Download Please do not remove this page. The periodic table, constants, and

Chem 1721/1821
Exam 2
March 27, 2014
ID#: 2013172 _____
Dr. Susan E. Bates
Name ____________________________________
Please do not remove this page. The periodic table, constants, and
equations you may need are on the last page of this exam. Feel free to
remove it, and use it as scrap paper. All pages must be turned in.
Be sure to fill out and bubble in the proper letters and numbers on the
ScanTron sheet for the following information:
your name
Test Form A
the 9-digit ID number given above (rightmost digit blank)
You should answer questions for Part II (1 - 15) on the Scantron sheet.
You will not have the Scantron returned to you, so if you would like to know
what you answered after the exam is returned, circle your answers on this form
as well.
Chem 1721/1821: Exam 2
Name _____________________________________
Part I (40 points): Short Answer.
1.
Write the balanced chemical equation (including physical states and charges on ions) for the base ionization of
pyridine, C5H5N.
2.
Consider an aqueous solution at 25ºC with pOH = 9.75.
a.
Determine [H3O+] in this solution.
b.
3.
[H3O+] = _________________ M
Is this solution acidic, basic, or neutral?
_________________________
Consider the following equilibrium at 350ºC: 2 A (g) + B (g)  C (g); KC = 5.6 x 10–2.
a.
Determine the value of KP for this reaction at 350ºC.
KP = ____________________
b.
If this reaction is started in a 2.00 L flask with 0.10 mol A, 0.12 mol B, and 0.05 mol C, in which direction will the
reaction proceed? Circle the one best answer below.
forward toward products
4.
reverse toward reactants
there will be no change
Consider the following proposed 3-step reaction mechanism:
step 1: S (g) + O2 (g)  SO2 (g)
step 2: NO2 (g) + SO2 (g)  NO (g) + SO3 (g)
step 3: NO (g) + 1/2 O2 (g)  NO2 (g)
a.
Which species in this mechanism is/are reaction intermediates? Circle the one best answer below.
SO2 & NO
b.
NO & NO2
Which species in this mechanism is/are catalysts? Circle the one best answer below.
SO2 & NO2
c.
step 2
NO2 only
step 3
not enough information to answer
For a reversible exothermic reaction, the activation energy for the forward reaction is __________________ the
activation energy of the reverse reaction. Which one answer below best fills in this blank to complete the statement?
less than
6.
NO & NO2
If the experimentally determined rate law for reaction S (g) + 3/2 O2 (g)  SO3 (g) is rate = k[S][O2], which step in
this mechanism is the rate determining step? Circle the one best answer below.
step 1
5.
NO only
greater than
equal to
Consider the following equilibrium: HClO2 (aq) + NH3 (aq)  NH4+ (aq) + ClO2– (aq). For HClO2, Ka = 1.1 x 10–2, and
for NH4+, Ka = 5.6 x 10–10. Are the reactants or products favored in this reaction?
________________________
7.
Consider the following reaction: 2 A + 2 B  2 C + D. For this reaction, the experimentally determined rate law
is: rate = k[A][B]2.
a.
What is the order of this reaction with respect to B?
b.
If the initial concentration of A is doubled, how will the rate of reaction change? Circle the one best answer.
increase by 2x
c.
increase by 4x
________________________
increase by 3x
decrease by 1/2
If concentration is measured in molarity, and time is measured in seconds, what are the units of the rate constant (k)
for this reaction?
8.
9.
10.
no change in rate
________________________
For each of the following, circle the sample that has the described characteristic:
a.
highest pOH:
0.10 M NH3
b.
greater % ionization in 0.25 M sol'n:
c.
higher concentration in reaction with KC = 5 x 10–5:
d.
rate constant (k) for reaction:
(at constant T)
HF (pKa = 3.14)
0.10 M Ca(OH)2
HClO2 (pKa = 1.96)
products
catalyzed reaction
0.10 M KOH
HC2H3O2 (pKa = 4.74)
reactants
uncatalyzed reaction
both are equal
k is the same for both
Indicate whether each of the following statements is True or False.
a.
