b_chemistry_frq_03

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AP® Chemistry

2003 Free-Response Questions

Form B

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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.

69e2r02e7r4e2H0001.N18.01A96.93K8.n)2.43514238X6381R22(031)l59F0r099.t0.7911C455.33B.395I65287A121(060)8O0.62e6.o961S0e69.423513S.87T7428P021(7458077051b7.i9.P93s2.13SN4103A9.45S28B.80d13712eTm11aN196C04i02e90n7.b2n. t.8221S.835EG5.2S18P70e12712y tMo1281N8l81a2798.l3E5B3.01A9.634n41I18T.§4012G.9612.KL194OE0n93d.g5).23.820O Z546C18H01§7701212(BE 77MH9u55g).7.9u.1O2C3476§276A07A19121RT1( F28 40)HF8i9..265§96CON6.6d8t0845P07P191211(A TE1E7o39h92t)..527.9DL2C845Rr07I29011M662 1(TBONA6e58u1.8s)5.416s2.0 2F541OT5R071O091H62(D C1I5n83)e2.7D2M93c.44T8597(R0h)2681B6251(O5r0o4I4048).R2C.22475M9.59W630g381S621(E45P3V91b19a9.5b)22.045N.32709T0811D62(092i0r222f44).f12T9.744Z.17H071R621(6111c99c30..2S.97a43Y9.85L748*381†A222082324e130Br63.02g.0a.8.6a.142037578a0.9M2C4S8B38R62129190711H70i4a91b49)..9.7.5s0391.121K93527r3.L16N23R58C38F221(

71u9).3r07L4071L621(40b0).2o97Y30711N52(399.d)86m1T80611M52(68r2.06E70m)75611F2(37o9).s26H4969E521(06y5).f16D2869C521(3)5b9.k76T87591B42(54d2.6G76m)74591C2(73u9.6E15m)34591A2()2u46m4.04S591P42(51p0.6m)53P419(N732430d20..6N42849U312149r90..501aP149P312248e1h0.5C.0049T2312sseeiirreeSS eeddiinnaithctnAa†L*

STANDARD REDUCTION POTENTIALS IN AQUEOUS SOLUTION AT 25°C Half-reaction

Co3++e−F2(g)+2e−→→→→→→→→→→→→→→2F−Co2+Au(s)2 Cl−2H2O(l)2 Br−Hg22+Hg(l)Ag(s)2 Hg(l)Fe2+Cu(s)Cu(s)Cu+ E°(V) 2.87182.1.501.361.231.070.920.850.800.790.770.520.340.150.150.140.00−0.13−0.14−0.25−0.28−0.34−0.40−0.41−0.44−0.74−0.76−118.−1.66−1.70−2.37−2.71−2.87−2.−2.90−2.92−2.92−2.92−3.05

Au3++3e−Cl2(g)+2e−Br2(l)+2e−2 Hg2++2e−Hg2++2e−Ag++e−Hg22++2e−Fe3++e−e−I22(s)+ +Cu+e−O2(g)+4H++4e−→−2I0.53Cu2++2e−Cu2++e−Sn4++2e−S(s)+2 H++2e−2H++2e−Pb2++2e−Sn2++2e−2++2e−Ni Co2++2e−→Sn2+ →H2S (g)→→→H2(g)Pb(s)Sn(s)→Ni(s) →Co(s)→Tl(s) →Cd(s)→→→→→→→→→→→→→→→→Cr2+Fe(s)Cr(s)Zn(s)Mn(s)Al(s)Be(s)Mg(s)Na(s)Ca(s)Sr(s)Ba(s)Rb(s)K(s)Cs(s)Li(s)Tl++e−Cd2++2e−Cr3++e−Fe2++2e−Cr3++3e−Zn2++2e−Mn2++2e−Al3++3e−Be2++2e−Mg2++2e−Na++e−Ca2++2e−Sr2++2e−Ba2++2e−Rb++e−K++e−Cs++e−Li++e−

