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Stochiometry and experimental analysis...

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AP® Chemistry 2007 Free-Response Questions Form B

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INFORMATION IN THE TABLE BELOW AND IN THE TABLES ON PAGES 4-6 MAY BE USEFUL IN ANSWERING THE QUESTIONS IN THIS SECTION OF THE EXAMINATION.

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STANDARD REDUCTION POTENTIALS IN AQUEOUS SOLUTION AT 25 C

F ( g)

2 H O( l)

7 2 0 6 1.23

Hg2

0.92

2e

3+

3+

Cl 2 ( g) 2 e O 2(g ) 4H+ Br2 ( l ) 2 e

4e

2+ 2+

g( l) g(s)

+ 2+ 2 3+

g( l) Fe

0.85 0.80 0.79 0.77

I 2 ( s) 2 e + 2+ 2+

Cu

4+

S(s) 2H

Sn +

2e

H S(g)

+

H ( g)

2 4 0.15 0.15 0.14 0.00

2+ 2+

4

2+ 2+ 2+ 3+

Cr

2+ 3+ 2+

0 – 0.41 4 4 6

2 H 2O( l) 2 e 2+ 3+ 2+ 2+

g(s)

8 6 0 – 2.37

+ 2+

7 9 0 2 2 2

2+ 2+ + + + +

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ADVANCED PLACEMENT CHEMISTRY EQUATIONS AND CONSTANTS ATOMIC STRUCTURE

E v

hv c lv h = p mu m 2.178 10 18 En joule n2 E

p

u n m

energy frequency wavelength momentum

Speed of light, c Planck’s constant, h

Ka

[H ][A ] [HA]

Kb

[OH ][HB ] [B]

Kw

Boltzmann’s constant, k

[OH ][H ] Ka Kb

Avogadro’s number

1.0

log [H ], pOH pH pOH

pH

pKa

pOH

pKb

pK a Kp where D

14

log [HB ] [B] log K a , p K b log K b

moles product gas

moles reactant gas

DH f reactants

DG

DGf products

DG f reactants

S reactants

T DS DH ln 2.303 RT logK RT K nᏲ E

ln k

23

JK

10

6.022 1.602

1023 mol 10

19

1 1

coulomb

ln A 0 1 A0 Ea 1 R T

S

standard entropy

H

standard enthalpy

G

standard free energy

E T n m q c Cp

standard reduction potential temperature moles mass heat specific heat capacity molar heat capacity at constant pressure

a

RT lnQ DG mcD T DH DT

1 A t

1.38

K c( RT ) Dn ,

DH f products

t

Js

K a (weak acid) K b (weak base) K w (water) K p (gas pressure) K c (molar concentrations)

[A ] [HA]

DH

ln A

34

log[OH ]

S products

Cp

6.63 10

Electron charge,

DS

DG q

10 m s

@ 25 C

THERMOCHEMISTRY/KINETICS

DG

3.0

1 electron volt per a

pH 14

log

10

velocity principal quantum number mass

DG

k

2.303 RT logQ

activation energy rate constant frequency factor

Faraday's constant,

kt

Gas constant,

96,500 coulombs per mole of electrons 8.31 J mol

K

0.0821 L atm mol

kt

1

K

1

8.31 volt coulomb mol 1 K

1

ln A

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GASES, LIQUIDS, AND SOLUTIONS PV

nRT

nb)

nRT

PA

Ptotal

P V T n D

2

n a V ( V2

P

Ptotal n K PV 1 1 T1 D urms KE per molecule KE per mole r1 r2 molarity,M molality Tf DTb p A

PA m M

X A ,where X A PB

PC

moles A total moles

...

C 273 P2V2 T2 m V 3 kT 3 RT m M 1 u2 m 2 3 RT 2 M2 M1 moles solute per liter solution moles solute per kilogram solvent iK f molality iK b molality iMRT abc

pressure volume temperature number of moles density mass velocity

Kf

root-mean-square speed kinetic energy rate of effusion molar mass osmotic pressure van't Hoff factor molal freezing -point depression constant

Kb A a b c Q I q t

molal boiling-point elevation constant absorbance molar absorptivity path length concentration reaction quotient current (amperes) charge (coulombs) time (seconds)

E

standard reduction potential equilibrium constant

urms KE r

i

OXIDATION-REDUCTION; ELECTROCHEMISTRY Gas constant, R

8.31 J mol

1

1

K

0.0821 L atm mol 1 K

Q

c

d

a

b

[C] [D]

, where a A b B

8.31 volt coulomb mol 1 K

c C d D

[A] [B] I Ecell log

Boltzmann's constant, k

q t Ecell

K f for H 2O

RT ln Q nᏲ

E cell

1

b

0.0592 log Q @ 25 C n

nE 0.0592

for H 2O 1 atm

STP Faraday's constant, Ᏺ

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1.38 10

23

1

JK

1.86 K kg mol

1

0.512 K kg mol 760 mm Hg 760 torr

1

0.0 96,500 coulombs per mole of electrons

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2007 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 the goldenrod booklet. Do NOT write your answers on the lavender insert. Answer Questions 1, 2, and 3. The Section II score weighting for each question is 20 percent. 1. A sample of solid U3O8 is placed in a rigid 1.500 L flask. Chlorine gas, Cl2(g), is added, and the flask is heated are given below. Kp

U

( pUO

Cl

)3 ( pO )

( pCl2 )3

your answer. your answer. (f) After a certain period of time, 1.000 mol of O2(g) is added to the mixture in the flask. Does the mass of U3O8(s) in the flask increase, decrease, or remain the same? Justify your answer.

