students and parents). 4

5 Question 2 (cont d.) (c) Calculate the number o moles o Fe 2 O 3 (s) produced when the reaction proceeds to completion. 1molFe2O3 n Fe2 O = 1.34 mol Fe 3 2molFe = mol Fe 2 O 3 1 point or number o moles o Fe 2 O 3 produced (d) The standard ree energy o ormation, (i) Calculate the standard entropy o ormation, your answer. G, o Fe 2 O 3 (s) is 740. kj mol 1 at 298 K. S, o Fe 2 O 3 (s) at 298 K. Include units with G = H T S 740. kj mol 1 = 824 kj mol 1 (298 K) +84 kj mol 1 = (298 K) S = + 84 kj mol 298 K 1 S = 0.28 kj mol 1 K 1 S 1 point or calculation o 1 point or correct units S (ii) Which is more responsible or the spontaneity o the ormation reaction at 298 K, the standard enthalpy o ormation, H, or the standard entropy o ormation, S? Justiy your answer. H is the more important actor. The reaction is exothermic, which avors spontaneity. S is negative, which means the system becomes more ordered as the reaction proceeds. Greater order will not increase the spontaneity o the reaction. 1 point or indicating that H is responsible and or an explanation that addresses the signs o H and S Visit apcentral.collegeboard.com (or AP proessionals) and (or AP students and parents). 5

6 Question 2 (cont d.) The reaction represented below also produces iron(iii) oxide. The value o per mole o Fe 2 O 3 (s) ormed. H or the reaction is 280. kj 2 FeO(s) O 2 (g) Fe 2 O 3 (s) (e) Calculate the standard enthalpy o ormation, H, o FeO(s). H rxn = Σ H (products) Σ H (reactants) H rxn = H Fe 2 O 3 (s) [2 H o FeO(s) + 1 H 2 O 2 (g)] 280. kj mol 1 = 824 kj mol 1 [2 H FeO(s) (0)] +544 kj mol 1 = 2 H FeO(s) 272 kj mol 1 = H FeO(s) 1 point or correct stoichiometry 1 point or correct calculation Visit apcentral.collegeboard.com (or AP proessionals) and (or AP students and parents). 6

7 Question 3 The irst-order decomposition o a colored chemical species, X, into colorless products is monitored with a spectrophotometer by measuring changes in absorbance over time. Species X has a molar absorptivity constant o cm 1 M 1 and the path length o the cuvette containing the reaction mixture is 1.00 cm. The data rom the experiment are given in the table below. [X] ( M ) Absorbance Time (min)? ? (a) Calculate the initial concentration o the colored species. A = abc c = A = ( cm M 1 )( 1.00 cm) ab = M OR 1 point or concentration o X A 0 = abc 0 A 1 = abc 1 A c A = c c 0 = M = c (b) Calculate the rate constant or the irst-order reaction using the values given or concentration and time. Include units with your answer. Using the irst two readings, ln[x] t ln[x] 0 = kt OR ln ln = k (35.0 min) [X] t [X] 0 = kt 1 point or magnitude and correct sign o rate constant ln (0.333) = k (35.0 min) 1 point or correct units 1.10 = k (35.0 min) k = min 1 Visit apcentral.collegeboard.com (or AP proessionals) and (or AP students and parents). 7

8 Question 3 (cont d.) (c) Calculate the number o minutes it takes or the absorbance to drop rom to [X] t ln = kt [X] ln = ( min 1 ) t ln (0.125) = ( min 1 ) t 2.08 = ( min 1 ) t t = 66.2 min 1 point or correct substitution 1 point or correct answer Note: students may use hal-lives to answer this question. (d) Calculate the hal-lie o the reaction. Include units with your answer. ln [X] t = kt [X] [X] 0 ln [X] 0 = ( min 1 ) t 1/2 ln (0.5) = ( min 1 ) t 1/ = ( min 1 ) t 1/2 1 point or correct magnitude 1 point or the correct units 22.1 min = t 1/2 OR t 1/2 = k t 1/2 = min = 22.1 min (1 point or the hal-lie equation i no k is given) (e) Experiments were perormed to determine the value o the rate constant or this reaction at various temperatures. Data rom these experiments were used to produce the graph below, where T is temperature. This graph can be used to determine the activation energy, E a, o the reaction. (i) Label the vertical axis o the graph. The vertical axis should be labeled ln k. 1 point Visit apcentral.collegeboard.com (or AP proessionals) and (or AP students and parents). 8