ADVANCED PLACEMENT CHEMISTRY EQUATIONS AND CONSTANTS

ATOMIC STRUCTURE DE=hvc=lvhl=mup=mu−2.178×10−18En=joule2n EQUILIBRIUM [H+][A−]Ka=[HA]Kb=[OH][HB][B]−+−14−+Evlpunm =======energyfrequencywavelengthmomentum velocityprincipal quantum numbermassSpeed of light, c=3.0×108m s−1Planck’s constant, h=6.63×10−34 J sBoltzmann’s constant, k=1.38×10−23 J K−1@25oCAvogadro’s number =6.022×10 moleculesmolElectron charge, e=−1.602×10−19 coulomb1 electron volt per atom=96.5 kJmol−123−1 Kw=[OH][H]=1.0×10=Ka×KbpH=−log[H+], pOH=−log[OH−]14=pH+pOH[A−]pH=pKa+log[HA][HB+]pOH=pKb+log[B]pKa=−logKa,pKb=−logKbKp=Kc(RT)Dn Equilibrium ConstantsKaKbKwKp(weak acid)(weak base)(water)(gas pressure) ,where Dn=moles product gas−moles reactant gasKc(molar concentrations) THERMOCHEMISTRY DSo=∑So products−∑So reactantsDHo So=standard entropyHo=standard enthalpyGo=standard free energyEoTnmqcCp=======standard reduction potentialtemperaturemoles massheatspecific heat capacitymolar heat capacity at constant pressure=DGo∑=∑DGfoDHfoproducts−∑products−∑DGforeactantsDHforeactants DGo=DHo−TDSo =−RTlnK=−2.303RTlogK=−nᏲEoDG=DGo+RTlnQ=DGo+2.303RTlogQq=mcDTDHCp=DT 1 faraday Ᏺ=96,500 coulombs

GASES, LIQUIDS, AND SOLUTIONS 󰀁󰀃P+na󰀄󰀆(V-nb)=nRT󰀂V󰀅22PV=nRT

PA=Ptotal󰀄XA,where XA=Ptotal=PA+PB+PC+...mn=MK=oC+273PVPV11=22T1T2mD=V3kT3RTurms==mM1KE per molecule=mu223KE per mole=RT2r1M2=r2M1moles Atotal moles

PVT nDmu

=======

pressurevolumetemperature

number of moles densitymassvelocity

urmsKErMpiKfKbQIqt

============

root-mean-square speedkinetic energyrate of effusionmolar mass

osmotic pressurevan’t Hoff factor

molal freezing-point depression constantmolal boiling-point elevation constantreaction quotientcurrent (amperes)charge (coulombs)time (seconds)

molarity, M= moles solute per liter solutionmolality= moles solute per kilogram solventDTf=iKf󰀄molalityDTb=iKb󰀄molalitynRTip=V

Eo=standard reduction potentialK=equilibrium constant

Gas constant,R=8.31 J mol-1K-1

=0.0821 L atm mol-1K-1=8.31 volt coulomb mol-1K-1

OXIDATION-REDUCTION; ELECTROCHEMISTRY cd[C][D]Q=, where Aa+bB→cC+dDab[A][B]qI=t o−RTlnQ=Eo−0.0592logQ@25oCEcell=EcellcellnᏲnnEologK=0.0592Boltzmann’s constant,k=1.38󰀄10-23 J K-1

Kf for H2O=186. K kg mol-1Kb for H2O=0.512 K kg mol-1

1atm=760mmHg

=760torr

STP=0.000oC and 1.000 atm

Faraday’s constant,Ᏺ=96,500 coulombs per mole

of electrons

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2003 AP® CHEMISTY FREE-RESPONSE QUESTIONS (Form B)

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 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 goldenrod cover. Do NOT write your answers on the lavender insert.

Answer Question 1 below. The Section II score weighting for this question is 20 percent.

→ H(g) + I(g) 2 HI(g) ←22 1. After a 1.0 mole sample of HI(g) is placed into an evacuated 1.0 L container at 700. K, the reaction represented

above occurs. The concentration of HI(g) as a function of time is shown below.

(a) Write the expression for the equilibrium constant, Kc, for the reaction.

(b) What is [HI] at equilibrium?

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2003 AP® CHEMISTY FREE-RESPONSE QUESTIONS (Form B)

(d) On the graph above, make a sketch that shows how the concentration of H2(g) changes as a function

of time. (e) Calculate the value of the following equilibrium constants at 700. K.

