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AP? Chemistry

2005 Free-Response Questions

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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.

D O N O T D

E T A C H

F R O M B O O K .

P E R I O D I C T A B L E O F T H E E L E M E N T S

311193785.4755132.9187(223)412203887.6256137.3388226.02213988.91

57138.9189

227.03

224091.2272

178.49234192.91

73180.95244295.9474

183.852543(98)75186.21

2644101.1

76190.22745102.9177192.2

2846106.4278

195.082947107.8779

196.973048

112.4180

200.5951331

49114.8281

204.38

614

3250118.7182

207.2

715

3351121.7583

208.98

8163452127.6084

(209)

1735

53126.9185

(210)

131.29

(222)

L i N a

K R b C s F r

B e

M g

C a S r B a R a

S c Y *L a A c

T i

Z r

H f

N b

T a

C r

M o

M n

T c

R e F e

R u

O s

C o

R h

I r

N i

P d

P t

C u

A g

A u

Z n

C d

H g

110

(269)

111

(272)

112

(277)

A l

G a

I n

T l

S i

G e

S n

P b

A s

S b

B i

S e

T e

P o

C l

B r

A t

N e

A r

K r

X e

R n

H e

104(261)

105(262)106(263)107(262)108

(265)

109

(266)

R f D b

S g B h H s M t

1.0079

6.94122.9939.109.01224.30

40.0844.96

47.9050.9452.0054.93855.8558.9358.6963.5565.39

10.81126.98

69.72

12.01128.09

72.59

14.007

30.974

74.92

16.00

32.06

78.96

19.00

35.45379.90

20.179

39.948

83.80

4.0026

5859606162636465666768697071

909192

939495

100

101

102

C e

P r

N d P m

S m

E u

G d

T b

D y

H o

E r

T m

Y b

L u

T h P a U

N p P u

A m

C m

B k

C f

E s

F m

M d

N o

L r

103

140.12140.91144.24

(145)

150.4

151.97157.25158.93162.50164.93167.26168.93173.04174.97

232.04231.04238.03237.05

(244)

(243)

(247)

(251)

(252)

(257)

(258)

(259)

(260)

*L a n t h a n i d e S e r i e s

?A c t i n i d e S e r i e s ?§N o t y e t n a m e d

STANDARD REDUCTION POTENTIALS IN AQUEOUS SOLUTION AT 25°C

Half-reaction

E °()V

2F ()2g e -+ ? 2F - 2.87 3+Co e -+

? 2Co + 1.82 3+Au 3e -+ ? Au()s 1.50 2Cl ()2g e -+

? 2Cl - 1.36 +2O ()4H 4g e -++? 22H O()l

1.23 2Br ()2l e -+ ? 2Br -

1.07 2+2Hg 2e -+ ? 2+

2Hg 0.92 2+Hg 2e -+ ? Hg()l 0.85 +Ag e -+

? Ag()s 0.80 2+2Hg 2e -+ ? 2Hg()l 0.79 3+Fe e -+

? 2+Fe

0.77 2I ()2s e -+ ? 2I - 0.53 +Cu e -+

? Cu()s 0.52 2+Cu 2e -+ ? Cu()s 0.34 2+Cu e -+ ? +Cu 0.15 4+Sn 2e -+ ? 2+Sn

0.15 +S()2H 2s e -++? 2H S()g 0.14 +2H 2e -+ ? 2H ()g 0.00 2+Pb 2e -+ ? Pb()s –0.13 2+Sn 2e -+ ? Sn()s

–0.14 2+Ni 2e -+ ? Ni()s –0.25 2+Co 2e -+ ? Co()s –0.28 2+Cd 2e -+ ? Cd()s –0.40 3+Cr e -+ ? 2+Cr

–0.41 2+Fe 2e -+ ? Fe()s –0.44 3+Cr 3e -+ ? Cr()s –0.74 2+Zn 2e -+ ? Zn()s

–0.76 22H O()2l e -+? 2H () + 2OH g -–0.83 2+Mn 2e -+ ? Mn()s –1.18 3+Al 3e -+ ? Al()s –1.66 2+Be 2e -+ ? Be()s –1.70 2+Mg 2e -+ ? Mg()s

–2.37 +Na e -+ ? Na()s –2.71 2+Ca 2e -+ ? Ca()s –2.87 2+Sr 2e -+ ? Sr()s –2.89 2+Ba 2e -+ ? Ba()s –2.90 +Rb e -+ ? Rb()s –2.92 +K e -+ ? K()s –2.92 +Cs e -+ ? Cs()s

