Practice Questions for Exam 2
CH 1020 Spring 2017
1. Pick all of the statements which are true about a
reaction mechanism?
I.
A rate law can be written from the
molecularity of the slowest elementary step.
II. The final rate law can include intermediates.
III. The rate of the reaction is dependent on the
fastest step in the mechanism.
IV. A mechanism can never be proven to be the
correct pathway for a reaction.
2. Consider the reaction:
2A(g) + B(g)  2C(g)
When the concentration of substance B in the
reaction is doubled (all other factors held constant),
it is found that the rate of the reaction remains
unchanged. The most probable explanation for this
observation is that:
A. The reactant with the smallest coefficient in
the balanced equation usually has little or no
effect on the rate of reaction.
B. Substance B is probably a catalyst which
means that its effects on the rate do not depend
on its concentration
C. Substance B is not involved in the ratedetermining step of the mechanism, but is
involved in other steps.
D. Substance B is not involved in any of the steps
in the mechanism of the reaction.
4. Which of the following is a catalyst for this
reaction?
A.
B.
C.
D.
E.
NO2
Cl2
Cl
NO2Cl
There is no catalyst in this mechanism.
5. What is the rate law for this mechanism?
A.
B.
C.
D.
E.
Rate = k[NO2][Cl2]
Rate = k[NO2]2[Cl2]
Rate = k[NO2][Cl]
Rate = k[NO2Cl][Cl]
Rate = k[NO2Cl]2
6. Which is the best progress of reaction diagram for
the reaction?
A.
C.
B.
D.
For the next four questions, consider the following
proposed mechanism for an exothermic reaction.
NO2 + Cl2 → NO2Cl + Cl
NO2 + Cl→ NO2Cl
(slow)
(fast)
3. Which of the following is a reaction intermediate?
A.
B.
C.
D.
E.
NO2
Cl2
Cl
NO2Cl
There is no intermediate.
7. ∆H for an uncatalyzed reaction is 5.0 kJ/mol. If
the activation energy, Ea, is 20.0 kJ/mol, which
reaction has the larger activation energy?
A.
B.
C.
D.
the forward reaction
the reverse reaction
both reactions occur at the same rate
The reaction will not take place without a
catalyst.
Practice Questions Exam 2 Spring 2017
Page 2 of 6
8. Consider the reaction and its rate law:
3A + B  2C
3
rate = k[A] .
Which will not increase the rate of reaction?
A.
B.
C.
D.
E.
Increasing the concentration of A
Adding a suitable catalyst
Increasing the temperature
Increasing the concentration of B
All will increase the rate.
9. An aqueous reaction occurs by the following
mechanism:
Step 1: A2X2 + Y → A2X + XY
Step 2: A2X2 + XY → A2X + X2 + Y
An increase in the concentration of compound Y
will result in a(n) [decrease/increase] in the
activation energy of the reaction which will result
in a(n) [decrease/increase] in the reaction rate .
Compound Y is a(n)
[reactant/product/intermediate/catalyst].
10. The reaction profile represents the following
chemical equation: A + B  C + D Calculate the
activation energy (Ea) and H of the reaction at
200 K.
11. A radioisotope decays to give an alpha particle and
Pb-208. What was the original element?
12. When 226
decays, it emits 2 α particles, then a β
88 Ra
particle, followed by an α particle. The resulting
nucleus is:
A.
B.
C.
Bi
222
86
Rn
214
82
Pb
D.
E.
212
85
At
214
83
Bi
287
Uuq
13. The, as yet unnamed, superheavy element 114
was made by firing a beam of Ca-48 ions at
Pu-242. How many neutrons are produced in the
reaction?
14. What nuclear process involves the splitting of a
heavier nuclide into smaller nuclides?
15. The nuclear reaction that takes place in our sun is
_________. The nuclear reaction which can be
used to produce heat for the generation of
electricity is ______.
16. What type of radiation do you think the isotope
He-6 would emit and why?
A.
B.
C.
D.
1200 kJ
212
83
positron emission; n:p ratio too low
positron emission; n:p ratio too high
beta emission; n:p ratio too low
beta emission; n:p ratio too high
17. All of the following would be expected to be stable
except…
A. Ne–20
B. Es–243
C. He–4
500 kJ
200 kJ
A.
B.
C.
D.
E.
F.
Ea
700 kJ
700 kJ
1200 kJ
1200 kJ
300 kJ
300 kJ
H
−300 kJ
+300 kJ
−300 kJ
+300 kJ
−700 kJ
+700 kJ
D. O–16
E. Ca–40
18. What type of decay would you expect for the
unstable nuclide in the previous question?
A.
B.
C.
D.
E.
alpha decay
fission
beta decay
fusion
positron emission
Practice Questions Exam 2 Spring 2017
19. What type of radioactive emission should be
28
expected for 13
Al ?
0
1
A. 11 H
D.
B. 01 e
C. None, its stable.
E. 42 He
e
Page 3 of 6
25. Kc = 2 for the reaction
X(g) + Y2(g)  XY(g) + Y(g)
Which of the reaction mixtures below represents a
mixture at equilibrium? Assume that dark spheres
represent Y atoms and light spheres represent X
atoms.
20. What is the binding energy (in J/mol) for Mo-98
(nuclear mass = 97.9055 amu)? Assume that the
mass of a neutron is 1.00855 amu and the mass of
a proton is 1.00728 amu.
Recall that 1 J = 1 kgm2/s2.
