Chem 106 Spring 2011 Hour Exam 2
Chap 15-18 Kotz 7th Ed
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Exam Directions
♣ This is a closed book, 90-minute exam. That means use nothing but the materials supplied to answer the
questions.
♣ There are a total of 11 pages in this exam, including the cover, back sheet, and scratch paper.
♣ The back sheet contains reference information including constants and periodic table.
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1
Part I. Multiple Choice (4 pts each; 108 pts for 27 questions)
Identify the choice that best completes the statement or answers the question. CAREFULLY fill in the correct answer on
the BUBBLE SHEET provided. Marks on this exam paper will NOT be graded, even if a mark here appears correct and
the answer on the bubble sheet is incorrect.
____
1. Which statement concerning relative rates of reaction is correct for the chemical equation given below?
2 NOBr(g) → 2 NO(g) + Br2(g)
a. The rate of disappearance of NOBr is equal to the rate of appearance of Br2.
b. The rate of disappearance of NOBr is two times the rate of appearance of NO.
c. The rate of disappearance of NOBr is half the rate of appearance of Br2.
d. The rate of appearance of NO is equal to the rate of appearance of Br2.
e. The rate of appearance of NO is two times the rate of appearance of Br2.
____
2. Given the initial rate data for the reaction 2A + B → C, determine the rate expression for the reaction.
[A], M
0.180
0.180
0.720
[B], M
0.250
0.500
0.500
∆[C]/∆t (initial) M/s
1.36 × 10–3
2.72 × 10–3
1.09 × 10–2
a.
b.
c.
d.
e.
____
3. For the reaction 2A + B → C, the rate law is
.
Which of the factor(s) will affect the value of the rate constant for this reaction?
1. decreasing the temperature
2. adding a catalyst
3. decreasing the concentration of reactant A
a.
b.
c.
d.
e.
1 only
2 only
3 only
1 and 2
2 and 3
2
____
4. How are the exponents in a rate law determined?
a. They are equal to one for gas phase reactants and zero for solid phase reactants.
b. They are determined by experimentation.
c. They are equal to the coefficients in the overall balanced chemical equation.
d. They are equal to the reactant concentrations.
e. They are equal to one for gas phase reactions and two for aqueous reactions.
____
5. For the second-order reaction below, the rate constant of the reaction is 3.9 × 10–5 M–1s–1. How long (in
seconds) is required to decrease the concentration of A from 1.07 M to 0.24 M?
rate = k[A]2
2A → B
a.
b.
c.
d.
e.
5.8 × 103 s
6.5 × 103 s
2.1 × 104 s
3.8 × 104 s
8.3 × 104 s
____
6. For a chemical reaction, the activation energy for the forward reaction is +155 kJ and the activation energy
for the backward reaction is +117 kJ. What is the overall energy change for the forward reaction?
a. –272 kJ
b. –38 kJ
c. +38 kJ
d. +155 kJ
e. +272 kJ
____
7. Which of the following statements is/are CORRECT?
1. For a chemical system, if the reaction quotient (Q) is greater than K, products must be
converted to reactants to reach equilibrium.
2. For a chemical system at equilibrium, the forward and reverse rates of reaction are
equal.
3. For a chemical system at equilibrium, the concentrations of products divided by the
concentrations of reactants equals one.
a.
b.
c.
d.
e.
1 only
2 only
3 only
1 and 2
1, 2, and 3
3
____
8. Write the expression for K for the reaction below.
Mg3(PO4)2(s)
3 Mg2+(aq) + 2 PO43–(aq)
a.
b.
c.
d.
e.
____
9. For which one of the following reactions does Kp equal Kc?
a. 2 CO(g) + O2(g)
2 CO2(g)
b. CH4(g) + 2 O2(g)
CO2(g) + 2 H2O(g)
c. CaCO3(s)
CaO(s) + CO2(g)
d. NH3(g)
3/2 H2(g) + 1/2 N2(g)
e. 2 O3(g)
3 O2(g)
____ 10. A 5.0 L flask is filled with 0.25 mol SO3, 0.50 mol SO2, and 1.0 mol O2, and allowed to reach equilibrium.
Assume the temperature of the mixture is chosen so that Kc = 0.12. Predict the effect on the concentration of
SO3 as equilibrium is achieved by using Q, the reaction quotient.
2 SO2(g) + O2(g)
2 SO3(g)
a. [SO3] will decrease because Q > K.
b. [SO3] will decrease because Q < K.
c. [SO3] will increase because Q < K.
d. [SO3] will increase because Q > K.
e. [SO3] will remain the same because Q = K.
