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Chemistry 111 Sect. 2
Exam #3
May 9, 2002
KEY
Name: ________________________________________________
Student #: ______________________________________________
Please put your name and student number on all pages in case they become separated. The last
page of the exam has some useful information. The exam lasts 60 minutes. There are a total of
100 points.
MULTIPLE CHOICE QUESTIONS
Place an X in the box corresponding to the correct answer.
1. (5 pts)
†
†
†
†
X
†
The First Law of Thermodynamics states that for all processes:
the potential energy of the system is constant.
the kinetic energy of the system is constant.
the total energy of the system is constant.
the total energy of the surroundings is constant.
the total energy of the universe is constant.
2. (5 pts) Consider a sample of oxygen gas at 800K. Kinetic molecular theory tells us that:
all of the O2 molecules have the same translational kinetic energy. Recall the Boltzmann
distribution of molecular
all of the O2 molecules have the same molecular speed.
speeds.
the O2 molecules may have speeds ranging from zero to very large values.
the average translational kinetic energy of O2 molecules is less than that for T = 1600 K.
the rms speed of O2 molecules is greater than that of Ne at T = 800 K. Higher T → faster;
more mass → slower.
I made a typo in the 4th answer. I meant to say greater than. So either
rd
th
the 3 or 4 answers were counted as correct since both are correct.
†
†
X
†
X
†
†
3. (5 pts) What is the correct net ionic equation for the reaction (if a reaction occurs) between nitric
acid and potassium hydroxide?
†
†
†
X
†
†
HNO3 (aq) + KOH (aq) → KNO3 (aq) + H2O (l )
HNO3 (aq) + OH- (aq) → NO3- (aq) + H2O (l )
NO3- (aq) + K+ (aq) → KNO3 (aq)
H+ (aq) + OH- (aq) → H2O (l)
This is a reaction between a strong acid and a strong base; both
no reaction
completely dissociate in water before the reaction starts.
Page 1
KEY
Chemistry 111 Sect. 2
Name: ________________________________________________
Exam #3
Student #: ______________________________________________
May 9, 2002
4. (5 pts) A helium-filled weather balloon has a volume of 800 L at 27.0 oC and 760 mmHg. It is
released and rises to an altitude of 2.60 km, where the pressure is 600 mm Hg and the
temperature is 2.0 oC. The volume of the balloon at this altitude is
†
†
†
X
†
†
75 L
730 L
800 L
930 L
1010 L
At constant amount of gas (n), PV/T is a constant, so
P1V1/T1 = P2V2/T2
(760 mmHg)(800 L)/(300 K) = (600 mmHg)V2/(275 K)
or
V2 = (800)(760)(275)/[(300)(600)] L = 929 L
5. (5 pts) You mix together solutions of (NH4)2CO3 and LiCl. To this mixture you add a
solution of H2SO4. What do you expect to observe after the first and second mixings of
solutions?
†
†
†
†
X
†
first mixing result second mixing result
no reaction
no reaction
white precipitate forms
no reaction
gas forms
no reaction
white precipitate forms
gas forms
no reaction
gas forms
Alkali metal (Li+) compounds are soluble, so
nothing happens for the first mixing.
Carbonates in the presence of acids produce
H2O and CO2, forming a gas on the second
mixing.
6. (5 pts) If the enthalpy change in the reaction A → B is ∆H = +150 kJ and the enthalpy change in
the reaction A → 2 C is ∆H = +100 kJ, then the enthalpy change in the reaction 2 C → B is
†
†
X
†
†
†
-250 kJ
-50 kJ
+50 kJ
+250 kJ
+350 kJ
Apply Hess’ Law to this problem. Reverse the second reaction (and reverse
the sign of its enthalpy change) and add to it the first reaction in the direction
given, also adding the enthalpy changes:
A→B
2C → A
2C → B
∆H = +150 kJ
∆H = -100 kJ
∆H = +50 kJ
Page 2
KEY
Chemistry 111 Sect. 2
Name: ________________________________________________
Exam #3
Student #: ______________________________________________
May 9, 2002
7. (5 pts) The standard enthalpy of formation of liquid water is -285.8 kJ/mol. For which of the
following reactions would the standard enthalpy change be equal to -285.8 kJ?
†
†
†
†
X
†
2 H (g) + O (g) → H2O (l)
H2O (s) → H2O (l)
2 H2 (g) + O2 (g) → 2 H2O (l)
H2O (l) → 2 H (g) + O (g)
H2 (g) + 1/2 O2 (g) → H2O (l)
This must be for the reaction that combines elements to
form one mole of water.
