Chemistry 1312
Exam 1, Spring, 2004
I. Multiple Choice
(100 points) Select
the best response for each of the following and record on the scantron. Do not
erase; erasures may result in a misgrading of your
response.
1.
Consider the equilibrium system:
4 NH3(g) + 5 O2(g) «
4 NO(g) + 6 H2O(g).
Which of the following statements is incorrect?
a. When NO(g) is added to the system, the amount of
H2O(g) decreases.
b. When NH3(g) is added to the system, the amount of
NO(g) increases.
c. When O2(g)
is removed from the system, the amount of NH3(g) decreases.
d. When NH3(g) is removed from the system, Kc
remains constant..
2.
Which of the following is a weak
acid?
a. HF(aq) d. HCN(aq)
b. HCOOH(aq) e. all are weak
acids
c. C6H5COOH(aq)
3.
Qc
is the reaction quotient for a reaction system at any stage of its progress
from reactants to products (or vice versa).
Kc
is a constant for a reaction system at equilibrium at a defined
temperature. What can be concluded from
the following statements?
a. If Qc >> Kc,
the reaction is “over”
b. If Qc >> Kc,
reactant concentrations are dominant
c. If Qc = Kc,
the reaction system has not yet moved beyond the preparation stage
d. If Qc <
Kc,
the reaction system is not at
equilibrium—the reaction proceeds to produce more product
e. If Qc > Kc, the reaction system is not at equilibrium—the reaction proceeds to produce more product
4.
An “A” amount of NOCl is placed in a
1.0 L reaction vessel and heated until equilibrium with the products Cl2
and NO is established: 2 NOCl(g) « Cl2(g)
+ 2 NO(g). Assuming “x” is the
molar concentration of Cl2(g) at equilibrium, what is the form of
the equilibrium constant expression?
a. Kc = d. Kc =
b. Kc = e. Kc =
c. Kc =
5.
Consider the gaseous endothermic
equilibrium: CH4(g) + 2 H2S(g) « CS2(g) + 4 H2(g)
Which of the following statements is false?
a. A decrease in the volume of the
reaction vessel favors the formation of CH4(g)
b. Addition of CH4(g) shifts the equilibrium to the right.
c. Removal of
heat favors the formation of CS2(g).
d. Removal of heat decreases the value
of Kc.
e. Removal of CS2(g) increases the amount of H2(g).
6.
Consider the gaseous equilibrium:
COCl2(g) « CO(g)
+ Cl2(g), Kc = 0.082 at 900°C
A 1.0 L reaction vessel is analyzed to have 0.33 mol COCl2,
0.11 mol CO, and 0.22 mol Cl2. Which of the following statements best
describes the state of the system?
a. The COCl2, CO, and Cl2
concentrations will all increase.
b. Only the COCl2
concentration will increase.
c. No change will occur,
the reaction system is at equilibrium.
d. The CO and
Cl2 concentrations will increase.
e. The CO concentration will increase
and the COCl2 and Cl2 concentrations will decrease.
7.
Identify the aqueous solution with the highest pH.
a. 0.010 M H2SO4 d. 0.010 M NH3
b. 0.010 M CH3COOH e. need more information
c. 0.010 M KOH
8.
Which of the following is not
a conjugate acid-base pair?
a. H2O/
b. NH4+/NH2- e.
c. H2SO4/HSO4-
9.
The Ka for HClO is 3.5 x 10-8. What is(are) the
major specie(s) in the aqueous system (besides H2O) at equilibrium?
a. HClO d. H3O+
b. HClO and
H3O+ e. H3O+ and OH-
c. H3O+ and ClO-
10. Consider the following acids and Ka
values. Which acidic solution has the highest pH?
a. 0.001 M C6H5COOH, Ka = 6.5 x 10-5 d. 0.001 M HCN, Ka =
4.9 x 10-10
b. 0.001 M HNO2, Ka
= 4.3 x 10-4 e. 0.001 M HCl, Ka =
¥
c. 0.001 M HIO3 Ka
= 1.7 x 10-1
11. In which of the following equilibrium systems does an increase in the
volume of the reaction vessel favor the formation of products?
a. MgO(s)
+ CO2(g) « MgCO3(s) d. N2(g) + O2(g) « 2 NO(g)
b. PCl5(g) «
PCl3(g) + Cl2(g) e. N2(g) + 3 H2(g) « 2 NH3(g)
c. H2(g) + F2(g) « 2 HF(g)
12. The Kw
for water at 0°C is 1.6 x 10-15.
What is the pH and pOH of water at 0°C?
a. pH = pOH
= 7.00 d. pH = pOH = 14.8
b. pH =
pOH = 7.40 e. pH = 6.60, pOH =
7.40
c. pH = 7.40, pOH
= 6.60
13. Identify the acid having the weakest
conjugate base:
a. C6H5COOH, Ka = 6.5 x 10-5 d. HCN, Ka
= 4.9 x 10-10
b. HNO2, Ka = 4.3 x 10-4 e. HCl, Ka
=
¥
c. HIO3 Ka = 1.7 x 10-1
14. All of the following statements about chemical equilibria
are true except:
a. At equilibrium the reactant and
product concentrations remain unchanged.
b. At equilibrium the rate of the
forward reaction equals the rate of the back reaction.
c. An
equilibrium condition can only be achieved by starting with reactants.
d. At equilibrium the equilibrium
constant expression is constant at a given temperature.
e. An equilibrium condition is
unchanged with the addition of a catalyst.
II. Exercises (65 points). The organization of all exercises must be
shown clearly and legibly in order to receive any partial credit. Answers without a logical problem setup will receive
only 10% credit.
1. (30 points) a.
(10 pts) Write a Brønsted equation for perchloric acid, HClO4(aq), in an
aqueous solution.
What is the pH and pOH
of a 0.0204 M HClO4
solution?
pH = 1.68; pOH = 12.31
b. (15 pts) Write a Brønsted equation for hypochlorous acid, HClO(aq), in an
aqueous solution.
What is the pH and pOH
of a 0.0204 M HClO solution? Ka
for HClO is 3.5 x 10-8.
pH = 4.57; pOH = 9.43
c. (5 pts) What is the percent dissociation of a
0.0204 M HClO
solution?
0.13% dissociation
2. (10 points) Write the Brønsted
equation and the equilibrium constant
expression for pyridine, C5H5N. Kb
= 1.8 x 10-9
C5H5N + H2O «
C5H5HH+
+ OH-
Kb = 1.8 x 10-9 = [C5H5NH+][OH-]/[C5H5N]
3. (10 points)
Given the equilibrium system: 4
NH3(g) + 7 O2(g) « 2 N2O4(g) + 6 H2O(g)
Initially, [NH3] = [O2] =3.00 mol/L. At equilibrium, [N2O4]
= 0.50 mol/L
a. (5 pts) What is the equilibrium molar
concentration of O2?
1.25 mol/L O2
b. (5 pts) What is the equilibrium molar
concentration of H2O?
1.50 mol/L H2O
4. (15 points)
At 300K Kc
= 2.5 x 10-9 for Q2X2(g) « 2 QX(g)
A 2.00 mol sample of Q2X2 is placed into a
10.0 L vessel and heated at 300 K until equilibrium is
established. What is the equilibrium
molar concentration of QX?
[QX] = 2.2 x 10-5 mol/L