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Unformatted text preview: AP® Chemistry
2007 Scoring Guidelines The College Board: Connecting Students to College Success
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2007 SCORING GUIDELINES
Question 1
Æ
HF(aq) + H2O( l ) ¨ H3O+(aq) + F −(aq) Ka = 7.2 ¥ 10− 4 Hydrofluoric acid, HF(aq), dissociates in water as represented by the equation above.
(a) Write the equilibriumconstant expression for the dissociation of HF(aq) in water.
Ka = [H3O+ ][F − ]
[HF] One point is earned for the correct expression. (b) Calculate the molar concentration of H3O+ in a 0.40 M HF(aq) solution. Ka = [H3O+ ][F − ]
( x)( x)
=
= 7.2 ¥ 10− 4
[HF]
0.40 − x One point is earned for the correct setup
(or the setup consistent with part (a)). Assume x << 0.40, then x2 = (0.40)(7.2 ¥ 10− 4)
One point is earned for the correct concentration. x = [ H3O+ ] = 0.017 M HF(aq) reacts with NaOH(aq) according to the reaction represented below.
HF(aq) + OH−(aq) → H2O(l) + F −(aq)
A volume of 15 mL of 0.40 M NaOH(aq) is added to 25 mL of 0.40 M HF(aq) solution. Assume that
volumes are additive.
(c) Calculate the number of moles of HF(aq) remaining in the solution.
mol HF(aq) = initial mol HF(aq) − mol NaOH(aq) added
= (0.025 L)(0.40 mol L−1) − (0.015 L)(0.40 mol L−1)
= 0.010 mol − 0.0060 mol = 0.004 mol One point is earned for
determining the initial number
of moles of HF and OH− .
One point is earned for setting up
and doing correct subtraction. (d) Calculate the molar concentration of F −(aq) in the solution. mol F −(aq) formed = mol NaOH(aq) added = 0.0060 mol F −(aq)
0.0060 mol F − (aq)
= 0.15 M F −(aq)
(0.015 + 0.025) L of solution One point is earned for
determining the number
of moles of F −(aq).
One point is earned for dividing
the number of moles of F −(aq)
by the correct total volume. © 2007 The College Board. All rights reserved.
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2007 SCORING GUIDELINES Question 1 (continued)
(e) Calculate the pH of the solution. [HF] =
Ka = ⇒ 0.004 mol HF
= 0.10 M HF
0.040 L [H3O+ ][F − ]
[HF] × K a
⇒
= [H3 O+ ]
−]
[HF]
[F 0.10 M (7.2 × 10
0.15 M −4 ) = 4.8 × 10− 4 ⇒ pH = − log (4.8 × 10− 4) = 3.32 One point is earned for indicating
that the resulting solution is a buffer
(e.g., by showing a ratio of [F −] to [HF]
or moles of F − to HF ). OR
pH = pKa + log [F − ]
[HF] = − log (7.2 × 10− 4) + log One point is earned for the correct calculation
of pH.
0.15 M
0.10 M = 3.14 + 0.18
= 3.32 © 2007 The College Board. All rights reserved.
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2007 SCORING GUIDELINES
Question 2
N2(g) + 3 F2(g) → 2 NF3(g) ΔH 298 = − 264 kJ mol−1; ΔS298 = − 278 J K−1 mol−1 The following questions relate to the synthesis reaction represented by the chemical equation in the box
above.
(a) Calculate the value of the standard free energy change, ΔG298 , for the reaction.
One point is earned for correct
substitution. ΔG298 = ΔH298 − T ΔS298 = − 264 kJ mol−1 − (298 K)(−0.278 kJ mol−1 K−1)
= −181 kJ mol−1 One point is earned for the value of
ΔG298 (including kJ or J). (b) Determine the temperature at which the equilibrium constant, Keq , for the reaction is equal to 1.00 .
(Assume that ΔH ° and ΔS ° are independent of temperature.)
When Keq = 1, then ΔGT = −RT ln(1) = 0
If ΔGT = 0 , then 0 = ΔH °− TΔS ° ⇒
T= T= One point is earned for indicating
that if Keq = 1, then ΔGT = 0. ΔH298
ΔS298
 264 kJ mol 1
= 950. K
 0.278 kJ K 1mol 1 One point is earned for the answer
(including the unit K). (c) Calculate the standard enthalpy change, ΔH °, that occurs when a 0.256 mol sample of NF3(g) is
formed from N2(g) and F2(g) at 1.00 atm and 298 K.