____ 11. At 2010 K, the equilibrium constant, Kc, for the following reaction is 2.5 × 103.
2 NO(g)
N2(g) + O2(g)
If the equilibrium concentrations of N2 and O2 are 0.28 mol/L and 0.38 mol/L at 2010 K, what is the
equilibrium concentration of NO?
a. 1.8 × 10–9 M
b. 4.3 × 10–5 M
c. 3.8 × 10–3 M
d. 6.5 × 10–3 M
e. 153 M
4
____ 12. Assume that the following endothermic chemical reaction is at equilibrium.
C(s) + H2O(g)
H2(g) + CO(g)
Which of the following statements is/are CORRECT?
1. Increasing the concentration of H2O(g) will cause the reaction to proceed in the
forward direction, increasing the equilibrium concentration of CO(g).
2. Increasing the temperature will cause the reaction to proceed in the forward direction,
increasing the equilibrium concentration of CO(g).
3. Increasing the amount of C(s) will cause the reaction to proceed in the forward
direction, increasing the equilibrium concentration of CO(g).
a.
b.
c.
d.
e.
1 only
2 only
3 only
1 and 2
1, 2, and 3
____ 13. Which of the following substances is never a Brønsted-Lowry acid in an aqueous solution?
a. hydrogen fluoride, HF(g)
b. sodium phosphate, Na3PO4(s)
c. ammonium chloride, NH4Cl(s)
d. hydrogen bromide, HBr(g)
e. sodium bicarbonate, NaHCO3(s)
____ 14. Which of the following molecules or ions is amphiprotic in water?
a.
b.
c.
d.
e.
NH4+
HF
CH3CO2H
HC2O4–
CN–
____ 15. A solution is prepared by diluting 0.055 mol HCl with water to a volume of 2.5 L. What is the pH of the
solution?
a. 0.0220
b. 1.26
c. 1.66
d. 2.90
e. 3.82
5
____ 16. What is the OH– concentration of an aqueous solution with a pH of 4.45? (Kw = 1.0 × 10–14)
a. 2.8 × 10–10 M
b. 3.5 × 10–5 M
c. 7.1 × 10–5 M
d. 9.55 M
e. 2.8 × 104 M
____ 17. What is the equilibrium constant for the following reaction,
CN–(aq) + HNO2(aq)
HCN(aq) + NO2–(aq)
and does the reaction favor the formation of reactants or products?
a.
b.
c.
d.
e.
K = 1.00. The reaction favors neither the formation of reactants nor products.
K = 8.9 × 10–7. The reaction favors the formation of products.
K = 8.9 × 10–7. The reaction favors the formation of reactants.
K = 1.1 × 106. The reaction favors the formation of products.
K = 1.1 × 106. The reaction favors the formation of reactants.
____ 18. What is the H3O+ concentration in 0.66 M NH4+(aq)?
a. 3.7 × 10–10 M
b. 8.5 × 10–10 M
c. 1.9 × 10–5 M
d. 2.4 × 10–5 M
e. 2.9 × 10–5 M
____ 19. Which of the following chemical equations corresponds to Ka2 of phosphoric acid?
a. HPO42–(aq) + H2O( )
PO43–(aq) + H3O+(aq)
3–
b. PO4 (aq) + H2O( )
HPO42–(aq) + OH–(aq)
c. H3PO4(aq) + H2O( )
H2PO4–(aq) + H3O+(aq)
d. H3PO4(aq) + 2 H2O( )
HPO42–(aq) + 2 H3O+(aq)
–
e. H2PO4 (aq) + H2O( )
HPO42–(aq) + H3O+(aq)
____ 20. Which of the following molecules or ions is the strongest acid?
a. CH3CO2–
b. CH3CO2H
c. CFH2CO2H
d. CF2HCO2H
e. CF3CO2H
6
____ 21. An acid-base equilibrium system is created by dissolving 0.10 mol HF in water to a volume of 1.0 L. What is
the effect of adding 0.050 mol F–(aq) to this solution?
a. The pH of the solution will decrease.
b. Some F–(aq) will react with H3O+, increasing the concentration of HF(aq) and
reestablishing the solution equilibrium.
c. The addition of F–(aq) will have no effect on the pH or the concentration of HF(aq).
d. Some HF(aq) will ionize, increasing the concentration of F–(aq) and decreasing the pH.
e. Some HF(aq) will ionize, increasing the concentration of F–(aq) and increasing the pH.
