Answer Key
Final Exam - Solutions Chemistry 12 2011
-
University:
Amherst College -
Course:
CHEM 161 | Chemical Principles Academic year:
2011
-
Views:
415
Pages:
8
Author:
Nathaly Davidson
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Final Exam
Chem 12
May 12, 2011
Name
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2 (30)
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6 (30)
7 (35)
8 (20)
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1 (15 points) Summer is just around the comer. Let's make liquid nitrogen ice cream. (Well,
we'll have to approximate ice cream as frozen water.) How many grams of liquid N2 would be
required to freeze 100 g of water, initially at 25.0 °C? Assume that the all the water ends up as ice
at 0 °C. Also assume that the liquid nitrogen is at its boiling point, 77.3 K, and the cold gas
formed when it vaporizes escapes immediately and is unable to provide any cooling. (Water's heat
capacity is in the table provided. Water's enthalpy of fusion is 6.01 kJ/mol, and nitrogen's
enthalpy of vaporization is 5.56 kJ/mol. Molecular weights are 18.0 g/mol for H2O and 28.0
g/mol for N2.)
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JfL 2. (18 points) The standard enthalpies of vaporization of 1-bromobutane, 1-chlorobutane, and 1butanol are given below.
CH3CH2CH2CH2-Br
51 kJ/mol
33.6
36.7
AH°vap:
CH3CH2CH2CH2-OH
CH3CH2CH2CH2-C1
(a) Explain briefly why the alcohol (the 3rd cmpd) has a much higher AH0vap than the other two
compounds.
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(c) The alcohol also has a higher entropy of vaporization (S vap = 1 3 1 J/molK) than the other two
compounds (S°vap ~ 97 J/molK for each). Why?
(d) The branched isomer of each compound, (CH3)3C-X (X = Br, Cl, or OH), has AH°vap about 5
kJ/mol /ower than the "straight chain" isomers shown above. Why?
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3. (12 points) When each compound below is dissolved in water, would the solution be acidic,
basic, or neutral?
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(a) HOCH2CH2CH2OH
(b) (CH3)2CHC02K
(c) (CH3)3NHBr
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(d) NaNH2
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4 (15 points) Which of the following substances can reduce aqueous Cr to chromium metal
under standard conditions? (circle)
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5 (30 points) Methylamine, CH3NH2, has Kb = 4.40 x 10 . (a) Write the equilibrium that
defines the Kb, showing all structures clearly.
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(b) Calculate the pH of a solution made by dissolving 0.00800 mol of methylamine in 1.00 L of
water.
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(c) The solution from part b is titrated with 0.100 M hydrochloric acid. What volume of aq. HC1
is required to reach the equivalence point?
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(d) What are the major species present at the equivalence point, and what are their
concentrations? Draw all structures clearly .
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(e) What is the pH of the solution at the equivalence point?
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(f ) What was the pH of the solution when exactly half of the required amount of aq . HC1 had been
added?
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6 X°l ) 6. (13 points) You may have noticed the following trend within the Kb values listed in the table.
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Kb = 4.4 x 10 6
Kb = 3.0 x 10-8
Kb = 1.1 x 10
H3 C-NH2
H2N-NH2
HO-NH2
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(a) In the process that defines Kb is the NH2 group in these compounds protonated or
deprotonated ?
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(b) Which compound above is the strongest base, and which is the weakest base?
(c) Is Kb a thermodynamic or a kinetic parameter?
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(d) Provide a brief structural explanation for the variation in Kb above.
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7 (32 points) (a) Hydrazine, NH2NH2, and formic acid, HCO2H, undergo an acid-base reaction.
Illustrate this reaction in the space below, being sure to show all structures clearly.
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(c) Label the stronger acid and the weaker acid .
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(d) Label the stronger base and the weaker base.
(e) Based on your stronger/weaker labels, which side of the reaction is favored at equilibrium?
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(f ) What is Keq for the reaction?
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(h) Suggest two experimental methods by which one could determine the AH° for the reaction.
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(g) What is AG° at 25 °C?
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(b) Label the acids with their Ka values.
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' 8. (35 points) (a) Use the data in the tables provided to calculate AH° and AS° for the dissociation
of liquid water to form aqueous H+ and HO . Be sure to include units.
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(d) In the lab you measured the AH° for an exothermic reaction, and on the second midterm, you
calculated the AH° for an endothermic reaction. Your AH° from part a was not zero, so this
reaction is either exothermic or endothermic. So if you placed a sample of water in a coffee cup
calorimeter, why would you not observe a change in temperature due to the reaction in part a?
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(e) Based on your AG°, what is Keq at 25°C?
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(f ) Based on your Keq (= "Kw") what is the pH of pure water at 25°C?
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(g) Calculate the pH of pure water at 100°C.
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J 9. (30 points) An electrochemical cell is constructed with a zinc electrode and aqueous ZnCk in
one compartment and a chromium electrode and aqueous Cr2(S04)3 in the other. Both salts are
very soluble in water.
(a) Write the balanced redox reaction. The sulfate and chloride ions do not participate in the redox
chemistry and may be omitted.
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(e) What is the equilibrium concentration of Cr when [Zn ] = 1.00 M?
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/ 10. (17 points) Calcium fluoride is a very slightly water-soluble solid. Oxidation of calcium
metal in the presence of fluoride ion produces CaF2 directly, rather than aqueous Ca . Ksp for
CaF2, is 4.0 x 10 at 25°C. Calculate the value of £° for the reaction below.
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Cc
tt-
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+
CaF2
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11. (18 points) Many organic molecules undergo free radical-induced reactions with molecular
oxygen, called autooxidation. Write the rate law for the autooxidation whose mechanism is shown
below. Assume that the third step is rate limiting, and use the equilibrium approximation to write
an expression for the rate of formation of the major peroxide product, PhCFbOOFl, formed in the
4th step.
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CH3-0
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PhCH3
PhCH2»
+
02
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2 PhCH2»
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h
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fast
CH3-OH
PhCH2*
+
PhCH2 O-O*
fast
PhCH3
—
PhCH2 OOH
+
PhCH2*
PhCH2-CH2Ph
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-« -x 12. (20 points) Recall that the activation energy for rotation around a C=C K-bond is 270 kJ/mol .
As a consequence, alkenes, which contain the C=C unit, can exist in geometrically distinct
"stereoisomeric" forms that do not interconvert at ordinary temperatures, as shown below for 2butene, CH3-CH=CH-CH3.
\
H
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H
\
C
c c
CH3
CH3
H
/
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= C\
H
CH3
CH3
Organic compounds that contain one or more silicon atoms in place of carbon are expected to have
unusual properties that could potentially lead to new materials. The Ea for rotation around the
Si=C 7t-bond is 165 kJ/mol, and the Ea for rotation around the Si=Si 7t-bond is about 95 kJ/mol.
(a) Calculate the rate constant and the timescale for rotation around a Si=C 7t-bond at 100 °C.
Assume an Arrhenius A-factor of 1013 2 sec-1.
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(b) Repeat part a for rotation around an Si=Si 7t-bond at 25 °C.
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(c) If you prepared two separate samples of the stereoisomeric forms of CH3-SiH=SiH-CH3
(analogous to the structures drawn above), at what temperature (in °C) would you need to store
them to ensure no more than 1% interconversion within a year?
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