Chem 112  Exam II  ::::::  Spring 1999:  Day/Whelan

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            Information for your use:
            K = °C + 273 
            R = 0.08206 Latm/mol·K 
            K = 8.314 J/mol· K 
            [A] = [A]o- kt 
            ln[A] = ln[A]o - kt 
            ln ([A]/[A]o) = -kt 
            kt½ = ln2 = 0.693 
            1/[A] = 1/[A]o+ kt 
            k = Ae-Ea/RT
            ln (k2/k1) = (-Ea/R)(1/T2 - 1/T1
            pH = - log [H3O+
            pOH = - log [OH-
            At 25 ° C:     [H3O+] [OH-] = 1.0x10-14
            pH + pOH = 14 

1.     (4 Points)
A catalyst increases the rate of a chemical reaction by

raising the temperature 
increasing the concentration
increasing the fraction of molecules with a given energy 
providing a lower energy pathway for the reaction 
decreasing DH for the reaction

2.     (6 points)
For the reaction in aqueous solution, 2UO2+ + 4H+® U4+ + UO22+ + 2H2O , the following initial rates were obtained: From these data, the rate law for the reaction is: 
 

[UO2+],M [H+],M Initial Rate, Ms-1
0.0030 0.200 2.34 x 10-4
0.0030 0.400 4.68 x 10-4
0.0015 0.400 1.17 x 10-4

 

Rate = k[UO2+][H+]
Rate = k[UO2+]2[H+]
Rate = k[UO2+][H+]
Rate = k[UO2+]2[H+]4
Rate = k[UO2+]1/2[H+]

3.    (5 Points)
The rate constant for the zero order decomposition of NH3 on a tungsten surface at 1100 ºC is 2.0 x 10-4 M min-1 . If the initial concentration of NH3 is 0.0060 M, how many minutes are required for the concentration to drop to 0.0050 M? 

5.09.5142142

4.     (4 Points)
The isotope iodine-131 has a half-life of 8.05 days and decays by first order kinetics. What is the rate constant, including units, for the decay reaction? 

8.05 day
6.96 x 105 sec 
1.44 x 10-6 sec-1
1.24 x 10-3 day-1
8.61 x 10-2 day-1

5.    (5 Points)
For the decomposition of hypobromite ion in aqueous solution, 3BrO-® BrO3- + 2Br-, it is observed that a plot of 1 / [BrO-] versus time in seconds gives a straight line with a slope of 0.056 M-1s-1. The rate law for this reaction is: 

Rate = 0.056
Rate = 0.056[BrO-]
Rate = 0.056[BrO-]2
Rate = 12[BrO-]
Rate = 12[BrO-]2

6.    (5 Points)
The following data are for the gas phase decomposition of di-t-butyl peroxide (DTBP) to form acetone and ethane, C8H18O2(g) ® 2C3H6O(g) + C2H6(g), at 420K. The order of the reaction with respect to DTBP is: 
 

[DTBP], M 24.8 12.4 6.2 3.1
time, min 0 80 160 240

zerofirstsecondminus oneminus two

Questions 7-9 are based on the following two-step mechanism proposed for the gas phase reaction of nitrogen dioxide with fluorine.
NO2 + F2 ® NO2F + F slow 
F + NO2 ® NO2F fast

7.    ( 4 Points)
The overall reaction is

NO2 + F ® NO2 F + F 
F + NO ® NO2 F
2NO + F ® 2NO2 F
NO + F ® NO + OF
NO + F ® NO + O2 F

8.    (3 Points)
The catalyst is: 

NO
F
NO2 F
F
there is none

9.    (4 Points)
The rate law consistent with this mechanism is: 

Rate = k[NO2]
Rate = k[NO2]2
Rate = k[NO2]2[F2]
Rate = k[F][NO2]
Rate = k[NO2][F2]

10.     (4 Points)
At 298K he equilibrium constant for the reaction NH4HS(s) « NH3(g) + H2S(g) is 1.8 x 10-4. If an equilibrium mixture of the three compounds in a 1.00 L container contains 0.50 mol NH4HS(s) and 0.012 mol H2S(g), how many moles of NH3(g) are present? 

