Chemistry
1312
Exam 3, Spring 2002
I. Multiple Choice (82 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. A catalyst increases
the rate of a chemical reaction by
a. increasing the rate of a chemical
reaction but does not participate in the reaction
b. accelerating a chemical reaction by
lowering the DHrxn
c.
increasing the order of the reaction
d. changing
the activated complex to one of lower activation energy
e.
yelling and hollering, “go
faster!”
2. All of the following
changes can potentially increase the rate of a chemical reaction except:
a.
increasing the volume of a gaseous reaction system
b.
adding a catalyst to the reaction system
c. increasing the surface area of the
reactant molecules
d.
increasing the partial pressure of a reactant gas
e. increasing the temperature of the
reaction system
3. The reaction, 2 N2O(g)
+ O2(g)
® 2
N2O4(g) occurs
in the presence of a gaseous catalyst.
Which of the following statements is true?
a. The reaction is 2nd
order in N2O(g) and 1st order in
O2(g).
b. The reaction occurs
rapidly.
c. The reaction
must be conducted at 25°C.
d. The reaction is exothermic.
e. The activation energy for the
forward reaction is greater than that for the back
reaction.
4. A reaction mixture has
concentrations such that DG = zero. However, DG° for the
reaction is –10 kJ. This
information means that
a.
further reaction will take place to form more products
b. the
reaction mixture is at equilibrium but the concentration of the products is
large
c. the reaction mixture is at
equilibrium but the concentration of the products is small
d. the reverse reaction will take
place to form more reactants
e.
who knows?
5.
The rate data for, NO(g) +
Cl2(g) ®
NOCl2(g), is
[NO]o
[Cl2]o
initial rate (mol/L•s)
0.01
0.05
0.05
0.01
0.10
0.20
0.02
0.15
0.90
0.03
0.15
1.35
The rate law for the reaction is
a. rate = k
[NO]2[Cl2]2
b. rate = k
[NO]1.5[Cl2]2
c. rate = k
[NO]2[Cl2]
d. rate = k
[NO][Cl2]2
e. rate = k
[Cl2]2
6.
For the reaction profile at right,
a. the DHrxn is
represented by C – B
b. the
reaction is endothermic
c. the activated complex is
B
d. the activation energy for the
forward reaction is A + B
e.
the activation energy for the back reaction is C –
A
7.
Which of the following parameters is least affected by temperature
changes?
a. Free energy
change, DG
b. Rate constant, k
c. Kinetic
energy of molecules
d. Activation energy, Ea
e.
Equilibrium constant, Kc
8. The decomposition of
the highly toxic pesticide parathion follows 1st order kinetics with
a half‑life of about 2 weeks. What
fraction of an original sample will remain after 9 1/2
weeks?
a. > 1/2
b. <
1/2 but > 1/4
c. <
1/4 but > 1/8
d. < 1/8 but
> 1/16
e. < 1/16 but
> 1/32
9. One of the statements
concerning free energy is false. Which one?
a. The standard free energy change for
a reaction = DH°rxn – TDS°sys
b. The standard free energy change for
a reaction = –RT ln K,.
c.
Free energy change is a measure of the “available work or energy”
possible from a system.
d. A positive free energy change for a
reaction infers a spontaneous reaction.
e. The
free energy change for a reaction is directly proportional to the negative value
of the total entropy change for a reaction.
10. Only one of the following statements
regarding the 2nd Law of Thermodynamics is false. Which one?
a. DStot = 0
for any system at equilibrium.
b.
+DSsurr
must be greater than –DSsys for
a spontaneous process.
c. DSsys is always
positive for a spontaneous reaction.
d.
DStot for any spontaneous process must be greater than
zero.
e. DSsurr is
negative for an endothermic, spontaneous reaction.
11. One of the following reactions shows a
decrease in entropy of the system.
Which one?
a.
C(gr) +
CO2(g) ® 2 CO(g)
b. 2 NH3(g)
®
N2(g) +
3 H2(g)
c. CH3OH(l)
+ CO(g)
®
CH3COOH(l)
d. CaCO3(s) ® CaO(s) +
CO2(g)
e. CH4(g)
+ NH3(g)
® HCN(g)
+ 3 H2(g)
12. For reactants to form an activated
complex in the transition state for a chemical reaction, all of the following
conditions must be met except:
a. the reactant species
(molecules/atoms/ions) must collide
b. the reactant species must collide
with an energy equal to or greater than the activation energy for the
reaction
c. the reactant
species must collide with a “proper” orientation
d. the reactant species must be at
temperatures above absolute zero
e. the reactant species must be in
their standard states
13. The rate law for C ® products is, rate = k [C]. The activation energy, Ea for the reaction can be
determined from the slope of a line for a plot of
a. [C] vs. time
b. ln [C]
vs.1/T(K)
c. ln k vs. 1/T(K)
d. 1/[C]
vs. time
e. 1/[C]
vs. 1/T(K)
14. The reaction, CS2(g)
+ 3 Cl2(g)
®
CCl4(g) +
S2Cl2(g)
DH = –238 kJ,
is
a. spontaneous at all
temperatures
b. spontaneous at low temperatures but
nonspontaneous at high temperatures
c.
nonspontaneous at low temperatures but
spontaneous at high temperatures
b. nonspontaneous at all temperatures
e. an endothermic
reaction
II. Exercises (55 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. (20 points) The thermal decomposition of phosphine, PH3, 4 PH3(g)
®
P4(g) +
6 H2(g), follows
first‑order kinetics. The half‑life
for the decomposition is 35.0 s at 680°C.
a. (5 pts) Write the rate law for the
reaction.
rate =
k[PH3]
b. (5 pts) What is the reaction rate constant for
the thermal decomposition of phosphine at
680°C?
k = 0.0198
s-1
c. (10 pts) Determine the time lapse for the
decomposition of 90% of a phosphine sample at
680°C.
time = 116
seconds
2. (35 points) For the
reaction, 2 N2O5(g) ® 4
NO2(g) +
O2(g), consider the
following data
|
|
N2O5(g) |
NO2(g) |
O2(g) |
|
S°
(J/mol•K) |
346 |
239.9 |
205.0 |
|
DH°f (kJ/mol) |
11.0 |
33.2 |
0 |
a. (10 pts) Determine the
entropy change of the reaction system.
State the significance of your answer.
DS°sys
= +472.6 J/K; an increase in the
disorder of the system
b. (10 pts) Determine the entropy change of the
surroundings for the reaction.
State the significance of your answer.
DH°rxn = +110.8 kJ; an endothermic
reaction
DS°surr = +372 J/K; a decrease in the disorder
(more order) of the surroundings
c.
(10 pts) Determine the
standard free energy change for the reaction. State the significance of your
answer.
DG°rxn
= ‑ 30.3 kJ; reaction is
spontaneous at standard conditions
d.
(5 pts) Determine the
equilibrium constant for the reaction at 25°C. R = 8.314 J/mol•K
K = 1.8 x 105