Chemistry 1312
Exam 2, Spring, 2004
I. Multiple Choice (90 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.
Identify
the coordinates of a data plot for the 1st order kinetic reaction of
“A” that yields a linear relationship.
a. [A] vs. time
d. ln [A] vs. time
b. 1/[A] vs.
time e. ln 1/[A]
vs. time
c. ln [A]
vs. 1/T
2.
The
specific rate constant, k, is 2.3 x
10-4 L/mol•sec for a chemical
reaction. What is the overall order of
the reaction?
a. zero d. third
b. first e. unknown, need more data
c. second
3.
Consider
the rate law, rate = k[A]2[B]. If the concentrations of both A and B are doubled, what happens the
rate of the reaction?
a. Increases by a factor of 2
d. Increases by a factor of 8
b. Increases by a factor of 4 e. unknown, need more data
c. Increases by a factor of 6
4.
Which
of the following relationships is always
correct for a spontaneous
reaction?
a.
DSsys
> 0 and
DSsurr > 0 d.
DSsys
> 0 and
DSsurr < 0
b.
DSsys
< 0 and
DSsurr > 0 e.
DSsys
= 0 and
DSsurr = 0
c.
DSsys
> 0
5.
The
rate of a chemical reaction can be affected by which of the following conditions?
a. changing the concentration of a
reactant d. changing the surface area of reactants
b. adding a catalyst to the reaction
system
e. all of the above affect reaction rates
c. changing the temperature of the
reaction system
6.
A
catalyst
a. increases the activation energy for
a reaction
b. alters the mechanism of a reaction
c. increases the average kinetic
energy of the reactants
d. increases the surface area of the
reactants
e. increases the collision frequency
of the reactants
7.
Consider
the reaction, N2(g) + 3 H2(g)
® 2 NH3(g) ∆H° = –92.6 kJ, ∆S° = –199 J/K
At standard conditions this reaction
a. is a spontaneous reaction at all
temperatures
b. can only be spontaneous at high
temperatures
c. can only be spontaneous at low
temperatures
d. can never be spontaneous at any
temperatures
e. can only be spontaneous at high
pressures
8.
The
standard free energy of formation, ∆G°f, for CS2(l),
a common solvent, is +65.27 kJ/mol. This
means that
a. CS2(l) has a negative entropy d. CS2(l) is thermodynamically stable
b. CS2(l) has a positive entropy e. CS2(l) has a negative enthalpy of formation
c. CS2(l) is thermodynamically unstable
9.
Which
of the following statements best
describes the second law of thermodynamics?
a. the total entropy change, ∆S°total,
always increases for all spontaneous reactions
b. all exothermic reactions are
spontaneous
c. all reactions for which the entropy
change of the system, ∆S°sys, increases are spontaneous
d. the free energy change, ∆G°,
always becomes more positive for all spontaneous reactions
e. the standard entropy, S°, for all
pure substances is zero
10.
The
activation energy of a chemical reaction can be calculated from the slope of a
linear data plot of
a. ln [A]
vs. time
d. ln k vs. 1/T
b. 1/[A] vs.
time e. ln rate vs. ln T
c. [A] vs. 1/T
11.
A
32 mg sample of a thermally unstable compound decomposes via 1st
order kinetics with a half-life of 30s.
How much of the sample will remain after 2 minutes?
a. 0.5 mg d. 4.0 mg
b. 1.0 mg e. 8.0 mg
c. 2.0 mg
12.
Which
of the following quantities is least
affected by temperature?
a. free energy change,
DG° d. rate constant, k
b. kinetic energy of molecules e. the pressure of a gas
c. activation energy, Ea
13.
Which
of the following equations represents the greatest
entropy change in the chemical reaction?
a. PCl3(g)
+ Cl2(g)
® PCl5(g) d. 2 N2(g) + O2(g)
® 2 N2O(g)
b. CO2(g)
® O2(g) + C(gr)
e. 2 CO2(g)
® 2 CO(g) + O2(g)
c. H2O(g)
® H2O(l)
14.
The
calculated value of
DS° is +314.1
J/K for the exothermic reaction,
2 CH3OH(l) + 3 O2(g)
® 2 CO2(g) + 3 H2O(g)
Therefore this reaction is
a. spontaneous only at high
temperatures d. nonspontaneous at
all temperatures
b. spontaneous only at low
temperatures
e. spontaneous at all temperatures
c. nonspontaneous
only at high temperatures
15.
For
the reaction, PCl5(g)
® PCl3(g)
+ Cl2(g),
DH° = +92.5 kJ and
DS°
= 182 J/K. Assuming
DH° and
DS°
do not change with temperature, this reaction is spontaneous
a. if T > 0.508 K
d. T > 508 K
b. if T > 1.97 K e. T > 197 K
c. if T < 274 K
II. Exercises (75 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. (37 points) Methanol, used int he production
of plastics, is prepared industrially by the reaction:
CO(g) + 2 H2(g)
® CH3OH(g)
Given the following data:
|
|
CO(g) |
H2(g) |
CH3OH(g) |
|
∆H°f(kJ/mol) |
–110.5 |
0 |
–201.2 |
|
S°(J/K•mol) |
197.6 |
130.6 |
238 |
R = 8.314 J/mol•K
a. (8 pts) Calculate the enthalpy change for the
reaction at 25°C. What does your answer
indicate about the reaction?
∆H°rxn = -90.7 kJ, an
exothermic reaction
b. (8 pts) Calculate the entropy change for the
reaction at 25°C. What does your answer
indicate about the reaction?
∆S°sys
= -220.8 J/K, a decrease in disorder of the system
c. (6 pts) Calculate the free energy change for
the reaction at 25°C. What does your
answer indicate about the reaction?
∆G°rxn
= -24.9 kJ, reaction occurs spontaneously at
25°C
d. (5 pts) Calculate the entropy change for the
surroundings at 25°C.
∆S°surr
= + 304 J/K
e. (10 pts) Calculate the equilibrium constant for
the reaction.
K = 2.32 x 104
2. (19 points) Consider the
following initial rate data for the reaction NO(g) + Cl2(g)
® NOCl2(g):
[NO]o [Cl2]o Initial rate (mol/L•s)
0.010 0.05 0.050
0.010 0.10 0.20
0.020 0.15 0.90
0.030 0.15 1.35
0.037 0.14 ???
a. (3 pts) The
order of the reaction with respect to NO is __1___.
b. (3 pts) The
order of the reaction with respect to Cl2 is __2__.
c. (3 pts) Write the rate law for the
reaction.
rate = k[NO][Cl2]2
d. (10 pts) What
is the predicted reaction rate for the 5th reaction listed?
k = 2.0 x 103 L2/mol2
s
rate = 1.45 mol/L s
3. (19 points)
The highly toxic pesticide parathion decomposes in the soil via 1st
order kinetics with a rate constant of 0.0495 days-1.
a. (3 pts) What is the half-life for the
decomposition of parathion?
14 days
b. (8 pts) How many days will lapse for a sample
of parathion to be 88% decomposed?
42.8 days
c. (8 pts) How many milligrams of a 20.0 mg
sample will remain after 30.0 days?
4.53 mg