One point is earned for multiplying ΔH298
by the number of moles of NF3 formed.
0.256 mol NF3(g) × −264 kJ
= −33.8 kJ
2.00 mol NF3 ( g ) One point is earned for recognizing that
2.00 mol of NF3 are produced for the
reaction as it is written.
One point is earned for the answer
(including kJ or J). © 2007 The College Board. All rights reserved.
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2007 SCORING GUIDELINES
Question 2 (continued)
The enthalpy change in a chemical reaction is the difference between energy absorbed in breaking bonds in
the reactants and energy released by bond formation in the products.
(d) How many bonds are formed when two molecules of NF3 are produced according to the equation in the
box above?
One point is earned for the correct answer. There are six N–F bonds formed. (e) Use both the information in the box above and the table of average bond enthalpies below to calculate
the average enthalpy of the F  F bond. Bond
N≡N 946 N–F 272 F –F ΔH 298 = Average Bond Enthalpy
(kJ mol−1) ? Σ Ebonds broken − Σ Ebonds formed = −264 kJ mol−1 = [ BEN ≡ N + (3 × BE F  F) ] − (6 × BENF)
= [946 kJ mol−1+ (3 × BE F  F) ] − 6(272 kJ mol−1)
= −264 kJ mol−1
⇒ 3 mol BE F  F = (− 264 − 946 + 1,632) kJ mol−1 ⇒ BE F  F = 141 kJ mol−1 One point is earned for the correct
number of bonds in all three compounds
multiplied by the average bond
enthalpies.
One point is earned for the answer
(including kJ or J).
Note: A total of one point is earned if
an incorrect number of bonds is
substituted in a correct equation and the
answer is reasonable (i.e., positive). © 2007 The College Board. All rights reserved.
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2007 SCORING GUIDELINES
Question 3 An external directcurrent power supply is connected to two platinum electrodes immersed in a beaker containing
1.0 M CuSO4(aq) at 25°C, as shown in the diagram above. As the cell operates, copper metal is deposited onto
one electrode and O2(g) is produced at the other electrode. The two reduction halfreactions for the overall
reaction that occurs in the cell are shown in the table below.
HalfReaction E°(V) O2(g) + 4 H+(aq) + 4 e− → 2 H2O(l) +1.23 Cu2+(aq) + 2 e− → Cu(s) +0.34 (a) On the diagram, indicate the direction of electron flow in the wire.
The electron flow in the wire is from the right toward the left (counterclockwise). One point is earned
for the correct direction. (b) Write a balanced net ionic equation for the electrolysis reaction that occurs in the cell. 2 H2O(l) + 2 Cu2+(aq) → 4 H+(aq) + 2 Cu(s) + O2(g) One point is earned
for all three products.
One point is earned
for balancing the equation. (c) Predict the algebraic sign of ΔG° for the reaction. Justify your prediction.
The sign of ΔG° would be positive because
the reaction is NOT spontaneous. One point is earned for indicating
that ΔG° is greater than zero
and supplying a correct explanation. © 2007 The College Board. All rights reserved.
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2007 SCORING GUIDELINES
Question 3 (continued)
(d) Calculate the value of ΔG° for the reaction.
E ° = −1.23 V + 0.34 V = − 0.89 V = − 0.89 J C−1
ΔG° = − n F E ° = − 4 (96,500 C mol−1)(− 0.89 J C−1) = +340,000 J mol−1 = + 340 kJ mol−1 One point is earned for calculating E°.
One point is earned for calculating ΔG°
(consistent with the calculated E°). An electric current of 1.50 amps passes through the cell for 40.0 minutes.
(e) Calculate the mass, in grams, of the Cu(s) that is deposited on the electrode.
One point is earned for calculating q.
q = (1.50 C s−1)(40.0 60 s
min) ×
= 3,600 C
1 minute mass Cu = (3,600 C) × 1 mol e −
1 mol Cu
63.55 g Cu
×
×
1 mol Cu
96,500 C
2 mol e = 1.19 g Cu One point is earned for calculating
the mass of copper deposited.
