June 14, 2005
Dr. Nash
100 Points Possible
Chemistry 111
Practice Exam 2
NAME:__________________
Potentially useful constant: R= 8.314 J/Mol K= 0.08206 Latm/mol K
1. Given the following data
H of (H2O(l ) )  285.840 kJ /mol

H of (OH(aq)
)  229.94 kJ /mol



H of (H(aq)
)0
(a) Calculate Ho for the autoionization of water.
H 2O  H   OH 


(b) What is the Kw of water at 25.0 C? (you shouldn’t need to do a calculation here)
(c) What is the Kw of water at 40 C?
(d) What is the pH of pure water at 40 C? Is the water acidic, basic, or neutral?
2. What is the pH of 0.050 M KCN at 25.0 C? Ka(HCN)=6.2 x 10-10.
3. An aqueous 0.10 M solution of Acetylsalicylic acid (aspirin-HC9H7O4), ionizes to
the extent of 1.25%.
(a) What is the pH of a 0.10 M solution of acetylsalicylic acid?
(b) What is the Ka of acetylsalicylic acid?
(c) What is the extent of ionization of a 0.010 M solution of acetylsalicylic acid?
4. Why is a solution of NaF basic while a solution of NaCl is neutral?
5. Formic acid, HCOOH, has a Ka of 1.17 x 10-4. A solution is prepared by
dissolving 1.25 moles of formic acid and 2.25 moles potassium formate (KHCOO) in
2 L of solution.
(a) What is the pH of this buffer solution?
(b) What is the pH of this solution if 1.0 moles of HCl(g) is dissolved in this solution
(assume that the volume of the buffer solution does not change).
(c) What would the be pH of a solution prepared by dissolving 1.0 moles of HCl(g)
in 2.0 L of water?
6. Consider the following reaction which takes place in the gas phase.
CH3CN(aq)  CH3NC(aq)
with the following data
25C
time(s)
25C
35C
35C
[CH3CN]
[CH3NC]
[CH3CN]
[CH3NC]
0
3.000000
0.000000
3.000000
0.000000
1
2.448980
0.551020
2.390438
0.609562
2
2.068966
0.931034
1.986755
1.013245
3
1.791045
1.208955
1.699717
1.300283
4
1.578947
1.421053
1.485149
1.514851
5
1.411765
1.588235
1.318681
1.681319
6
1.276596
1.723404
1.185771
1.814229
7
1.165049
1.834951
1.077199
1.922801
8
1.071429
1.928571
0.986842
2.013158
9
0.991736
2.008264
0.910470
2.089530
10
0.923077
2.076923
0.845070
2.154930
11
0.863309
2.136691
0.788436
2.211564
12
0.810811
2.189189
0.738916
2.261084
13
0.764331
2.235669
0.695249
2.304751
14
0.722892
2.277108
0.656455
2.343545
15
0.685714
2.314286
0.621762
2.378238
16
0.652174
2.347826
0.590551
2.409449
17
0.621762
2.378238
0.562324
2.437676
18
0.594059
2.405941
0.536673
2.463327
19
0.568720
2.431280
0.513259
2.486741
20
0.545455
2.454545
0.491803
2.508197
1000
10000
0.296574
0.296574
2.703426
2.703426
0.232367
0.232367
(a). What is the order of this reaction? Show your work.
(b) What is the rate constant at 25 C?
2.767633
2.767633
(c) What is the rate constant at 35 C
(d) What is the activation energy of this reaction?
(e) Which of the following is a plausible mechanism for this reaction?
i.
CH3CN  CH3 + CN (slow)
CH3 + CN  CH3NC (fast)
ii.
CH3CN  CH3 + CN (fast, equilibrium)
CH3 + CN  CH3NC (slow)
iii.
CH3CN + CH3CN  C2H6 + 2CN (fast, equilibrium)
2 CN + C2H6  2 CH3NC (slow)
iv.
CH3CN + CH3CN  C2H6 + 2CN (slow)
2 CN + C2H6  2 CH3NC (fast)
v.
CH3CN  CH3NC
(f) What does it mean that after at most 1000 seconds the concentrations of reactant
and product do not change?
(g) What is the value of the equilibrium constant, Kc, at 25 C? Is it the same as Kp
in this case? Why?
(h) What is the Go of this reaction at 25 C? (4 pts)
(i) What is the value of the equilibrium constant, Kc(=Kp) at 35 C? (4 pts)
(j) What is the Go of this reaction at 35 C? (4 pts)
(k) Based on your values of Kp at the two temperatures above, What is the Ho for
this reaction? (4 pts)
(l) What is the So for this reaction at 25C?