(i) Kc (ii) Kp

(f) At 1,000 K, the value of Kc for the reaction is 2.6 󰀄 10–2. In an experiment, 0.75 mole of HI(g),

0.10 mole of H2(g), and 0.50 mole of I2(g) are placed in a 1.0 L container and allowed to reach

equilibrium at 1,000 K. Determine whether the equilibrium concentration of HI(g) will be greater than, equal to, or less than the initial concentration of HI(g). Justify your answer.

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2003 AP® CHEMISTY FREE-RESPONSE QUESTIONS (Form B)

Answer EITHER Question 2 below OR Question 3 printed on page 9. Only one of these two questions will be graded. If you start both questions, be sure to cross out the question you do not want graded. The Section II score weighting for the question you choose is 20 percent.

2. Answer the following questions that relate to chemical reactions.

(a) Iron(III) oxide can be reduced with carbon monoxide according to the following equation.

Fe2O3(s) + 3 CO(g) 󰀅 2 Fe(s) + 3 CO2(g)

A 16.2 L sample of CO(g) at 1.50 atm and 200.󰀆C is combined with 15.39 g of Fe2O3(s). (i) How many moles of CO(g) are available for the reaction?

(ii) What is the limiting reactant for the reaction? Justify your answer with calculations. (iii) How many moles of Fe(s) are formed in the reaction?

(b) In a reaction vessel, 0.600 mol of Ba(NO3)2(s) and 0.300 mol of H3PO4(aq) are combined with deionized

water to a final volume of 2.00 L. The reaction represented below occurs.

3 Ba(NO3)2(aq) + 2 H3PO4(aq) 󰀅 Ba3(PO4)2(s) + 6 HNO3(aq)

(i) Calculate the mass of Ba(PO)(s) formed.

342 (ii) Calculate the pH of the resulting solution.

(iii) What is the concentration, in mol L–1, of the nitrate ion, NO3–(aq), after the reaction reaches

completion?

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2003 AP® CHEMISTY FREE-RESPONSE QUESTIONS (Form B)

3. In an experiment, a sample of an unknown, pure gaseous hydrocarbon was analyzed. Results showed that the

sample contained 6.000 g of carbon and 1.344 g of hydrogen.

(a) Determine the empirical formula of the hydrocarbon.

(b) The density of the hydrocarbon at 25󰀆C and 1.09 atm is 1.96 g L–1.

(i) Calculate the molar mass of the hydrocarbon. (ii) Determine the molecular formula of the hydrocarbon.

In another experiment, liquid heptane, C7H16(l), is completely combusted to produce CO2(g) and H2O(l), as

represented by the following equation.

C7H16(l) + 11 O2(g) 󰀅 7 CO2(g) + 8 H2O(l)

oThe heat of combustion, DHcomb, for one mole of C7H16(l) is -4.85 󰀄 103 kJ.

Compound CO2(g)

H2O(l)

(d) A 0.0108 mol sample of C7H16(l) is combusted in a bomb calorimeter.

(i) Calculate the amount of heat released to the calorimeter.

DHfo (kJ mol–1)

–393.5 –285.8

(ii) Given that the total heat capacity of the calorimeter is 9.273 kJ 󰀆C –1, calculate the temperature change

of the calorimeter.

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2003 AP® CHEMISTY FREE-RESPONSE QUESTIONS (Form B)

CHEMISTRY

Part B

Time—50 minutes

NO CALCULATORS MAY BE USED FOR PART B.

Answer Question 4 below. The Section II score weighting for this question is 15 percent.

4. Write the formulas to show the reactants and the products for any FIVE of the laboratory situations described

below. Answers to more than five 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 if the substances are extensively ionized. Omit formulas for any ions or molecules that are unchanged by the reaction. You need not balance the equations.

Example: A strip of magnesium is added to a solution of silver nitrate.

(a) Hot hydrogen gas is passed over heated copper(II) oxide solid.

(b) Solid sodium hydride is added to water.

(d) A solution of lead(II) nitrate is added to a solution of potassium sulfate.

(e) Ammonia gas is mixed with hydrogen chloride gas.

(f) Sulfur trioxide gas is bubbled into water.

