CHEMISTRY 121
Name____________________________
WINTER 2004
Student Number___________________
Midterm Exam #2
Signature_________________________
Wednesday, March 17
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Make sure you have all 7 pages (including this one).
Make a note of the point value of each question, and allocate your time accordingly.
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♣ I’m looking over a three-leafed clover that I’ve overlooked bethree.
Acid Dissociation Constants at 25ºC
Total Points: 60
Total Time: 80 minutes
Potentially Helpful Data, Equations, and Constants
Constants
Equations
absolute zero = –273.15ºC
1 atm = 760 torr
pH = pKa + log
= 101.325 kPa
PV = nRT
= 1.01325 bar
H = E + PV
1 Latm = 101.325 J
NA = 6.022 × 10 mol
[acid]
E=q+w
1 eV = 96485 J/mol
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[base ]
w = –P∆V
–1
R = 0.082057 Latm/molK
= 8.3145 J/molK
Kw (298K) = 1.0 × 10–14
q = mC∆T
∆H = Hfinal – Hinitial
∆S = qrev/T
∆G = ∆H – T∆S
∆Gº = –RTlnK
∆G = ∆Gº + RTlnQ
ln
K2
K1
=
∆Hº  1
1 
 – 
R  T1 T2 
acid
H2SO3
HClO2
H3PO4
HF
HNO2
CH3CO2H
H2S
H2PO4–
HSO3–
HClO
H3BO3
HCN
HPO42–
H2O
Ka
1.3 × 10–2
1.1 × 10–2
7.1 × 10–3
7.2 × 10–4
4.5 × 10–4
1.8 × 10–5
1.0 × 10–7
6.2 × 10–8
6.2 × 10–8
2.9 × 10–8
6.3 × 10–10
6.2 × 10–10
3.6 × 10–13
1.0 × 10–14
Base Dissociation Constants at 25ºC
base
CH3CH2NH2
(CH3)2NH
CH3NH2
(CH3)3N
NH3
C5H5N
C6H5NH2
H2O
Kb
6.4 × 10–4
5.4 × 10–4
4.4 × 10–4
6.4 × 10–5
1.8 × 10–5
1.7 × 10–9
4.3 × 10–10
1.0 × 10–14
2
Chem 121, Midterm#2, Winter ’04
1. NOW I KNOW MY ABCS (12 POINTS, 2 EACH)
Circle the one best answer for each question.
a) Two bars of equal mass, one of Fe and one of Cu, start at a temperature of 150ºC and lose
500 J of heat. The heat capacity of Fe is greater than that of Cu. Which statement is true?
a) The iron has a lower final temperature than copper.
b) The iron has a higher final temperature than copper.
c) The two bars have the same temperature, since they lose the same amount of heat.
d) The work for both bars is +500J, because energy is conserved.
e) I don't know. Please give me half a point.
b) Which acid-base pair would be the best choice to prepare a buffer solution with pH = 8?
a) H3PO4 and NaH2PO4
b) H3BO3 and NaH2BO3
c) [C5H5NH]Cl and C5H5N
d) NaH2PO4 and Na2HPO4
e) I don't know. Please give me half a point.
c) Two aqueous solutions are mixed in a flask and an exothermic reaction takes place.
Which of the following is true?
a) The products have more enthalpy than the reactants.
b) The mixed solution gets colder, because the reaction loses heat.
c) The mixed solution gets warmer, because the reaction loses heat.
d) The mixed solution stays the same temperature, because energy is conserved.
e) I don't know. Please give me half a point.
d) A gas absorbs 20 kJ of heat, and ∆E of the gas is 15 kJ. Which of the following is true?
a) q = –20 kJ
b) The gas expands, doing work on the surroundings.
c) The system is at constant volume.
d) q = –5 kJ
e) I don't know. Please give me half a point.
e) The standard heat of formation of N2(l) is
a) equal to the NN bond strength.
b) zero, because N2 is an element.
c) a positive value, because N2 is stable as a gas under standard conditions.
d) a negative value, because heat must be removed from N2(g) for it to condense.
e) I don't know. Please give me half a point.
f) Water freezes at –5ºC. Which of the following statements regarding this change is false?
a) The change is spontaneous, because the entropy of the universe increases.
b) The entropy lost from the water equals the entropy gained by the surroundings.
c) The entropy of the surroundings increases, because the change is exothermic.
d) The entropy of the water decreases, because the molecular motion becomes less random.
e) I don't know. Please give me half a point.