–2.92

+Li e -+

Li()s

–3.05

ADVANCED PLACEMENT CHEMISTRY EQUATIONS AND CONSTANTS

P V T n D m =======pressure volume temperature

number of moles density mass velocity

u KE r i K K Q I q t

E K rms f b A a b c ============

======root-mean-square speed kinetic energy rate of effusion molar mass

osmotic pressure van't Hoff factor

molal freezing-point depression constant molal boiling-point elevation constant reaction quotient current (amperes)charge (coulombs)time (seconds)

standard reduction potential equilibrium constant

absorbance

molar absorptivity path length concentration M p D

Gas constant, J mol K L atm mol K volt coulomb mol K Boltzmann's constant,J K for H O K kg mol for H O K kg mol STP C and atm

Faraday's constant,coulombs per mole

of electrons

atm mm Hg

torr

R k K K f b ====￥===----------===83100821831138101860512000010009650011

1111

231

2121

1760760........,D ?

2005 AP? CHEMISTRY FREE-RESPONSE QUESTIONS

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 pink cover. Do NOT write your answers on the green insert.

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

HC3H5O2(aq)→← C3H5O2–(aq) + H+(aq) K a = 1.34 × 10–5

1. Propanoic acid, HC3H5O2 , ionizes in water according to the equation above.

(a) Write the equilibrium-constant expression for the reaction.

(b) Calculate the pH of a 0.265 M solution of propanoic acid.

of propanoic acid. Assuming that no change in the volume of the solution occurs, calculate each of the

following.

(i) The concentration of the propanoate ion, C3H5O2–(aq), in the solution

(ii) The concentration of the H+(aq)ion in the solution

The methanoate ion, HCO2–(aq), reacts with water to form methanoic acid and hydroxide ion, as shown in the following equation.

HCO2–(aq) + H2O(l) →← HCO2H(aq) + OH–(aq)

(d) Given that [OH–] is 4.18 × 10–6M in a 0.309 M solution of sodium methanoate, calculate each of the

following.

(i) The value of K b for the methanoate ion, HCO2–(aq)

(ii) The value of K a for methanoic acid, HCO2H

(e) Which acid is stronger, propanoic acid or methanoic acid? Justify your answer.

Answer EITHER Question 2 below OR Question 3 printed on page 8-9. 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 about a pure compound that contains only carbon, hydrogen, and oxygen.

(a) A 0.7549 g sample of the compound burns in O2(g)to produce 1.9061 g of CO2(g) and

0.3370 g of H2O(g).

(i) Calculate the individual masses of C, H, and O in the 0.7549 g sample.

(ii) Determine the empirical formula for the compound.

(b) A 0.5246 g sample of the compound was dissolved in 10.0012 g of lauric acid, and it was determined that

the freezing point of the lauric acid was lowered by 1.68°C. The value of K f of lauric acid is 3.90°C m–1.

Assume that the compound does not dissociate in lauric acid.

(i) Calculate the molality of the compound dissolved in the lauric acid.

(ii) Calculate the molar mass of the compound from the information provided.

the answers to parts (a)(ii) and (b)(ii).

(d) Further tests indicate that a 0.10 M aqueous solution of the compound has a pH of 2.6. Identify the organic

functional group that accounts for this pH.

3. Answer the following questions related to the kinetics of chemical reactions.

I–(aq) + ClO–(aq) OH-

???? IO–(aq) + Cl–(aq)

Iodide ion, I–, is oxidized to hypoiodite ion, IO–, by hypochlorite, ClO–, in basic solution according to the equation above. Three initial-rate experiments were conducted; the results are shown in the following table.

Experiment

[I–]

(mol L?1)

[ClO–]

(mol L?1)

Initial Rate of

Formation of IO–

(mol L?1 s–1)

1 0.017 0.015 0.156

2 0.052 0.015 0.476

3 0.016 0.061 0.596

(a) Determine the order of the reaction with respect to each reactant listed below. Show your work.

(i) I–(aq)

(ii) ClO–(aq)

(b) For the reaction,

(i) write the rate law that is consistent with the calculations in part (a);

(ii) calculate the value of the specific rate constant, k, and specify units.

The catalyzed decomposition of hydrogen peroxide, H2O2(aq), is represented by the following equation.

2 H2O2(aq2O(l) + O2(g)

The kinetics of the decomposition reaction were studied and the analysis of the results show that it is a first-order reaction. Some of the experimental data are shown in the table below.

[H2O2] (mol L?1)

Time (minutes)

1.000.0 0.78 5.0 0.6110.0

(i) Label the vertical axis of the graph.

(ii) What are the units of the rate constant, k, for the decomposition of H2O2(aq) ?

(iii) On the graph, draw the line that represents the plot of the uncatalyzed first-order decomposition of 1.00 M H2O2(aq).

S T O P

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.

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) A strip of zinc is placed in a solution of nickel(II) nitrate.