OR
Two points are earned for
calculating the mass of copper
in one step. (f) Calculate the dry volume, in liters measured at 25°C and 1.16 atm, of the O2(g) that is produced.
nO = (1.19 g Cu) × 1 mol Cu × 1 mol O2 = 0.00936 mol O2
2
63.55 g Cu 2 mol Cu
V= nRT
(0.00936 mol)(0.0821 L atm mol −1 K −1 )(298 K)
=
P
1.16 atm = 0.197 L One point is earned for
calculating the number of
moles of O2.
One point is earned for
calculating V (consistent with
previous calculations). © 2007 The College Board. All rights reserved.
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2007 SCORING GUIDELINES
Question 4
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) A solution of sodium hydroxide is added to a solution of lead(II) nitrate.
(i) Balanced equation:
2 OH− + Pb2+ → Pb(OH)2 One point is earned for the correct reactants.
Two points are earned for the correct product.
One point is earned for balancing the equation
for mass and charge. (ii) If 1.0 L volumes of 1.0 M solutions of sodium hydroxide and lead(II) nitrate are mixed together, how
many moles of product(s) will be produced? Assume the reaction goes to completion.
A total of 0.5 mol of Pb(OH)2 will be produced. One point is earned for the correct number of
moles. (b) Excess nitric acid is added to solid calcium carbonate.
(i) Balanced equation:
2 H+ + CaCO3 → Ca2+ + H2O + CO2 One point is earned for the correct reactants.
Two points are earned for all three of the correct
products; one point is earned for any one or two
of the three.
One point is earned for balancing the equation
for mass and charge. (ii) Briefly explain why statues made of marble (calcium carbonate) displayed outdoors in urban areas are
deteriorating.
The H+ ions in acid rain react with the
marble statues and the soluble compounds
of Ca that are formed wash away. One point is earned for a correct answer
involving acid precipitation. © 2007 The College Board. All rights reserved.
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2007 SCORING GUIDELINES
Question 4 (continued) (c) A solution containing silver(I) ion (an oxidizing agent) is mixed with a solution containing iron(II) ion
(a reducing agent).
(i) Balanced equation:
Ag+ + Fe2+ → Ag + Fe3+ One point is earned for the correct reactants.
One point is earned for each of the two
correct products.
One point is earned for balancing the
equation for mass and charge. (ii) If the contents of the reaction mixture described above are filtered, what substance(s), if any, would
remain on the filter paper?
The precipitated solid silver will remain on the filter paper. One point is earned for the
correct substance. © 2007 The College Board. All rights reserved.
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2007 SCORING GUIDELINES
Question 5
5 Fe2+(aq) + MnO4−(aq) + 8 H+(aq) → 5 Fe3+(aq) + Mn2+(aq) + 4 H2O(l)
The mass percent of iron in a soluble iron(II) compound is measured using a titration based on the balanced
equation above.
(a) What is the oxidation number of manganese in the permanganate ion, MnO4−(aq) ?
+7 One point is earned for the correct oxidation number. (b) Identify the reducing agent in the reaction represented above.
Fe2+(aq) One point is earned for the correct iron ion. The mass of a sample of the iron(II) compound is carefully measured before the sample is dissolved in
distilled water. The resulting solution is acidified with H2SO4(aq). The solution is then titrated with
MnO4−(aq) until the end point is reached.
(c) Describe the color change that occurs in the flask when the end point of the titration has been reached.
Explain why the color of the solution changes at the end point.
The solution in the flask changes from colorless to faint purplepink
at the endpoint of the titration.
At the endpoint there is no Fe2+(aq) left in the flask to reduce the
colored permanganate ion, so when a small amount of permanganate
ion is added after the endpoint, the unreacted permanganate ion
present in the solution colors the solution faint purple/pink. One point is earned for stating
that a faint pink color appears
(unless indication of acidbase
reaction).
One point is earned for a
correct explanation involving
excess MnO4− after all Fe2+
has reacted. (d) Let the variables g, M, and V be defined as follows:
g = the mass, in grams, of the sample of the iron(II) compound
M = the molarity of the MnO4−(aq) used as the titrant
V = the volume, in liters, of MnO4−(aq) added to reach the end point
In terms of these variables, the number of moles of MnO4−(aq) added to reach the end point of the
titration is expressed as M × V. Using the variables defined above, the molar mass of iron (55.85 g
mol−1), and the coefficients in the balanced chemical equation, write the expression for each of the
following quantities.