The % ionization of a weak base tends to decrease as the concentration of the base increases.
_____________
b.
In the reaction Fe3+ (aq) + 6 :C≡N:– (aq)  Fe(CN)63+ (aq), Fe3+ is best described as a Bronsted-Lowry acid._______
c.
When a catalyst is added to a reaction, both activation energy and rate increase.
d.
For a reaction at equilibrium it will always be true that kforward = kreverse.
____________
_____________
Consider the following equilibrium: C (s) + CO2 (g)  2 CO (g); ∆H = 173 kJ
a.
Which of the following changes in conditions would result in an increase in the mass of C (s) present?
Circle the one best answer below.
increase pressure
b.
decrease pressure
changing P will have no effect on the mass of C (s)
Which of the following changes in conditions would result in a decrease in the value of KC? Circle the one best
answer below.
increase temperature
decrease temperature
changing temperature will not change KC
/40
Part II (45 points): Multiple Choice. For each of the following quesitons, choose the ONE best answer. Bubble-in your
answer for these questions (1 - 15) on the Scan-Tron sheet provided.
1.
Consider the balanced equation given for the reaction of iodate ion with iodide ion in acidic solution:
IO3– (aq) + 5 I– (aq) + 6 H+ (aq)  3 I2 (aq) + 3 H2O (l)
At a particular instant in time, the value of ∆[I–]/∆t = 4.0 x 10–3 M•s–1. What is the value of ∆[I2]/∆t at the same instant in
time?
a. 2.4 x 10–3 M•s–1
b. 1.2 x 10–2 M•s–1
d. 6.6 x 10–5 M•s–1
e. 4.0 x 10–3 M•s–1
c. 6.6 x 10–3 M•s–1
2.
For the reaction A  products, the concentration of A was monitored with time. From the following graph of data
collected in studying this reaction, determine the order of the reaction, and its rate constant.
a. 1st; 0.01 s–1
b. 1st; –2.30 s–1
c. 2nd; 0.01 s–1
d. 2nd; 3.52 s–1
e. 2nd; –2.30 s–1
3.
Consider the following data collected to study the reaction H2O2 (aq) + 3 I– (aq) + 2 H+ (aq)  I3– (aq) + 2 H2O (l) :
[H2O2]0, M
0.100
0.100
0.200
0.400
[I–]0, M
5.00 x 10–4
1.00 x 10–3
1.00 x 10–3
1.00 x 10–3
[H+]0, M
0.010
0.010
0.010
0.020
initial rate, M/s
0.137
0.274
0.548
1.096
Determine the rate law for this reaction.
4.
5.
a. rate = k[H2O2][I–][H+]
b. rate = k[H2O2]2[I–]2[H+]2
d. rate = k[H2O2][H+]
e. rate = k[H2O2][I–]
A particular chemical reaction has a rate constant that is three times greater at 40.0°C than at 20.0°C. Determine the
activation energy for this reaction.
a. 3.00 kJ/mol
b. 366 kJ/mol
d. 3.20 kJ/mol
e. none of these
c. 41.9 kJ/mol
Determine the pH at 25°C of a 0.036 M solution of a weak acid with Ka = 3.9 x 10–6.
a. 2.55
6.
c. rate = k[I–][H+]
b. 3.43
c. 5.80
d. 6.85
e. 7.15
Which of the following are stronger acids than HClO2?
i. HCl
a. i & ii
ii. HBr
b. i, ii & iii
iii. HClO
c. i, ii & iv
iv. HBrO
v. HBrO2
d. i, ii, iv & v
e. all of them
7.