3
Chem 121, Midterm#2, Winter ’04
2. ENEMY BASES HAVE BEEN NEUTRALIZED (8 POINTS)
Consider 500.0 mL of an aqueous solution that contains 0.250 M of aniline (C6H5NH2) and
0.180 M of the anilinium ion ( [C6H5NH3]+ ).
a) What is the pH of the solution ? (2)
b) What is the resulting pH after 0.050 moles of HCl are added to the solution? (3)
c) What is the resulting pH if more HCl is added, to a total of 0.200 moles? (3)
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Chem 121, Midterm#2, Winter ’04
3. HESS AND HERS (6 POINTS)
A long-sought achievement in synthetic chemistry is the catalytic and selective oxidation of
methane to form methanol:
CH4(g) + ½ O2(g) → CH3OH(l)
Given the thermochemical data at below, calculate ∆Hºrxn for the reaction above.
2 H2O(g) → 2 H2(g) + O2(g)
∆Hº = +483.6 kJ
2 H2O(l) → 2 H2(g) + O2(g)
∆Hº = +571.6 kJ
3 H2(g) + CO(g)
CH4(g) + H2O(l)
∆Hº = –250.1 kJ
CO(g) → C(s) + ½ O2(g)
∆Hº = +110.5 kJ
∆Hºf(CH3OH(l)) = –238.9 kJ/mol
5
Chem 121, Midterm#2, Winter ’04
4. FICKLE NICKEL (8 POINTS)
The dissolution and reaction of Ni(OH)2(s) in water has K = 6.5 × 1010 at 25ºC.
Ni(OH)2(s) + 2 H3O+(aq)
4 H2O(l) + Ni2+(aq)
a) What is ∆Gº for the above reaction of Ni(OH)2 in water at 25ºC, in kJ? (2)
b) What is the value of Q for the reaction of Ni(OH)2 in a solution of 0.050 M Ni(NO3)2(aq) at
pH 7.80 ? (3)
c) Does Ni(OH)2 dissolve under these conditions? Justify your answer quantitatively. (3)
6
Chem 121, Midterm#2, Winter ’04
5. AN EQUAL AND OPPOSITE REACTION (8 POINTS)
Tin tetrachloride was once called the Fuming Liquor of
Libavious, because it reacts with atmospheric moisture to
give a visible white smoke.
SnCl4(l) + 2 H2O(g)
SnO2(s) + 4 HCl(g)
Consider the thermochemical data given at right and answer
the questions below.
SnCl4(l)
SnO2(s)
H2O(l)
H2O(g)
HCl(g)
HCl(aq)
∆Hºf
(kJ/mol)
–511.3
–580.7
–285.8
–241.8
–92.0
–167.2
Sº
(J/molK)
258.6
52.3
69.9
188.7
186.8
56.5
a) Calculate ∆Hº and ∆Sº for this reaction of SnCl4 with atmospheric water. (4)
b) Calculate K of this reaction at 35ºC (2)
c) Under what range of temperatures is this reaction reactant-favoured? Justify your answer.
(Possible answers might include things like “all temperatures”, “no temperatures”, or “above
78ºC”). (2)
7
Chem 121, Midterm#2, Winter ’04
6. MERCEDES BENZOATE (8 POINTS)
14
Consider the titration curve shown at right, for
the titration of 50.00 mL of a benzoic acid
(C6H5CO2H, Ka = 6.3 × 10–5) solution with
0.200 M NaOH(aq).
12
10
equivalence point
= 35.00 mL
8
pH
a) What is the pH of the solution when 20.00
mL of NaOH(aq) has been added? (3)
6
4
2
0
0.00
10.00
20.00
30.00
40.00
50.00
60.00
volume added NaOH (mL)
b) Explain briefly why the pH at equivalence is not that of neutral water. (Preferably, your
explanation will be in words, although I suppose a numerical calculation of the pH would
suffice.) (2)
c) Qualitatively sketch on the graph the corresponding titration
curve of an equimolar weak acid with Ka of 1.0 × 10–4. Indicate
clearly whether the pH values at 0, 35, and 60 mL added NaOH
are greater, lesser, or equal to those on the benzoic acid curve.
(3)
Question
Points
1
12
2
8
3
6
4
8
5
8
6
8
TOTAL
50
Score