(b) Solid aluminum hydroxide is added to a concentrated solution of potassium hydroxide.

(d) Solid calcium carbonate is added to a solution of ethanoic (acetic) acid.

(e) Lithium metal is strongly heated in nitrogen gas.

(f) Boron trifluoride gas is added to ammonia gas.

(g) Sulfur trioxide gas is bubbled into a solution of sodium hydroxide.

(h) Equal volumes of 0.1 M solutions of lead(II) nitrate and magnesium iodide are combined.

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. Answer the following questions that relate to laboratory observations and procedures.

(a) An unknown gas is one of three possible gases: nitrogen, hydrogen, or oxygen. For each of the three

possibilities, describe the result expected when the gas is tested using a glowing splint (a wooden stick with one end that has been ignited and extinguished, but still contains hot, glowing, partially burned wood).

(b) The following three mixtures have been prepared: CaO plus water, SiO2plus water, and CO2 plus water.

For each mixture, predict whether the pH is less than 7, equal to 7, or greater than 7. Justify your answers.

nitrate, or sodium sulfide. The three beakers are labeled randomly as solution 1, solution 2, and solution 3.

Shown below is a partially completed table of observations made of the results of combining small amounts of different pairs of the solutions.

(i) Write the chemical formula of the black precipitate.

(ii) Describe the expected results of mixing solution 1 with solution 3.

(iii) Identify each of the solutions 1, 2, and 3.

6. Answer the following questions that relate to chemical bonding.

(a) In the boxes provided, draw the complete Lewis structure (electron-dot diagram) for each of the three

molecules represented below.

CF4 PF5SF4

(b) On the basis of the Lewis structures drawn above, answer the following questions about the particular

molecule indicated.

(i) What is the F–C–F bond angle in CF4 ?

(ii) What is the hybridization of the valence orbitals of P in PF5 ?

(iii) What is the geometric shape formed by the atoms in SF4 ?

(i) How many sigma bonds and how many pi bonds are in structure 1 ?

(ii) Which one of the two structures best represents a molecule of OPF3? Justify your answer in terms of formal charge.

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. Use principles of atomic structure, bonding, and/or intermolecular forces to respond to each of the following.

Your responses must include specific information about all substances referred to in each question.

(a) At a pressure of 1 atm, the boiling point of NH3(l) is 240 K, whereas the boiling point of NF3(l)

is 144 K.

(i) Identify the intermolecular force(s) in each substance.

(ii) Account for the difference in the boiling points of the substances.

(b) The melting point of KCl(s) is 776°C, whereas the melting point of NaCl(s) is 801°C.

(i) Identify the type of bonding in each substance.

(ii) Account for the difference in the melting points of the substances.

Element First Ionization Energy

(kJ mol?1)

Si786

P1,012

Cl1,251

(i) For each of the three elements, identify the quantum level (e.g., n = 1, n = 2, etc.) of the valence

electrons in the atom.

(ii) Explain the reasons for the trend in first ionization energies.

(d) A certain element has two stable isotopes. The mass of one of the isotopes is 62.93 amu and the mass of

the other isotope is 64.93 amu.

(i) Identify the element. Justify your answer.

(ii) Which isotope is more abundant? Justify your answer.

AgNO3(s) → Ag+(aq) + NO3?(aq)

8. The dissolving of AgNO3(s) in pure water is represented by the equation above.

(a) Is ?G for the dissolving of AgNO3(s) positive, negative, or zero? Justify your answer.

(b) Is ?S for the dissolving of AgNO3(s) positive, negative, or zero? Justify your answer.

(i) What is the sign of ?H for the dissolving process? Justify your answer.

(ii) Is the answer you gave in part (a) consistent with your answers to parts (b) and (c) (i) ? Explain.

The compound NaI dissolves in pure water according to the equation NaI(s) → Na+(aq) + I?(aq) . Some of the information in the table of standard reduction potentials given below may be useful in answering the questions that follow.

Half-reaction E° (V)

O2(g) + 4 H+ + 4 e?→ 2 H2O(l) 1.23

I2(s) + 2 e?→ 2 I?0.53

2 H2O(l) + 2 e?→ H2(g) + 2 OH??0.83

Na+ + e?→ Na(s)?2.71

(d) An electric current is applied to a 1.0 M NaI solution.

(i) Write the balanced oxidation half-reaction for the reaction that takes place.

(ii) Write the balanced reduction half-reaction for the reaction that takes place.

(iii) Which reaction takes place at the anode, the oxidation reaction or the reduction reaction?

(iv) All electrolysis reactions have the same sign for ?G°. Is the sign positive or negative? Justify your answer.

END OF EXAM

ap05_frq_chemistry

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