Consider the following reactions and their associated equilibrium constants:
A (g) + B (g)  2 C (g)
K1
2 C (g)  A (g) + B (g)
K2
2 A (g) + 2 B (g)  4 C (g)
K3
Which of the following statements is not true?
a. K1 = (K3)1/2
8.
b. K1 = K2K3
10.
b. HClO4
c. Ba(OH)2
a. Ka of HX = Ka of HZ
b. Ka of HZ > Ka of HX
d. Ka of HZ is too large to measure
e. HX is a strong acid
d. HI
e. H2SO3
c. Ka of HZ < Ka of HX
Which of the following are conjugate acid/base pairs?
a. i & ii
iii. HC2H3O2 and C2H3O2–
ii. HClO and HCl
b. i & iii
c. ii, iii & iv
iv. NH4+ and NH2–
d. only i
e. all of these
Consider the following equilibrium: 2 NO (g) + Br2 (g)  2 NOBr (g). 0.0524 mol NO (g) and 0.0262 mol Br2 (g) are
combined in a 1.00 L flask, and the system is allowed to reach equilibrium. It is determined that, at equilibrium,
0.0311 mol NOBr are present. How much NO is present at equilibrium?
a. 0.0370 mol
12.
e. all of these are true
Consider 0.78 M aqueous solutions of two monoprotic acids, HX and HZ at 25°C. If [OH–] is less in HZ (aq), what must
be true?
i. H2O and OH–
11.
d. K3 = K1K2
Which one of the following is not a strong acid or strong base?
a. CsOH
9.
c. K1 = (K2)–1
b. 0.0156 mol
c. 0.0107 mol
d. 0.0213 mol
e. 0.0262 mol
Which one of the following statements is not true?
a. HF is a stronger acid than H2O because F is more electronegative than O.
b. For 1.0 M solutions of any 2 weak bases, the solution of the base with the larger Kb will have the greater [OH–].
c. A Lewis acid is an electron pair acceptor.
d. For a polyprotic acid, Ka1 is always greater than Ka2.
e. The pH of a neutral solution is 7.00 at all temperatures.
13.
A 0.072 M solution of a weak acid, HA, has pH = 2.90 at 25°C. Determine the percent ionization in this solution of HA.
a. 2.5%
14.
b. 9.1%
c. 1.7%
d. 3.5%
e. 0.91%
In your Rate Law of the Iodine Clock Reaction lab, you measured reaction times for different initial concentrations of
reactants. In order to determine the order of reaction with respect to IO3–, what data plot did you prepare?
a. 1/t vs. 1/[IO3–]
b. t vs. [IO3–]
d. log(1/t) vs. log [IO3–]
e. log t vs. log [IO3–]
c. 1/t vs. [IO3–]
15.
In your Determination of an Equilibrium Constant lab, you studied the iron thiocyanate equilibrium:
Fe3+ (aq) + SCN– (aq)  FeSCN2+ (aq)
–3
In one trial 10.0 mL of 1.00 x 10 M Fe3+ (aq) is combined with 50.0 mL of 0.0200 M SCN– (aq) and diluted to a total
volume of 100.0 mL. The concentration of FeSCN2+ (aq) at equilibrium is determined via an absorbance measurement to
be 5.75 x 10–5 M. What is [Fe3+] at equilibrium?
a. 5.75 x 10–5 M
b. 1.00 x 10–4 M
c. 4.25 x 10–5 M
d. 9.42 x 10–4 M
e. 1.58 x 10–4 M
/45
Part III (15 points): Problems. Work all of the following problems. Show all work; work with units in all steps; report
all answers with appropriate units and significant figures.
1.
The reaction of NOBr (g) to form NO (g) and Br2 (g) is 2nd order: 2 NOBr (g)  2 NO (g) + Br2 (g).
It takes 88.0 minutes at 95.0°C for [NOBr] to decrease from 0.1000 M to 0.0675 M.
a.
Determine the value of k at 95.0°C. Be sure to include units!
k = _______________________
b.
How much time (in min) is required for [NOBr] to decrease from 0.1000 M to 0.0875 M?
time = ______________________ min
2.