8. (5 pts) Which of the following has the highest density?
†
†
†
†
X
†
1 L of He gas at T = 25 ºC and P = 1 atm.
5 L of He gas at T = 25 ºC and P = 1 atm.
1 L of He gas at T = 25 ºC and P = 5 atm.
1 L of Ne gas at T = 25 ºC and P = 1 atm.
1 L of Ne gas at T = 25 ºC and P = 5 atm.
Gas density d = PM/RT. Of the first three
possibilities, the one with P = 5 atm is densest.
Comparing that to the last line, the higher
molar mass gas is the most dense.
MULTIPLE MULTIPLE CHOICE QUESTIONS
Place an X in the boxes corresponding to the correct answers. Any number of answers may be
correct, including none of them.
9. (10 pts) 2 grams of octane are burned in a calorimeter. The calorimeter has a heat capacity of
900 J/K. There are 1200 grams of water in the calorimeter surrounding the bomb. The
temperature of the water rises from 25 º C to 40 ºC. The specific heat capacity of water is 4.184
J/g K. Which of the following is TRUE?
q = mC∆T is the heat flowing into the water.
† the temperature of the calorimeter remains at 25 ºC This calculates to q = (1200 g)(4.184 J/g K)
† combustion of octane is an endothermic reaction (15 K) = 75,300 J = 75.3 kJ. Heat arises
from the reaction, which is therefore
X
†
the amount of heat flowing into the water is 75.3 kJ
exothermic. The ∆H for an exothermic
X
†
∆H for the combustion of octane is negative
reaction is negative. This is a combustion
† the enthalpy of formation of octane equals the enthalpy change in the combustion
reaction; the final products are not the
reaction
elements making up octane, so the enthalpy
change won’t be the same as ∆Hºf.
Page 3
KEY
Name: ________________________________________________
Student #: ______________________________________________
Chemistry 111 Sect. 2
Exam #3
May 9, 2002
10. (10 pts) Which of the following gasses has the same average molecular kinetic energy as a
gas with molar mass of 200 g/mol at 100 K?
†
†
X
†
†
X
†
a gas with molar mass 100 g/mol at 200 K Kinetic Molecular Theory says that all gases at the
same temperature have the same molecular kinetic
a gas with molar mass 20 g/mol at 10 K
energy.
a gas with molar mass 10 g/mol at 100 K
a gas with molar mass 2000 g/mol at 10 K
a gas with molar mass 2000 g/mol at 100 K
11. (10 pts) Which of the following are weak electrolytes?
†
X
†
†
X
†
†
sodium acetate
acetic acid (hydrogen acetate)
sodium carbonate
lead carbonate
ammonium carbonate
Weak electrolytes do not strongly ionize in aqueous
solution. Sodium and ammonium salts are highly soluble.
Many carbonates are poorly soluble (including lead). Most
organic acids are weakly ionizable.
SHORT ANSWER QUESTIONS
Answer in the spaces indicated.
12. (10 pts) Nitrogen monoxide has recently been found to be involved in a wide range of
biological processes. NO gas reacts with oxygen to form brown NO2 gas as shown, with an
enthalpy change for the reaction as given. What is the enthalpy of formation of NO2 (g)? Note
that thermodynamic data are given on the last page of the exam.
∆Hºrxn = -114 kJ
2 NO (g) + O2 (g) → 2 NO2 (g)
Use Hess’ law here. We need use a set of reactions to make the
formation reaction for one mol of NO2 (g):
½ N2 (g) + O2 (g) → NO2 (g)
Use half of the given reaction plus the reaction for the formation of NO
(see table on p. 6)
NO (g) + ½ O2 (g) → NO2 (g)
½ N2 (g) + ½ O2 (g) → NO (g)
½ N2 (g) + O2 (g) → NO2 (g)
∆Hºf =
33.9
kJ/mol
Page 4
∆Hº = (-114 kJ)/2
∆Hºf = +90.9 kJ
∆Hºf = -57 kJ + 90.9 kJ
Chemistry 111 Sect. 2
Exam #3
May 9, 2002
KEY
Name: ________________________________________________
Student #: ______________________________________________
13. (10 pts) Insoluble Mg(OH)2 is placed into a solution of nitric acid, HNO3. Write the net
ionic equation that occurs. If no reaction occurs, say so.