____ 22. What is the pH of a solution that results from adding 50.0 mL of 0.100 M HCl to 50.0 mL of 0.330 M NH3?
(Kb of NH3 = 1.8 × 10–5)
a. 1.00
b. 1.30
c. 4.38
d. 9.62
e. 11.16
____ 23. Which of the following combinations would be best to buffer an aqueous solution at a pH of 9.0?
a. H3PO4 and H2PO4–, Ka1 = 7.5 × 10–3
b. HNO2 and NO2–, Ka = 4.5 × 10–4
c. CH3CO2H and CH3COO–, Ka = 1.8 × 10–5
d. H2PO4– and HPO42–, Ka2 = 6.2 × 10–8
e. NH4+ and NH3, Ka = 5.7 × 10–10
____ 24. What is the pH of the buffer that results when 12 g sodium formate (NaHCO2) is mixed with 250 mL of 0.50
M formic acid (HCO2H) and diluted with water to 1.0 L? (Ka of HCO2H = 1.8 × 10–4)
a. 3.29
b. 3.60
c. 3.74
d. 3.89
e. 5.12
____ 25. If the ratio of base to acid in a buffer increases by a factor of 10, the pH of the buffer
a. increases by 1.
b. decreases by 1.
c. increases by 10.
d. decreases by 10.
e. remains unchanged.
7
____ 26. Which of the following conditions is/are met at the equivalence point of the titration of a monoprotic weak
base with a strong acid?
1. The moles of acid added from the buret equals the initial moles of weak base.
2. The volume of acid added from the buret equals the volume of base titrated.
3. The pH of the solution is greater than 7.00.
a.
b.
c.
d.
e.
1 only
2 only
3 only
1 and 3
2 and 3
____ 27. Which of the following equations is the solubility product for lead(II) iodate, Pb(IO3)2?
a.
b.
c.
d.
e.
SCRATCH PAPER
8
Part II.
For full credit on numeric problems, you must show all conversion factors and the final answer with
complete units, use the correct number of significant figures, and round off correctly. WRITE
CLEARLY: if the grader cannot read your writing, no credit will be assigned.
28.
(9 pts)
t-Butyl bromide (C4H9Br) reacts with water at 30.0 °C to give t-butyl alcohol (C4H9OH) and HBr per the
following equation:
C4H9Br + H2O C4H9OH + HBr
A kinetic experiment was carried out by measuring the concentration of HBr at times after mixing the
reactants. Some the data points are shown below for a reaction where the initial [C4H9Br] = 0.200 M.
(Water is the solvent, whose concentration is high, and essentially constant throughout the reaction.)
(a) Determine (algebraically or graphically) the rate law of the reaction, and
(b) Calculate the rate constant.
Hint: start by calculating [C4H9Br] values.
Time
(min)
0.00
1.00
2.00
3.00
4.00
5.00
[HBr]
mol/L
0.000
0.059
0.100
0.130
0.150
0.165
9
29. (9 pts) The equilibrium constant at 25 °C for the dissolution of silver iodide is 8.5 x 10-17.
AgI(s)
Ag+(aq)
+ I-(aq)
If an excess quantity of AgI(s) is added to water and allowed to equilibrate, what is the equilibrium
concentration of I- ?
30. (9 pts) Calculate the molar solubility of Fe(OH)3 (units of mol/L) in a solution that is buffered at pH 3.50
at 25 °C.
10
May be removed – but please do not rip out the staple.
Avogadro’s number = 6.0221 x 1023
Specific heat capacity of water = 4.184 J/(g-°C)
En = –Rhc/n2 ;
Speed of light (c) = 2.998 x 108 m s-1
R = 1.097 x 107 m-1
Circumference of circle = 2πr
Gas constant (R) = 0.08206 L-atm/(K-mol)
= 8.314 J/(mol-K)
Ion Product of Water @ 25 °C
Kw = 1.0x 10-14
.
Acid Dissociation Constants
Acid
Ka
HNO2
4.5 x 10-4
H2CO2H
1.8 x 10-4
CH3CO2H
1.8 x 10-5
H2PO4-
6.2 x 10-8
NH4+
5.7 x 10-10
HCN
4.0 x 10-10
Base Hydrolysis Constant
Base
F
-
Kb
1.4 x 10-11
Solubility Product Constants
Compound
11
Ksp
PbF2
3.3 x 10-8
AgI
8.5 x 10-17
Cu(OH)2
2.2 x 10-20
Fe(OH)3
6.3 x 10-38