At 25°C, Kb = 7.3 x 10–5 for trimethylamine, (CH3)3N. If an aqueous solution of this weak base has pH = 11.05,
determine the initial concentration of (CH3)3N.
[(CH3)3N] = _______________________ M
3.
Consider the equilibrium: A (s) + B (g)  C (g). In a flask at equilibrium at 250°C, PB = 0.240 atm and PC = 0.195 atm.
a.
Determine the value of KP at 250°C.
KP = ______________________
b.
If all of the B (g) is removed from the system, and the reaction is allowed to return to equilibrium, what will be the
new equilibrium partial pressure of B (in atm)?
PB = _______________________ atm
/15
Some information that you may find useful:
ln[A] = —kt + ln[A]0
1/[A] = kt + 1/[A]0
[A] = —kt + [A]0
t1/2 = 1/k[A]0
t1/2 = [A]0/2k
t1/2 = .693/k
!
!
!" !! = !!
!
!
!
!!
− !
k = Ae—Ea/RT
!
—1
R = 8.314 J•K •mol
II A
—1
OR 0.08206 L•atm•K •mol
III B
IV B
VB
VI B
VIII
VII B
IB
II B
III A
!!± ! ! !!"#
!"
—1
Kw = [H3O+][OH—] = 1.0 x 10—14 at 25°C
pX = —log X
IA
—1
!=
KP = KC(RT)∆n
pKw = pH + pOH
IV A
VA
VI A
1
VII A inert
gases
2
H
He
1.008
3
4
Li
Be
6.941 9.012
11
12
K
Ca
6
7
8
9
B
C
N
O
F
Ne
10.81 12.01 14.01 16.00 19.00 20.17
13
14
15
16
17
18
Na Mg
22.98 24.31
19
20
5
4.003
10
Al
21
22
23
24
Sc
Ti
V
Cr Mn Fe
25
26
27
28
Co
Ni
29
30
Si
P
S
Cl
Ar
26.98 28.09 30.97 32.06 35.45 39.95
31
32
33
34
35
36
Cu Zn Ga Ge
As
Se
Br
Kr
39.10 40.08 44.96 47.90 50.94 52.00 54.94 55.85 58.93 58.71 63.55 65.37 69.72 72.59 74.92 78.96 79.90 83.80
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
Rb
Sr
Y
Zr
Nb Mo Tc
Ru Rh Pd Ag Cd
In
Sn Sb
Te
I
Xe
85.46 87.62 88.91 91.22 92.91 95.94 98.91 101.0 102.9 106.4 107.8 112.4 114.8 118.6 121.8 127.6 126.9 131.3
55
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
132.9 137.3 138.9 178.4 180.9 183.8 186.2 190.2 192.2 195.0 196.9 200.5 204.3 207.2 208.9 (210)
87
88
89
104
105
106
107
108
109
110
111
112
113
114
115
116
*
(210) (222)
117
118
(223) 226.0 (227)
(294)
Fr
Ra
Ac
Rf
(267)
Db Sg Bh Hs
(268)
58
59
Ce
Pr
(271) (272)
60
61
Mt
(270)
(276)
62
63
Nd Pm Sm Eu
140.1 140.9 144.2 (147) 150.4 151.9
90
91
92
93
94
95
Th
Pa
U
Ds Rg Cn Uut
(269)
(280)
64
65
Gd
Tb
(242)
(243)
66
67
Dy Ho
Fl Uup Lv Uus Uuo
(289)
(288)
(293)
68
69
70
Er Tm Yb
71
Lu
157.3 158.9 162.5 164.9 167.3 168.9 173.0 174.9
96
97
98
99
100
101
102
103
Np Pu Am Cm Bk
232.0 (231) 238.0 (237)
(285) (286)
(247)
(247)
Cf
(251)
Es Fm Md No
(254)
(253)
(256) (254)
Lr
(262)
(294)