(m) Draw a reaction coordinate diagram for the mechanism you chose in part (h).
Be sure to indicate what the effect of introducing a catalyst would be.
7. At 25C what is the fraction of collisions with energy equal or greater than an
activation energy of 100 kJ/mol?
8. A slightly bruised apple will rot extensively in about 4 days at room temperature
(~20 C). If it is kept in the refrigerator at 0 C, the same extent of rotting takes
about 16 days. What is the activation energy of the rotting reaction?
9. A 0.050 M solution of triethylamonium chloride (HN(C2H5)3+ Cl- ) has a pH of
4.33.
(a) What is the Ka of triethylamonium ion?
(b) What is the Kb of triethylamine (N(C2H5)3)
(c) What is the pH of a 0.020 M solution of triethylamine (N(C2H5)3 ? (data at @25
C)
(d) Using whatever means you can (pictures are always good) describe why
trimethylamine can be a base even though it does not actually contain OH- ions.
10. A buffer is prepared by dissolving 12.5g of HCN and 12.5 g of NaCN in water to
make 1.25 L of solution. The KB of CN- is 8.0 x 10-9.
(a) What is the pH of this buffer solution?
(b) What is the pH of the buffer solution after I add 5.5 g of solid NaOH to it
(assume that the volume of the solution does not change).
(c) What would the pH of the solution resulting from dissolving 5.5 g NaOH in 1.25
L of water?
11. The Ksp of MgF2 is 6.4 x 10-9.
(a) What is [Mg2+] and [F-] in a saturated (solid in equilibrium with aqueous ions)
solution of MgF2?.
(b) What is [Mg2+ ] in a solution containing 0.020 M [F-]?
12. Consider the following reaction taking place in aqueous solution
2 C3H4  C6H8
Time(s)
[C3H4]@25 C [C3H4]@35 C
0
2.5000
2.5000
2
2.1518
2.0468
4
1.8520
1.6758
6
1.5941
1.3720
8
1.3720
1.1233
10
1.1809
0.9197
12
1.0164
0.7530
14
0.8748
0.6165
16
0.7530
0.5047
18
0.6481
0.4132
20
0.5578
0.3383
22
0.4801
0.2770
24
0.4132
0.2268
26
0.3557
0.1857
28
0.3061
0.1520
30
0.2635
0.1245
(a) What is the rate law for the reaction?
rate=
(b) Is this a single step reaction? Why or why not?
(c) What is the rate constant at 25C?
k(25)=
(d) At 25C, how long will it be until the concentration of C 3H4 is 1.25 M.
(e) What is the rate constant at 35 C?
k(35)=
(f) At 35 C, how long will it be until the concentration of C 3H4 is 1/4 of its
original concentration?
(g) What is the activation energy of this reaction?
Ea=
(h) What is the rate constant of this reaction at 45 C?
13. Consider the following reaction taking place in aqueous solution
CH3I + Br-  CH3Br + IThe following data were obtained at 25C.
Experiment
1
2
3
4
Initial [CH3I]
M
0.015
0.030
0.015
0.045
Initial [Br---] M
Initial rate Ms-1
0.015
0.030
0.045
0.060
0.0667
0.1334
0.0667
0.2001
(a) Write a rate law for this reaction.
rate=
(b) What is the rate constant for this reaction?
k=
Given your rate law, which of the following is a plausible mechanism for
this reaction
i. one step:
CH3I + Br-  CH3Br + Iii. two steps:
CH3I  CH3+ + I- (fast, equilibrium)
Br- + CH3+  CH3Br (slow)
iii. two steps:
CH3I  CH3+ + I- (slow)
Br- + CH3+  CH3Br (fast)
iv. two steps:
CH3I + Br- CH3IBr (slow)
CH3IBr  CH3Br + I- (fast)
v. two steps:
CH3I + Br- CH3IBr (fast, equilibrium)
CH3IBr  CH3Br + I- (slow)
Draw a reaction coordinate diagram for the mechanism you’ve chosen. Be
sure to identify the locations of transitions states and the locations and
identities of reactive intermediates.
Potentially Useful Equations:
 K   H o
ln  2  
R
 K1 
1 1
  
 T2 T1 
 k   Ea  1 1 
  
ln  2  
R  T2 T1 
 k1 
 Ea
k  Ae
RT
1
1

 kt
 At A 0
ln A t  ln A0  kt
G  G  RT ln Q
o
 At  A0 e kt