Exam 2 Key

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Chemistry 102: Exam 2A 11/16/99

I pledge that I have neither given nor received unauthorized assistance on the exam:

1. (4 points each) Multiple Choice. Choose the one best answer, and circle it.

Use the following information for questions a and b. The Haber synthesis for ammonia is N2(g)+3H2« 2NH3(g); Kp is 0.113 for this reaction at 500 K. The initial pressure of N2 is 1 atm, the initial pressure of H2 is 5 atm, and the initial pressure of NH3 is 4 atm.

a. In what direction will the reaction proceed to reach equilibrium?

I. right II. left III. it's at equilibrium IV. can't be determined

 

b. The correct equilibrium expression is

I. II.

III. IV.

 

c. Which of the following is true for an equilibrium system having a Kc = 1.8?

I. The amount of reactants present will be very small relative to the amount of products present.

II. The reactant and product concentrations will be appreciable.

III. The equilibrium ratio of products to reactants is equal to 0.556, the inverse of 1.8.

IV. The reaction proceeds hardly at all.

d. Solution with the lowest pH:

I. 0.01 M HCl II. 0.01 M HC2H3O2 III. 0.01 M K2O IV. 0.01 M NaOH

e. Solution with the highest pH:

I. 0.01 M HCl II. 0.01 M HC2H3O2 III. 0.01 M K2O IV. 0.01 M NaOH

 

f. Which of the following reactions will proceed to the right if equal concentrations of all of the acids and bases present in these reactions are mixed?

I. HClO4(aq) + OH1-® ClO41-(aq) + H2O(l) II. F1-(aq) + NH3(aq)® HF(aq) + NH21-(aq)

III. NH41+ + NO3-(aq) ® NH3(aq) + HNO3(aq) IV. HCO31-(aq) + Cl1-(aq)® CO32-(aq) + HCl(aq)

 

g. Which of these is not characteristic of an acidic oxide?

I. have group 1A metal cations II. dissolve in strong base

III. oxidation number of O is -2 IV. react with water to produce H1+

#1 points

 

2. (18 points) Consider the system 3CO(g)+Fe2O3(s)« 2Fe(l)+3CO2(g); the reaction is exothermic. Use the table below to indicate if the equilibrium will shift right, shift left, or be unaffected as a result of the applied "stress". [This reaction is used in blast furnaces to reduce iron ore to iron; it accounts for the majority of oxygen used in industry.]

stress

shift right

shift left

no change
CO(g) is added

X

    
CO2(g) is removed

X

    
the volume of the reaction vessel is halved    

X
Ar(g) (argon) is added    

X
solid Fe2O3 is added    

X
the temperature of the reaction vessel is decreased by 100° C

X

    

#2 points

3. (10 points) KP equals 0.050 for the reaction N2(g)+O2 (g)« 2NO(g). Calculate the equilibrium partial pressures of all species present in a mixture that has PNO2=PNO=2.5 atm, and PN2O3=0. Show your work; prove that your answer is correct.

 

N2(g) +

O2 (g) «

2NO(g)

init

2.5

2.5

0
change

-x

-x

+2x
equil

2.5-x

2.5-x

2x

 

PNO=0.504=0.50 PN2=2.25=PO2

#3 points

4. (10 points) Write the balanced chemical equation for the reaction of K2O with water. Then calculate the pH, pOH, [H1+], and [OH1-] in a 5.49x10-4 M solution of K2O. Show your work. Your answers must have the correct number of significant digits.

 

K2O+H2O® 2KOH(aq)

#4 points

5. (10 points) Calculate the pH and equilibrium concentration of all species present in a 0.40 M solution of hydrazine, NH2NH2, which has Kb = 1.7x10-6. Show all your work. Explicitly state any assumptions you make, and justify the assumptions.
 

NH2NH2 +

H2O«

OH1-

NH2NH31+

init

0.40

      
change

-x

  

+x

+x
equil

0.40-x

  

x

x

 

[NH2NH2]=0.40 [OH1-]=[ NH2NH31+]=8.25x10-4 pOH=3.08 pH=10.92 [H1+]=1.2x10-11

#5 points

6. (15 points) Calculate the equilibrium concentration of all species present (except OH1-) in a 0.45M solution of carbonic acid (H2CO3), Ka1=4.3x10-7 and Ka2=5.6x10-11. Show all your work. Explicitly state any assumptions you make.
 

H2CO3«

H1+

HCO31-

init

0.45

    
change

-x

+x

+x
equil

0.45-x

x

x
 

HCO31-«

H1+

CO32-

init

4.4x10-4

4.4x10-4

  
change

-x

+x

+x
equil

4.4x10-4-x

4.4x10-4+x

x

[H2CO3]=0.45 [H1]=4.4x10-4=[ HCO31] [CO32]=5.6x10-11 pH=3.36

#6 points

7. (4 points each)

a. Sketch a graph showing the relationship between concentration (on the x-axis) and % ionization (on the y-axis) for a weak acid like acetic acid.

 

 

#7a points

b. Explain the appearance of your graph using LeChatelier's Principle. In other words, use LeChatelier's Principle to explain the relationship between concentration and % ionization for a weak acid.

 

Ka=[H1+][A1-]/[HA] If water is added, concentrations of all species fall by same fraction; product in numerator will decrease more than denominator; Qa falls below Ka; equilibrium must shift right (more ionized) to compensate.

#7b points

8. (1 point) What is your current lecture instructor's name, spelled correctly?

 

Dr. Hunnicutt #8 points