9 Question 3 (cont d.) (ii) Explain how to calculate the activation energy rom this graph. The slope o the line is related to the activation energy: E slope = a R To determine the activation energy or the reaction, multiply the slope by J mol 1 K 1. 1 point or recognizing that the slope must be measured 1 point or the correct explanation o how to obtain the activation energy Visit apcentral.collegeboard.com (or AP proessionals) and (or AP students and parents). 9

10 Question 4 Write the ormulas to show the reactants and the products or any FIVE o the laboratory situations described below. Answers to more than ive choices will not be graded. In all cases, a reaction occurs. Assume that solutions are aqueous unless otherwise indicated. Represent substances in solution as ions i the substances are extensively ionized. Omit ormulas or any ions or molecules that are unchanged by the reaction. You need not balance the equations. Example: A strip o magnesium is added to a solution o silver nitrate. (a) A solution o copper(ii) sulate is spilled onto a sheet o reshly polished aluminum metal. Cu 2+ + Al Cu + Al 3+ 1 point or reactant(s), 2 points or product(s) (b) Dimethyl ether is burned in air. C 2 H 6 O + O 2 CO 2 + H 2 O OR C 2 H 6 O + O 2 CO 2 + H 2 O + CO 1 point or reactant(s), 2 points or product(s) (c) A 0.1 M nitrous acid solution is added to the same volume o a 0.1 M sodium hydroxide solution. HNO 2 + OH H 2 O + NO 2 1 point or reactant(s), 2 points or product(s) (d) Hydrogen iodide gas is bubbled into a solution o lithium carbonate. 2 HI + CO 3 H 2 O + I + CO 2 OR 2 HI + CO 3 I + HCO 3 1 point or reactant(s), 2 points or product(s) (e) An acidic solution o potassium dichromate is added to a solution o iron(ii) nitrate. Cr 2 O Fe 2+ + H + Fe 3+ + Cr 3+ + H 2 O 1 point or reactant(s), 2 points or product(s) Visit apcentral.collegeboard.com (or AP proessionals) and (or AP students and parents). 10

11 Question 4 (cont d.) () Excess concentrated aqueous ammonia is added to a solution o nickel(ii) bromide. Ni 2+ + NH 3 [Ni(NH 3 ) 6 ] 2+ 1 point or reactant(s), 2 points or product(s) Note: complexes with numbers o NH 3 ligands less than 6 were also accepted. (g) A solution o sodium phosphate is added to a solution o aluminum nitrate. PO Al 3+ AlPO 4 1 point or reactant(s), 2 points or product(s) (h) Concentrated hydrochloric acid is added to a solution o sodium sulide. H + + OH + HS H 2 O + H 2 S OR H + + HS H 2 S 1 point or reactant(s), 2 points or product(s) Visit apcentral.collegeboard.com (or AP proessionals) and (or AP students and parents). 11

12 Question 5 In a laboratory class, a student is given three lasks that are labeled Q, R, and S. Each lask contains one o the ollowing solutions: 1.0 M Pb(NO 3 ) 2, 1.0 M NaCl, or 1.0 M K 2 CO 3. The student is also given two lasks that are labeled X and Y. One o these lasks contains 1.0 M AgNO 3, and the other contains 1.0 M BaCl 2. This inormation is summarized in the diagram below. (a) When the student combined a sample o solution Q with a sample o solution X, a precipitate ormed. A precipitate also ormed when samples o solutions Q and Y were combined. (i) Identiy solution Q. Solution Q is K 2 CO 3 1 point or correct identiication o solution Q (ii) Write the chemical ormulas or each o the two precipitates. Ag 2 CO 3 and BaCO 3 1 point each or correct ormulas o carbonate precipitates (b) When solution Q is mixed with solution R, a precipitate orms. However, no precipitate orms when solution Q is mixed with solution S. (i) Identiy solution R and solution S. Solution R is Pb(NO 3 ) 2 and solution S is NaCl. 1 point each or identiication o solutions R and S (ii) Write the chemical ormula o the precipitate that orms when solution Q is mixed with solution R. PbCO 3 1 point or correct ormula o the precipitate Visit apcentral.collegeboard.com (or AP proessionals) and (or AP students and parents). 12

13 Question 5 (cont d.) (c) The identity o solution X and solution Y are to be determined using only the ollowing solutions: 1.0 M Pb(NO 3 ) 2, 1.0 M NaCl, and 1.0 M K 2 CO 3. (i) Describe a procedure to identiy solution X and solution Y. The identities o solutions X and Y can be determined by adding a sample o NaCl to each solution. OR The identities o solutions X and Y can be determined by adding a sample o Pb(NO 3 ) 2 to solution X and solution Y. 1 point or a correct identiication o a reagent that will dierentiate between the two solutions (ii) Describe the observations that would allow you to distinguish between solution X and solution Y. NaCl will orm a white precipitate when added to a solution o AgNO 3, but will not orm a precipitate when added to a solution o BaCl 2. OR Pb(NO 3 ) 2 will orm a white precipitate when added to a solution o BaCl 2, but will not orm a precipitate when added to a solution o AgNO 3. 1 point or correct observation that a precipitate is ormed in one solution but not the other (iii) Explain how the observations would enable you to distinguish between solution X and solution Y. When NaCl is added to solution X, a precipitate o AgCl orms i solution X is AgNO 3. I no precipitate orms, solution X must be BaCl 2. The same logic can be used to identiy solution Y. OR When Pb(NO 3 ) 2 is added to solution X, a precipitate o PbCl 2 orms i solution X is BaCl 2. I no precipitate orms, solution X must be AgNO 3. The same logic can be used to identiy solution Y. 1 point or correct identiication o the solution that orms a precipitate Visit apcentral.collegeboard.com (or AP proessionals) and (or AP students and parents). 13

14 Question 6 An electrochemical cell is constructed with an open switch, as shown in the diagram above. A strip o Sn and a strip o an unknown metal, X, are used as electrodes. When the switch is closed, the mass o the Sn electrode increases. The hal-reactions are shown below. Sn 2+ (aq) + 2 e Sn(s) E = 0.14 V X 3+ (aq) + 3 e X(s) E =? (a) In the diagram above, label the electrode that is the cathode. Justiy your answer. The Sn (tin) electrode is the cathode. The increase in mass indicates that reduction occurs at the Sn electrode: Sn 2+ (aq) + 2 e Sn(s) Reduction occurs at the cathode. 1 point or identiying Sn as the cathode 1 point or reasoning based on increase in mass (b) In the diagram above, draw an arrow indicating the direction o the electron low in the external circuit when the switch is closed. Diagram should have arrow showing electrons lowing rom the anode towards the cathode. 1 point or correct direction o electron low Visit apcentral.collegeboard.com (or AP proessionals) and (or AP students and parents). 14

15 (c) I the standard cell potential, electrode? Question 6 (cont ) Ecell o, is V, what is the standard reduction potential, in volts, or the X 3+ /X Ecell o = Ecathode o Eanode o V = 0.14 V Eanode o Eanode o = 0.74 V 1 point or correct potential with correct sign (d) Identiy metal X. Cr 1 point or correct metal (e) Write a balanced net-ionic equation or the overall chemical reaction occurring in the cell. 3 Sn Cr 3 Sn + 2 Cr 3+ 1 point or correctly balanced net-ionic equation () In the cell, the concentration o Sn 2+ is changed rom 1.0 M to 0.50 M, and the concentration o X 3+ is changed rom 1.0 M to 0.10 M. (i) Substitute all the appropriate values or determining the cell potential, not do any calculations.) E cell, into the Nernst equation. (Do E cell = Ecell o [Cr ] log 2+ 3 n [Sn ] E cell = V [0.10] log 6 [0.50] point or using Ecell o 1 point or using n = 6 = V 1 point or substituting correctly into the Q expression based on the equation in part (e) (ii) On the basis o your response in part () (i), will the cell potential, or equal to the original Ecell o? Justiy your answer. E cell, be greater than, less than, E cell will be greater (more positive). Since the Q ratio is a number less than 1, the log o the ratio will be negative. A negative times a negative is positive. Thus, [0.10] log increases E 3 cell 6 [0.50] 1 point or the correct prediction with an explanation based on Q Visit apcentral.collegeboard.com (or AP proessionals) and (or AP students and parents). 15