0.00750.0150.0300.330.67

11.     (4 Points)
For the equilibrium N2(g) + O2(g) « 2NO(g) , the equilibrium constant has a value of 4.5 x 10-31 at 298K. Which of the following are true? Choose all that apply. 
  1. When a sample of NO is introduced into an evacuated container at 298K, at equilibrium most of the sample will have decomposed to form N2 and O2
  2. When equal amounts of N2 and O2 are mixed in an evacuated container at 298K, at equilibrium about half of each will have reacted to form NO. 
  3. Fortunately gaseous mixtures of N2 and O2 do not react to an appreciable extent at 298K or we would all be breathing NO. 


1 only2 only3 only1 and 21 and 3

Questions 12 through 14 are based on the following gas phase equilibrium and related data: 

2 H2(g) + CO(g) « CH3OH(g)     DH = - 103 kJ and Kc = 25.6 at 200° C. 


12.    (4 Points)
If the volume of the above equilibrium system is decreased at constant temperature, the amount of CH3OH will 

increasedecreasenot changedecrease then increaseincrease then decrease

13.    (4 Points)
If the temperature of the above equilibrium system is increased, the value of the equilibrium constant will 

increasedecreasenot changedecrease then increaseincrease then decrease

14.    (4 Points)
What is the value of Kc at 200 °C for the reaction  ½ CH3OH(g) « H2(g) + ½ CO(g) 

0.03910.1985.0625.6640

15.    (4 Points)
The following equilibrium takes place in aqueous solution. At very high chloride ion concentrations, when the total concentration of the cobalt species is about 1 M, the equilibrium system would appear: 

Co(H2O)62+ + 4Cl- « CoCl42- + 6H2O        Co(H2O)62+ is pink         CoCl42- is blue 

pink due to the pink color of the aquo complex 
blue due to the blue color of the chloro complex 
colorless due to the high concentration of the colorless chloride ion 
pinkish-blue since both colored species are present in comparable amounts 
green because chlorine gas, which is green, would form 

16.     (4 Points)
Which one of the following aqueous solutions is basic? 

pH = 4.0
[OH-] = 4.0x10-8
pH = 6.9
[H3O+] = 2.0x10-7
none are basic 

17.    (4 Points)
What is the hydroxide ion concentration in an aqueous solution that has a pH of 4.40 at 25 ° C? 

2.5 x 10-10
3.8 x 10-9
1.5 x 10-7
5.7 x 10-6
4.0 x 10-5

Questions 18 and 19 refer to the following equilibrium taking place in aqueous solution:

HF + H2O « H3O+ + F


18.    (4 Points)
The two acids in the equilibrium system are 

HF and H2O
H3O+ and F
HF and H3O+
H2O and F
there is only one acid 

19.    (4 Points)
The conjugate ______ of HF is ______ .

acid, H2O
base, F
acid, H3O+
base, H2
acid, F

Esssay Portion
20.    (5 Points)
 Draw a reaction coordinate diagram for a typical exothermic reaction, reactants ® products, in the space below. Label the diagram clearly, indicating the reactants, the products, and the activation energy. With reference to this diagram, explain why for most reactions the rate of the reaction increases with increasing temperature.
Answer:  
21.    (5 Points)
Do a calculation to show what activation energy corresponds to the rule of thumb that the rate of a reaction near 25 °C (298K) doubles for each 10 degree rise in temperature.
Answer:  
Questions 22 and 23 refer to the following gas phase equilibrium system at 500K:

CH3OH(g)   «   CO(g)  +  2H2(g)


22.    (5 Points)
When 1.000 mole of methanol, CH3OH(g), is introduced into an evacuated 10.0 L container at 500K, the equilibrium amount of CO(g) is found to be 0.753 moles. What is the value of Kc at 500K? 
Hint: ICE might help your thinking.
Answer:  
23.    (5 Points)
If a mixture of the three gases at 500K is introduced into a container so that the initial concentration of all three gases is 0.500 M, describe what will happen? Hint: Use your value of Kc from question 22.
Answer: