Download Exam 3, 2006

Name:
Chemical Principles II
April 26, 2006
Third Exam
To receive credit, all calculations must be shown when you are asked to calculate or
estimate.
Useful information: R = 8.314 J/mol or 0.08206 L•atm/mol•K, F = 96,485 J/V(C/mol e–)
Substance Ho298-f kJ/mol So298-f J/mol.K Go298-f kJ/mol
Br2(l)
Not given
152.3
Not given
Br2(g)
30.71
245.3
31.82
N2(g)
Not given
191.5
Not given
H2(g)
Not given
130.6
Not given
NH3(g)
-46.2
192.5
To be calculated
1 ( 6 pts) Estimate the normal boiling point of elemental bromine, Br2.
2 ( 6 pts each) A process for the decomposition of ammonia involves the following
equilibrium:
2NH3(g)
N2(g) + 3H2(g)
a) Calculate HoSoGo and K for this reaction at 298K. Under these conditions,
is the reaction at equilibrium or is it spontaneous to the left or to the right?
(continue calculations on next page)
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b) Calculate Go and K for this reaction at 500K. Under these conditions, is the
reaction at equilibrium, spontaneous to the left or to the right? Explain which
trend (shift) LeChâtelier’s principle would predict when raising the temperature
from 298 to 500K for this reaction.
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c) Calculate G for this reaction at 500K and all gases with partial pressure = 10.0
atm. Under these conditions, is the reaction at equilibrium or is it spontaneous to
the left or to the right? Explain which trend LeChâtelier’s principle would predict
when raising the total pressure from 1.0 to 10.0 atm for this reaction.
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3. (a)( 6 pts) Define entropy in more scientific terms than “randomness” or “disorder”. Name the
types of motion (energy) that can be quantized into energy levels.
(b) ( 2 pts) What does the symbol o mean in the term Go in terms of concentrations or
pressures?
4. Given the following two half-reactions and their potentials, write the balanced
equation for the reaction that is spontaneous and give the E° of the cell. ( 8 pts)
Sn2+(aq) + 2e–  Sn(s)
Cr3+(aq) + 3e–  Cr(s)
E° = –0.136 V
E° = –0.74 V
5. Calculate ∆G° for the cell in the above problem. (5 pt)
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3+
2+
6. Calculate ∆E for the cell in problem 4 if [Cr ] = 0.001 M and [Sn ] = 10. M. (8 pts)
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7. A diagram of the voltaic cell in question 4 connected to an electrolytic cell containing
hydrogen and iodide ions is drawn below. With arrowheads describe the flow of ions to
metal electrodes in the cells, the KCl ions in a salt bridge, and electrons in the wires.
Label the anodes and cathodes. ( 8 pts)
e?
-ode
-ode
-ode
?
-ode
e-
K+Cl-
?
?
H+
?
I?
Cr3+
Cr
?
Sn2+(aq) + 2e–  Sn(s)
Cr3+(aq) + 3e–  Cr(s)
Sn2+
Sn
?
E° = –0.136 V
E° = –0.74 V
Pt
Pt
2 H+ + 2e– 
2 I-  + 2e–
8. Complete and balance the equations on this page and the next. Credit will only be
given for the work shown clearly.( 5 pts each)
(a)
2Cr2O7
+ I  Cr3+ + IO3 (acidic solution)
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10. (b)
Pb(OH)4
2-
+ ClO  PbO2 + Cl (basic solution)
11. The half life of cesium-137 is 30 years. Calculate the percentage remaining after 200
years (6 pt).
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12. Give the reaction products. (If no reaction, write NR.)(1 pt each)
Na+(aq) + Cl-(aq) + e- electrolysis 
H3PO4 + heat 
NaHCO3 + H3C2O2H (acetic acid)
ZnS + O2 
Zn + OH -(conc.) 
Zn + H+ 
Fe2O3 + CO 
P + O2(excess) 
P (excess) + O2 
Fe2O3 + H2 
MgO(s) + H2O(l) 
13. A voltaic concentration cell is constructed with Ca2+ compartments. The interior cell
compartment has [Ca2+] = 1.0 M and the exterior cell compartment has [Ca2+] = 1.0 x 10-2
M.
Ca2+ 1.0x10-2 M
membrane
Ca2+ 1.0M
a) Which compartment is the anode? (interior or exterior)(1pt)
b) What is the standard emf of the cell? (2pts)
c) Calculate the cell emf for the concentrations given. (3pts)
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Energy
14. (2pts if you get the four next questions correct.)
Use the following figure.
a)
b)
c)
d)
Which figure represents band picture for
1. a metal
a.
b.
2. an insulator
c.
d.
a.
c.
d.
c.
d.
c.
d.
b.
3. an intrinsic semiconductor
a.
b.
4. a p-doped semiconductor
a.
b.
15. Complete the following (4 pt)
a.
210
84 Po
b.
121
53 I
c.
239
94 Pu
d.
14
7N




206
82 Pb
121
52Te


4
2 He 
1
0n 


1
0n
1
1p
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Bonuses
1) (5 pts) Calculate the temperature at which the system in problem 2b would be in
equilibrium.
2) (BONUS 2pts) How is the price of fertilizer tied to the price of oil?
3) (BONUS 2pts) Give an equation (that was not in your text book) with which an
equilibrium constant at a new temperature can be estimated if Ho is known; and
(BONUS 3pts) derive it from two well known thermodynamic equations.
4) (Bonus 1pt) Remove the page with the periodic chart, keeping all the other pages
stapled together.
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