16 Question 7 Use appropriate chemical principles to account or each o the ollowing observations. In each part, your response must include speciic inormation about both substances. (a) At 25 C and 1 atm, F 2 is a gas, whereas I 2 is a solid. Both F 2 and I 2 are nonpolar, so the only intermolecular attractive orces are London dispersion orces. I 2 is solid because the electrons in the I 2 molecule occupy a larger volume and are more polarizable compared to the electrons in the F 2 molecule. As a result, the dispersion orces are considerably stronger in I 2 compared to F 2. 1 point or indicating that both molecules have dispersion orces as IMFs 1 point or indicating that I 2 molecules are more polarizable than F 2 molecules (b) The melting point o NaF is 993 C, whereas the melting point o CsCl is 645 C. Both NaF and CsCl are ionic compounds with the same charges on the cations and anions. The ionic radius o Na + is smaller than the ionic radius o Cs + and the ionic radius o F is smaller than the ionic radius o Cl. Thereore, the ionic centers are closer in NaF than in CsCl. Melting occurs when the attraction between the cation and the anion are overcome due to thermal motion. Since the lattice energy is inversely proportional to the distance between the ion centers (Coulomb s Law), the compound with the smaller ions will have the stronger attractions and the higher melting point. 1 point or indicating that NaF and CsCl are both ionic compounds (or are composed o M + and X ions) 1 point or indicating that the strength o these orces is determined by the distance between the ionic centers (or the size o the ions) (c) The shape o the ICl 4 ion is square planar, whereas the shape o the BF 4 ion is tetrahedral. The central iodine atom in ICl 4 has our bonding pairs and two lone pairs o electrons on the central iodine atom, so the molecular geometry is square planar. BF 4 has our bonding pairs and no lone pairs on the central boron atom, so the molecular geometry is tetrahedral. 2 points or indicating that ICl 4 has two unshared electron pairs, but BF 4 has no unshared pairs Note: 1 point earned i student gives incorrect numbers o unshared electron pairs but indicates that dierence in number o unshared electron pairs determines dierence in geometry. Visit apcentral.collegeboard.com (or AP proessionals) and (or AP students and parents). 16

17 Question 7 (cont d.) (d) Ammonia, NH 3, is very soluble in water, whereas phosphine, PH 3, is only moderately soluble in water. Ammonia has hydrogen-bonding intermolecular orces, whereas phosphine has dipole-dipole and/or dispersion intermolecular orces. Water also has hydrogen-bonding intermolecular attractive orces. Ammonia is more soluble in water than phosphine because ammonia molecules can hydrogen-bond with water molecules, whereas phosphine molecules cannot hydrogen-bond with water molecules. 1 point or indicating that NH 3 can orm hydrogen bonds but PH 3 cannot 1 point or indicating that NH 3 can orm hydrogen bonds with water, but PH 3 cannot Visit apcentral.collegeboard.com (or AP proessionals) and (or AP students and parents). 17

18 Question 8 Answer the ollowing questions about carbon monoxide, CO(g), and carbon dioxide, CO 2 (g). Assume that both gases exhibit ideal behavior. (a) Draw the complete Lewis structure (electron-dot diagram) or the CO molecule and or the CO 2 molecule. 1 point or each correct, complete Lewis structure (b) Identiy the shape o the CO 2 molecule. CO 2 has a linear molecular geometry 1 point or correct molecular geometry (c) One o the two gases dissolves readily in water to orm a solution with a ph below 7. Identiy the gas and account or this observation by writing a chemical equation. The gas that produces a ph less than 7 when added to water is CO 2. The reaction that accounts or this is CO 2 (g) + H 2 O(l) HCO 3 (aq) + H + (aq) OR, CO 2 (g) + H 2 O(l) H 2 CO 3 (aq) 1 point or identiying CO 2 in a correct chemical equation (d) A 1.0 mole sample o CO(g) is heated at constant pressure. On the graph below, sketch the expected plot o volume versus temperature as the gas is heated. The graph should have a straight line with a positive slope. 1 point or drawing a correct line Visit apcentral.collegeboard.com (or AP proessionals) and (or AP students and parents). 18

19 Question 8 (cont d.) (e) Samples o CO(g) and CO 2 (g) are placed in 1 L containers at the conditions indicated in the diagram below. (i) Indicate whether the average kinetic energy o the CO 2 (g) molecules is greater than, equal to, or less than the average kinetic energy o the CO(g) molecules. Justiy your answer. The average kinetic energy is the same or both samples because the temperature is the same or both samples. Average kinetic energy is proportional to temperature. 1 point or correct answer and explanation (ii) Indicate whether the root-mean-square speed o the CO 2 (g) molecules is greater than, equal to, or less than the root-mean-square speed o the CO(g) molecules. Justiy your answer. The root-mean-square speed or CO 2 is lower than the root-meansquare speed or CO. The molar mass o CO 2 is higher than the molar mass o CO. The root-mean-square speed is inversely proportional to the square root o the molar mass o the gas. 1 point or correct answer and explanation (iii) Indicate whether the number o CO 2 (g) molecules is greater than, equal to, or less than the number o CO(g) molecules. Justiy your answer. There are ewer CO 2 molecules than CO molecules. The CO 2 molecules exert hal the pressure o the CO molecules (at the same T and V ), so there must be hal as many molecules present. 1 point or correct answer and explanation Visit apcentral.collegeboard.com (or AP proessionals) and (or AP students and parents). 19