This will be an acid-base reaction, so H2O is a product. Nitric acid is a strong acid, meaning
that it will start out completely ionized into H+ (aq) and NO3- (aq). The Mg(OH)2 will not
ionize much (it’s not soluble), so it starts out the reaction as Mg(OH)2. When the reaction
finishes, with the hydroxides of Mg(OH)2 combined with the H+ of the nitric acid, both nitrate
and Mg2+ will be left in solution. Nitrate is a spectator, so the overall NIE is
Mg(OH)2 (s) + 2 H+ (aq) → Mg2+ (aq) + 2 H2O (l)
14. (10 pts) Ammonia can break down into its elemental components as shown. If 0.25 mol
ammonia contained in a flask at T = 25 ºC and P = 0.5 atm is completely converted to N2 and H2,
what will the pressure in the flask be if the volume of the flask and T do not change?
2 NH3 (g) →
N2 (g) + 3 H2 (g)
Note the molar ratios of the gases that make up reactants (2) and products (3 + 1 = 4). There are
twice as many moles of gas present in the end as as the start. When V and T are constant, the
relationship between P and n is
P/n = constant, or
P1/n1 = P2/n2. We know n1 = 0.25 mol, n2 = 0.5 mol and P1 = 0.5 atm, so
P2 = (0.5 atm)(0.5 mol)/(0.25 mol) = 1.0 atm.
1.0
P = _________________________
atm
Page 5
KEY
Name: ________________________________________________
Student #: ______________________________________________
Chemistry 111 Sect. 2
Exam #3
May 9, 2002
Useful information
NA = 6.02 × 1023 molecules/mol
c = 3.0 × 108 m/sec
h = 6.6 × 10-34 J sec
Rhc = 2.2 × 10-18 J
me = 9.1 × 10-28 g
a0 = 0.0529 nm
2-linear, 3-trigonal planar, 4-tetrahedral
5-trigonal bipyramidal, 6-octahedral
R = 0.082 L atm/mol K
urms = √3RT/M
∆H = qp
d = m/V = PM/RT
E = -Rhc/n2
E = hν
c = λν
λ = h/(m v)
Z* = Z - ninner electons
∆Hrxn = Σ(bond ener)react - Σ(bond ener)prod
q = C m ∆T
q = (heat of fusion) m
∆H0net = Σ (∆H0f)prod - Σ (∆H0f)react
∆H0net = Σ (∆H0rxn)
Some useful thermodynamic data
Material
∆H0f (kJ/mol)
∆H0f (kJ/mol)
-74.8
H2O (l)
-285.8
-110.5
H2O (g)
-241.8
-393.5
H2O2 (l)
-187.8
-272.0
N (g)
472.7
-824.2
NH3 (g)
-46.1
-1118.4
NO (g)
90.9
Material
CH4 (g)
CO (g)
CO2 (g)
FeO (s)
Fe2O3 (s)
Fe3O4 (s)
PERIODIC TABLE OF THE ELEMENTS
IA
IIA
IIIB
IVB
VB
VIB
VIIB
VIIIB
IB
IIB
IIIA
IVA
VA
VIA
VIIA VIIIA
1
2
H
He
1.008
4.003
3
4
5
6
7
8
9
Li
Be
B
C
N
O
F
10
Ne
6.939
9.012
10.81
12.01
14.01
16.00
19.00
20.18
11
12
13
14
15
16
17
18
Na
Mg
Al
Si
P
S
Cl
Ar
22.99
24.31
19
20
21
22
K
Ca
Sc
Ti
39.10
40.08
44.96
37
38
39
Rb
Sr
85.47
55
26.98
28.09
30.97
32.07
35.45
39.95
24
25
26
27
28
29
30
31
32
33
34
35
36
V
Cr
Mn
Fe
Co
Ni
Cu
Zn
Ga
Ge
As
Se
Br
Kr
47.90
50.94
52.00
54.94
55.85
58.93
58.71
63.55
65.39
69.72
72.61
74.92
78.96
79.90
83.80
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
Y
Zr
Nb
Mo
Tc
Ru
Rh
Pd
Ag
Cd
In
Sn
Sb
Te
87.62
88.91
91.22
92.91
95.94
(99)
101.1
102.9
106.4
107.9
112.4
114.8
118.7
121.8
127.6
126.9
131.3
56
57
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
Cs
Ba
La
Hf
Ta
W
Re
Os
Ir
Pt
Au
Hg
Tl
Pb
Bi
Po
At
Rn
132.9
137.3
138.9
178.5
181.0
183.8
186.2
190.2
192.2
195.1
197.0
200.6
204.4
207.2
209.0
(209)
(210)
(222)
87
88
89
Fr
Ra
Ac
(223)
226.0
227.0
23
Page 6
I
Xe
Chemistry 111 Sect. 2
Exam #3
May 9, 2002
KEY
Name: ________________________________________________
Student #: ______________________________________________
Page 7