(g) Excess concentrated potassium hydroxide solution is added to a solution of nickel(II) chloride.

(h) Solid sodium acetate is added to 1.0 M hydrobromic acid.

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2003 AP® CHEMISTY FREE-RESPONSE QUESTIONS (Form B)

Your responses to the rest of the questions in this part of the examination 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.

Answer BOTH Question 5 below AND Question 6 printed on page 12. Both of these questions will be graded. The Section II score weighting for these questions is 30 percent (15 percent each).

5. Oxalic acid, H2C2O4 , is a primary standard used to determine the concentration of potassium permanganate,

KMnO4 , in solution. The equation for the reaction is as follows.

2 KMnO4(aq) + 5 H2C2O4(aq) + 3 H2SO4(aq) 󰀅 2 MnSO4(aq) + 10 CO2(g) + 8 H2O(l) + K2SO4(aq) A student dissolves a sample of oxalic acid in a flask with 30 mL of water and 2.00 mL of 3.00 M H2SO4 . The KMnO4 solution of unknown concentration is in a 25.0 mL buret. In the titration, the KMnO4 solution is added to the solution containing oxalic acid.

(a) What chemical species is being oxidized in the reaction?

(b) What substance indicates the observable endpoint of the titration? Describe the observation that shows the

endpoint has been reached. (c) What data must be collected in the titration in order to determine the molar concentration of the unknown

KMnO4 solution? (d) Without doing any calculations, explain how to determine the molarity of the unknown KMnO4 solution. (e) How would the calculated concentration of the KMnO4 solution be affected if 40 mL of water was added to

the oxalic acid initially instead of 30 mL? Explain your reasoning.

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2003 AP® CHEMISTY FREE-RESPONSE QUESTIONS (Form B)

6. Answer the following questions about electrochemistry.

(a) Several different electrochemical cells can be constructed using the materials shown below. Write the

balanced net-ionic equation for the reaction that occurs in the cell that would have the greatest positive

. value of Ecell

(b) Calculate the standard cell potential, Ecell, for the reaction written in part (a).

shown in the figure below.

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2003 AP® CHEMISTY FREE-RESPONSE QUESTIONS (Form B)

(d) Of the compounds NaOH, CuS, and NaNO3, which one is appropriate to use in a salt bridge? Briefly

explain your answer, and for each of the other compounds, include a reason why it is not appropriate. (e) Another standard cell is based on the following reaction.

Zn + Pb2+ 󰀅 Zn2+ + Pb

Answer EITHER Question 7 below OR Question 8 printed on page 14. Only one of these two questions will be graded. If you start both questions, be sure to cross out the question you do not want graded. The Section II score weighting for the question you choose is 15 percent.

7. Account for the following observations using principles of atomic structure and/or chemical bonding. In each

part, your answer must include specific information about both substances.

(a) The Ca2+ and Cl– ions are isoelectronic, but their radii are not the same. Which ion has the larger radius?

Explain. (b) Carbon and lead are in the same group of elements, but carbon is classified as a nonmetal and lead is

classified as a metal. (c) Compounds containing Kr have been synthesized, but there are no known compounds that contain He. (d) The first ionization energy of Be is 900 kJ mol–1, but the first ionization energy of B is 800 kJ mol–1.

If the concentration of Zn2+ is decreased from 1.0 M to 0.25 M, what effect does this have on the cell potential? Justify your answer.

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2003 AP® CHEMISTY FREE-RESPONSE QUESTIONS (Form B)

8. The decay of the radioisotope I-131 was studied in a laboratory. I-131 is known to decay by beta (-01e) emission.

(a) Write a balanced nuclear equation for the decay of I-131. (b) What is the source of the beta particle emitted from the nucleus?

The radioactivity of a sample of I-131 was measured. The data collected are plotted on the graph below.

(d) The data can be used to show that the decay of I-131 is a first-order reaction, as indicated on the graph

below.

(i) Label the vertical axis of the graph above.

(ii) What are the units of the rate constant, k, for the decay reaction?

(iii) Explain how the half-life of I-131 can be calculated using the slope of the line plotted on the graph. (e) Compare the value of the half-life of I-131 at 25󰀆C to its value at 50󰀆C.

END OF EXAMINATION

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