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2007 AP® CHEMISTRY FREE-RESPONSE QUESTIONS (Form B) 2. Answer the following problems about gases. (a) The average atomic mass of naturally occurring neon is 20.18 amu. There are two common isotopes of naturally occurring neon as indicated in the table below. Isotope

Mass (amu)

Ne-20 Ne-22

19.99 21.99

(i) Using the information above, calculate the percent abundance of each isotope. (ii) Calculate the number of Ne-22 atoms in a 12.55 g sample of naturally occurring neon.

wavelength, in nanometers, of light that corresponds to this line. (c) In the upper atmosphere, ozone molecules decompose as they absorb ultraviolet (UV) radiation, as shown by the equation below. Ozone serves to block harmful ultraviolet radiation that comes from the Sun. O3(g)

UV

(i) How much energy, in joules, does the O3(g) molecule absorb per photon?

answer with a calculation.

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2007 AP® CHEMISTRY FREE-RESPONSE QUESTIONS (Form B)

3. In a hydrogen-oxygen fuel cell, energy is produced by the overall reaction represented above.

Calculate the volume of H2(g) consumed during the same time period. Express your answer in liters

(b) Given that the fuel cell reaction takes place in an acidic medium, (i) write the two half reactions that occur as the cell operates, (ii) identify the half reaction that takes place at the cathode, and

(c) Calculate the charge, in coulombs, that passes through the cell during the 78.0 minutes of operation as described in part (a).

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.

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2007 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, 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) Solid ammonium carbonate decomposes as it is heated. (i) Balanced equation:

______________________________________________________________________________________ ______________________________________________________________________________________

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2007 AP® CHEMISTRY FREE-RESPONSE QUESTIONS (Form B) (b) Chlorine gas, an oxidizing agent, is bubbled into a solution of potassium bromide. (i) Balanced equation:

(ii) What is the oxidation number of chlorine before the reaction occurs? What is the oxidation number of chlorine after the reaction occurs? ______________________________________________________________________________________ ______________________________________________________________________________________ (c) A small piece of sodium is placed in a beaker of distilled water. (i) Balanced equation:

(ii) The reaction is exothermic, and sometimes small flames are observed as the sodium reacts with the water. Identify the product of the reaction that burns to produce the flames. ______________________________________________________________________________________ ______________________________________________________________________________________

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2007 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 laboratory situations involving acids, bases, and buffer solutions. (a) Lactic acid, HC3H5O3 , reacts with water to produce an acidic solution. Shown below are the complete Lewis structures of the reactants.

In the space provided above, complete the equation by drawing the complete Lewis structures of the reaction products. (b) Choosing from the chemicals and equipment listed below, describe how to prepare 100.00 mL of a 1.00 M appropriate. NH4Cl(s)

50 mL buret

100 mL graduated cylinder

100 mL pipet

Distilled water

100 mL beaker

100 mL volumetric flask

Balance

(c) Two buffer solutions, each containing acetic acid and sodium acetate, are prepared. A student adds 0.10 mol of HCl to 1.0 L of each of these buffer solutions and to 1.0 L of distilled water. The table below shows the pH measurements made before and after the 0.10 mol of HCl is added. pH Before HCl Added

pH After HCl Added

Distilled water

7.0

1.0

Buffer 1

4.7

2.7

Buffer 2

4.7

4.3

(i) Write the balanced net-ionic equation for the reaction that takes place when the HCl is added to buffer 1 or buffer 2. (ii) Explain why the pH of buffer 1 is different from the pH of buffer 2 after 0.10 mol of HCl is added. (iii) Explain why the pH of buffer 1 is the same as the pH of buffer 2 before 0.10 mol of HCl is added.

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2007 AP® CHEMISTRY FREE-RESPONSE QUESTIONS (Form B)

First Ionization Energy

Second Ionization Energy

Third Ionization Energy

Element 1

1,251

2,300

3,820

Element 2

496

4,560

6,910

Element 3

738

1,450

7,730

Element 4

1,000

2,250

3,360

6. The table above shows the first three ionization energies for atoms of four elements from the third period of the periodic table. The elements are numbered randomly. Use the information in the table to answer the following questions. (a) Which element is most metallic in character? Explain your reasoning. (b) Identify element 3. Explain your reasoning. (c) Write the complete electron configuration for an atom of element 3. (d) What is the expected oxidation state for the most common ion of element 2 ? (e) What is the chemical symbol for element 2 ? (f) A neutral atom of which of the four elements has the smallest radius?

STOP END OF EXAM

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