1
2
21. What amount of energy, in kJ, is released when a
mole of Co-57 undergoes electron capture? Recall
that 1 J = 1 kgm2/s2
57
27
Co + 01e 
57
26
Fe
mass of nucleus Co-57:
mass of nucleus Fe-57:
mass of electron:
3
56.93629 amu
56.93539 amu
0.000549 amu
22. A 1.00 g sample of carbon from wood at an
archeological dig in Arizona gave 7900
disintegrations in a period of 20 hours. In the same
period, 1.00 g of wood from a modern source
underwent 18,400 disintegrations. About how old
is the sample, to two significant figures? (The
half-life of C–14 is 5730 years.)
23. A 9.9 gram sample of iodine–131 is stored for
exactly 3 weeks. If the decay constant is
0.0861 day–1, what mass of the isotope remains?
24. For the hypothetical reaction,
2A(s) + B(g)  3C(g)
write the equilibrium constant expression.
A. Mixture 1
B. Mixture 2
C. Mixture 3
D. None of the above
26. If Kc = 46.9 at 700 K for the reaction,
PCl5(g)  PCl3(g) + Cl2(g)
what is Kc for the reverse reaction?
27. What is the value of Kp for the reaction shown in
Question 26?
28. In which of the following situations will Keq
change?
A.
B.
C.
D.
when reactant concentrations change
when temperature is changed
when product concentrations change
when a catalyst is added
29. Consider the following reaction at 1700 C.
2CH4(g)  C2H2(g) + 3H2(g)
Initially, the reaction mixture contains only
[CH4] = 0.320 M. At equilibrium, the mixture
contains [C2H2] = 0.070 M. What is the
equilibrium constant for this reaction at 1700 C?
Practice Questions Exam 2 Spring 2017
Page 4 of 6
30. If Kc = 1.70 x 103 for the reaction,
N2(g) + O2(g)  2 NO(g)
and the initial concentrations are [N2]= 0.515 M,
[O2] = 0.175 M, and [NO] = 0.135 M, what is
required for the reaction to reach equilibrium?
A. Nothing, because the reaction is already at
equilibrium.
B. More NO must form.
C. More O2 must form.
D. A catalyst must be added.
31. The diagram to the left represents an equilibrium
mixture for the reaction shown. What can be said
about the mixture to the right?
34. At a certain temperature the equilibrium constant,
Kc, equals 0.11 for the following reaction:
2 ICl  I2(g) + Cl2(g)
What is the equilibrium concentration of ICl if
0.25 mol of I2 and 0.25 mol of Cl2 are initially
mixed in a 2.0 L flask?
35. A decrease in volume will have what effect on the
reaction below?
C(s) + H2O(g)  CO(g) + H2(g)
A.
B.
C.
D.
E.
K decreases
more CO(g) and H2(g) are produced
no effect
more H2O(g) produced
K increases
36. Consider the following reaction:
CO(g) + 2H2(g)  CH3OH(g)
At room temperature, K is approximately 2  104,
but at a higher temperature K is substantially
smaller. Which of the following is true?
A. The system is at equilibrium.
B. The system is not at equilibrium the forward
reaction predominates.
C. The system is not at equilibrium, the reverse
reaction predominates.
32. For the reaction: 2 HI(g)  H2(g) + I2(g),
Kc = 0.020 at 720 K. If the initial concentrations of
HI, H2, and I2 are all 1.50 × 103 M at 720 K,
which one of the following is correct?
A. The system is at equilibrium.
B. The concentrations of H2 and I2 will decrease
as the system is approaching equilibrium.
C. The concentration of HI will decrease as the
system is approaching equilibrium.
33. Consider the following reaction at 100 C.
N2O4(g)  2NO2(g)
Kc = 0.36
A reaction mixture at 100 C initially contains
[NO2] = 0.100 M. What is the equilibrium
concentration of NO2?
A. The reaction is endothermic.
B. The value of Kc for this reaction is smaller at
all temperatures.
C. At the higher temperature, more CH3OH(g) is
produced.
D. The reaction is exothermic.
E. The reaction becomes spontaneous at higher
temperatures.
37. What is ΔG for the reaction below at 25 C if
ΔG = 141.8 kJ and the partial pressures of the
gases involved in the reaction are 10 atm SO2,
100 atm O2, and 1.0 atm SO3?
2SO2(g) + O2(g) → 2SO3(g)
Practice Questions Exam 2 Spring 2017
38. Consider the following gas-phase reaction of A2
(grey spheres) and B2 (black spheres) molecules.
A2 + B2  2AB
40. For the reaction below, S = 269 J/(mol·K) and
H = 103.8 kJ/mol. Calculate the equilibrium
constant at 25 C.
3C(s) + 4 H2(g)  C3H8(g)
Which of the following reaction mixtures (not at
equilibrium) has the largest (most positive) G of
reaction?
A. Mixture I
B. Mixture II
C. Mixture III
Page 5 of 6
I
I
I
I
D. Mixtures I and II
E. Mixtures I and III
39. What is G for the vaporization of hydrazine,
N2H4, at 70 C if 2.0 mol of hydrazine liquid are
present and the pressure of the hydrazine gas is
0.15 atm?
N2H4(l)  N2H4(g)
Gf = 159 kJ/mol for N2H4(g)
Gf = 149 kJ/mol for N2H4(l)
USEFUL INFORMATION
[A]t = kt + [A]0
t½ =
[A]0
2k
ln [A]t = kt + ln[A]0
t½ =
ln 2
k
E = mc2
c = 2.998  108 m/s
1 mol = 6.022  1023 particles
1 amu = 1.6605  1027 kg
Kp = Kc(RT)n
R = 0.08206 L • atm • mol1 • K1
G = Go + RT ln Q
Go = RT ln K
1/[A]t = kt + 1/[A]0
t½ =
1
[A]0 • k
J = kg • m2 • s2
R = 8.314 J/K·mol