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Exam 4A
Spring 2006

No electronic devices (calculators, PDAs, laptop computers, cell phones, iPods, MP3 players, etc.) are allowed for this exam.

Read all questions carefully. Numerical answers require units to obtain full credit. Answers without work shown will receive no credit. Remember to balance all chemical equations.

Possibly useful information:

S = Rln W

       G = wmax

G = –RTlnKeq

G = H – TS

Eo = Eoox + Eored

R = 8.314 J/mol•K

Go = –nFEo

F = 96500 C/mol


Reduction Half–Reaction

Standard Potential
Eored (V)

 
 

Substance

Hof (kJ/mol)

So (J/mol•K)

O3(g) + 2 H+(aq) + 2 e O2(g) + H2O(l)

2.075

 

Ba2+(aq)

–537.6

9.6

O2(g) + 4 H+(aq) + 4 e 2 H2O(l)

1.229

 

BaO(s)

–548.1

72.1

H2SO3(aq) + 4 H+(aq) + 4 e S(s) + 3 H2O(l)

0.449

 

Ca2+(aq)

–543.0

–55.2

SO42–(aq) + 4 H+(aq) + 2 e SO2(g) + 2 H2O(l)

0.17

 

CaO(s)

–635.1

39.8

 

 

 

H2O(g)

–241.8

188.7

 

 

 

H2O(l)

–285.8

69.9

 

 

 

HNO3(aq)

–207.4

146.4

 

 

 

HNO3(l)

–173.2

155.6

 

 

 

NO3(aq)

–205.0

146.4


1. Complete and balance the following reactions:

a. aqueous perchloric acid plus solid magnesium hydroxide

b. SrCl2(aq) + CO32–(aq)

c. HClO2(aq) HClO(aq) + HClO4(aq)

d. Cu(s) + BrO3(aq) Cu(OH)2(s) + Br(aq)

2. Metal oxides react with acids similarly to metal hydroxides, i.e. the oxide ion acts as a hydrogen ion acceptor (twice) to form water as a product. Consider the following at 25o C.

a. Complete and balance: BaO(s) + HNO3(aq)

b. Calculate Ho for the reaction in part a.

c. Calculate So for the reaction in part a.

d. Show how you would calculate Go for the reaction in part a. Plug all of the numbers into the appropriate equation, explicitly using the values you would plug into a calculator, if you had one available.

e. Predict the spontaneity of the reaction in part a. Briefly explain your reasoning.

f. Assuming Ho and So were constant at all temperatures (a poor assumption, of course), would you expect the spontaneity to be the same for the reaction in part a at all temperatures? Briefly explain your reasoning.

3. Consider the following electrochemical cell at standard conditions and 25o C.

Pt(s) | O3(g) | O2(g) | H+(aq) || SO2(g) | SO42–(aq) | H+(aq) | Pt(s)

a. Write the reduction half–reaction and give the standard reduction potential.

b. Write the oxidation half–reaction and give the standard oxidation potential.

c. Write the net reaction and give the standard potential.

d. Is the reaction spontaneous or nonspontaneous? Explain (very briefly).

e. Consider the cell if it were used under atmospheric conditions rather than standard conditions so that the partial pressure of oxygen gas were 0.2 atm rather than 1.0 atm. Assuming all other components remained at standard conditions, would the cell potential, increase, decrease, or remain the same? Explain (very briefly).

f. Ozone (O3) is a form of the element oxygen but not in its standard state. What is the oxidation number of O in ozone?