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2007 SCORING GUIDELINES
Question 5 (continued)
(i) The number of moles of iron in the sample
mol Fe2+ = 5 × M × V
OR
mol Fe2+ = 5 mol Fe One point is earned for either expression.
2+ 1 mol MnO 4 − ×M×V (ii) The mass of iron in the sample, in grams
mass Fe = 5 × M × V × 55.85 g mol−1
OR One point is earned for the answer in part (d)(i)
multiplied by 55.85. mass Fe = mol Fe2+ × 55.85 g mol−1 (iii) The mass percent of iron in the compound mass % Fe = 5 × M × V × 55.85
× 100
g One point is earned for the answer in part (d)(ii)
divided by g. OR
mass % Fe = mass Fe
× 100
g One point is earned for converting to percent. (e) What effect will adding too much titrant have on the experimentally determined value of the mass
percent of iron in the compound? Justify your answer.
The experimentally determined mass percent of iron in the
compound will be too large.
V is too large ⇒ expression in (d)(iii) above is too large One point is earned for stating that the
mass percent is too large, with
justification. © 2007 The College Board. All rights reserved.
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2007 SCORING GUIDELINES
Question 6
Answer the following questions, which pertain to binary compounds.
(a) In the box provided below, draw a complete Lewis electrondot diagram for the IF3 molecule. One point is earned for a correct Lewis diagram (can be done with dots or lines). (b) On the basis of the Lewis electrondot diagram that you drew in part (a), predict the molecular geometry of
the IF3 molecule.
One point is earned for the molecular geometry consistent with the Lewis diagram
Tshaped in part (a). (c) In the SO2 molecule, both of the bonds between sulfur and oxygen have the same length. Explain this
observation, supporting your explanation by drawing in the box below a Lewis electrondot diagram (or
diagrams) for the SO2 molecule. One point is earned for a correct diagram (can be done with dots or lines).
One point is earned for some indication or discussion of resonance (but the point is not earned for a
description of resonance as a dynamic process).
OR
One point is earned for a correct diagram
(can be done with dots or lines).
The bonds are the same length because they are both
double bonds. One point is earned for stating that both bonds
are double bonds. © 2007 The College Board. All rights reserved.
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2007 SCORING GUIDELINES
Question 6 (continued)
(d) On the basis of your Lewis electrondot diagram(s) in part (c), identify the hybridization of the sulfur
atom in the SO2 molecule.
s p2 One point is earned for hybridization consistent with part (c). The reaction between SO2(g) and O2(g) to form SO3(g) is represented below.
→
2 SO2(g) + O2(g) ← 2 SO3(g) The reaction is exothermic. The reaction is slow at 25°C; however, a catalyst will cause the reaction to
proceed faster.
(e) Using the axes provided below, draw the complete potentialenergy diagram for both the catalyzed and
uncatalyzed reactions. Clearly label the curve that represents the catalyzed reaction. One point is earned for an uncatalyzed reaction curve that must show that Ea > 0 and ΔH < 0.
One point is earned for a catalyzed reaction curve that must show Ea < uncatalyzed Ea ,
must be clearly labeled, and must begin and end at the same energies as the uncatalyzed curve. © 2007 The College Board. All rights reserved.
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2007 SCORING GUIDELINES
Question 6 (continued) (f) Predict how the ratio of the equilibrium pressures, pSO2
pSO3 , would change when the temperature of the uncatalyzed reaction mixture is increased. Justify your prediction. The ratio pSO2
pSO3 would increase as the temperature increases. Because the reaction is exothermic (ΔH < 0), as the temperature is raised the One point is earned for the
correct answer and explanation. reaction shifts to the left. (g) How would the presence of a catalyst affect the change in the ratio described in part (f)? Explain.
The catalyst would not affect the value of the two equilibrium ratios
but would increase the rate of the shifting of the system to the new
equilibrium position. The catalyst does this by providing an alternate
path with a lower activation energy. One point is earned for the
correct answer and explanation. © 2007 The College Board. All rights reserved.
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