Download Problem 1 from 2007 form B AP exam

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
Half-reaction
E ’(V)
2F

Co
2

Au(s)

2 Cl 
2 H 2 O(l )

2 Br 2 Hg2+ 2e 
Hg2 2+
Hg2+ 2 e 
Hg(l )

Ag(s)

2 Hg(l )

Fe 2+
I 2 (s ) 2 e 
2 I
Cu+ e 
Cu(s)

Cu(s)

Cu+
Sn 4+ 2e 
Sn2+
S(s) 2 H + 2 e F2 ( g ) 2 e
Co
3+
Au
e
3+

3e
Cl2 (g) 2 e
O2 (g) 4H + 4 e Br2 (l ) 2 e
+
Ag e
Hg2
Fe
2+
3+
2e
e
Cu
2+
2e
Cu
2+
e

H 2S(g)
+

H2 ( g)
Pb
2+
2e

Pb(s)
Sn
2+
2e

Sn(s)

Ni(s)
2e

Co(s)
2e

Cd(s)

Cr 2+
Fe 2+ 2 e 
Fe(s)

Cr(s)

Zn(s)

H 2 ( g) + 2 OH 
Mn(s)

Al(s)

Be(s)

Mg(s)
2 H 2e
Ni
2+
2e
Co
2+
Cd
2+
Cr
Cr
3+
e
3+
Zn
3e
2+
2e
2 H 2 O(l ) 2 e
Mn2+ 2e Al
3+
Be
3e
2+
Mg
2e
2+
Na e
Sr
2+
2+
Ba

Na(s)

Ca(s)

Sr(s)

Ba(s)

Rb(s)
2e
2e
2e
+
Rb e
2+
+
2e
+
Ca

K(s)
+

Cs(s)
+

Li(s)
K e
Cs e
Li e
-3-
2.87
1.82
1.50
1.36
1.23
1.07
0.92
0.85
0.80
0.79
0.77
0.53
0.52
0.34
0.15
0.15
0.14
0.00
– 0.13
– 0.14
– 0.25
– 0.28
– 0.40
– 0.41
– 0.44
– 0.74
– 0.76
– 0.83
– 1.18
– 1.66
– 1.70
– 2.37
– 2.71
– 2.87
– 2.89
– 2.90
– 2.92
– 2.92
– 2.92
– 3.05
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ADVANCED PLACEMENT CHEMISTRY EQUATIONS AND CONSTANTS
E
v
l
p
ATOMIC STRUCTURE
hv
c lv
h
l=
p mu
mu
2.178 – 10 18
joule
En
n2
E
EQUILIBRIUM
[H ][A ]
Ka
[HA]
[OH ][HB ]
Kb
[B]
K w [OH ][H ] 1.0 – 10 14 @ 25DC
K a – Kb
pH
14
log [H ], pOH
pH pOH
pH
pK a log
pOH
pK b log
pK a
Kp
where D n
Kc ( RT )
DG D
Ç DGfD products Ç DGfD reactants
DG D
DH D T D S D
RT ln K 2.303 RT log K
n Ᏺ E D
ln k
Boltzmann’s constant, k
1.38 – 10 23 J K 1
Avogadro’s number
6.022 – 1023 mol 1
1.602 – 10 19 coulomb
96.5 kJ mol 1
K c (molar concentrations)
moles product gas moles reactant gas
ln >A @ t ln > A @0
1
1
>A@ t >A@0
6.63 – 10 34 J s
K a (weak acid)
K b (weak base)
K w (water)
K p (gas pressure)
,
DH D
Cp
Planck’s constant, h
Equilibrium Constants
[A ]
[HA]
DG D RT ln Q
mcDT
DH
DT
3.0 – 108 m s1
log[OH ]
Ç SD products Ç SD reactants
Ç DHfD products Ç DH fD reactants
DG
q
velocity
principal quantum number
mass
Speed of light, c
1 electron volt per atom
THERMOCHEMISTRY/KINETICS
DS D
u
n
m
Electron charge, e
[HB ]
[B]
log K a , pKb log Kb
Dn
energy
frequency
wavelength
momentum
DG D 2.303 RT log Q
SD
standard entropy
HD
standard enthalpy
GD
standard free energy
ED
T
n
m
q
c
Cp
standard reduction potential
temperature
moles
mass
heat
specific heat capacity
molar heat capacity at constant pressure
Ea
k
A
activation energy
rate constant
frequency factor
Faraday's constant, Ᏺ
kt
Gas constant, R
96,500 coulombs per mole
of electrons
8.31 J mol 1 K 1
0.0821 L atm mol 1 K 1
kt
8.31 volt coulomb mol 1 K 1
Ea 1
ln A
R T
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GASES, LIQUIDS, AND SOLUTIONS
PV
nRT
È
n aØ
ÉÊ P 2 ÙÚ (V nb)
V
nRT
P
V
T
n
D
m
u
2
Ptotal – X A , where X A
PA
PA PB PC ...
m
M
Ptotal
n
K
PV
1 1
T1
D
urms
KE per molecule
KE per mole
r1
r2
molarity, M
molality
DT f
DTb
p
A
moles A
total moles
DC
273
P2V2
T2
m
V
3kT
3RT
m
M
1 2
mu
2
3
RT
2
M2
M1
moles solute per liter solution
moles solute per kilogram solvent
iK f – molality
urms
KE
r
M
p
i
Kf
iK b – molality
iMRT
abc
OXIDATION-REDUCTION; ELECTROCHEMISTRY
pressure
volume
temperature
number of moles
density
mass
velocity
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)
ED
K
standard reduction potential
equilibrium constant
Gas constant, R
8.31 J mol 1 K 1
0.0821 L atm mol 1 K 1
Q
[C] c [D] d
a
[A] [B]
I
Ecell
log K
b
8.31 volt coulomb mol 1 K 1
, where a A b B  c C d D
Boltzmann's constant, k
q
t
ED
cell
RT
ln Q
nᏲ
ED
cell
0.0592
log Q @ 25DC
n
nE D
0.0592
K f for H2 O
1.86 K kg mol 1
K b for H2 O
1 atm
0.512 K kg mol 1
760 mm Hg
760 torr
STP
Faraday's constant, Ᏺ
-5-
1.38 – 10 23 J K 1
0.00 D C and 1.0 atm
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
to 862qC. The equation for the reaction that takes place and the equilibrium-constant expression for the reaction
are given below.
 3 UO Cl (g) + O (g)
U3O8(s) + 3 Cl2(g) Ž
2 2
2
Kp
( pUO2Cl2 )3 ( pO2 )
( pCl2 )3
When the system is at equilibrium, the partial pressure of Cl2(g) is 1.007 atm and the partial pressure
of UO2Cl2(g) is 9.734 u 104 atm.
(a) Calculate the partial pressure of O2(g) at equilibrium at 862qC.
(b) Calculate the value of the equilibrium constant, Kp , for the system at 862qC.
(c) Calculate the Gibbs free-energy change, 'Gq, for the reaction at 862qC.
(d) State whether the entropy change, 'Sq, for the reaction at 862qC is positive, negative, or zero. Justify
your answer.
(e) State whether the enthalpy change, 'Hq, for the reaction at 862qC is positive, negative, or zero. Justify
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.
(b) A major line in the emission spectrum of neon corresponds to a frequency of 4.34 u 1014 s1. Calculate the
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.
UV
 O2(g) + O(g)
O3(g) A molecule of O3(g) absorbs a photon with a frequency of 1.00 u 1015 s1.
(i) How much energy, in joules, does the O3(g) molecule absorb per photon?
(ii) The minimum energy needed to break an oxygen-oxygen bond in ozone is 387 kJ mol1. Does a
photon with a frequency of 1.00 u 1015 s1 have enough energy to break this bond? Support your
answer with a calculation.
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2007 AP® CHEMISTRY FREE-RESPONSE QUESTIONS (Form B)
2 H2(g) + O2(g) o 2 H2O(l )
3. In a hydrogen-oxygen fuel cell, energy is produced by the overall reaction represented above.
(a) When the fuel cell operates at 25qC and 1.00 atm for 78.0 minutes, 0.0746 mol of O2(g) is consumed.
Calculate the volume of H2(g) consumed during the same time period. Express your answer in liters
measured at 25qC and 1.00 atm.
(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
(iii) determine the value of the standard potential, E q, of the cell.
(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:
(ii) Predict the algebraic sign of 'Sq for the reaction. Explain your reasoning.
______________________________________________________________________________________
______________________________________________________________________________________
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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
aqueous solution of NH4Cl (molar mass 53.5 g mol1). Include specific amounts and equipment where
appropriate.
NH4Cl(s)
Distilled water
50 mL buret
100 mL beaker
100 mL graduated cylinder
100 mL volumetric flask
100 mL pipet
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
(kJ mol1)
Second
Ionization Energy
(kJ mol1)
Third
Ionization Energy
(kJ mol1)
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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