Chem 128, Exam III
April 23, 2004
Name
I. (35 points) A. (10 points) Consider an aqueous solution of PbI2 with solid lead(II)
iodide present. Ksp = 8.4 x 10−9 .
1. Write a balanced net ionic equation for the equilibrium established between the
solid and its corresponding ions in solution.
2. What is the Ksp expression?
3. What is the concentration of I− at equilibrium in a saturated solution of PbI2 ?
B. (10 points) Aluminum hydroxide solid (Ksp = 2 x 10−31 ) reacts with an excess of
33
hydroxide ions to form the complex ion Al(OH)−
4 (Kf = 1 x 10 ).
1. Write a balanced net ionic equation for the reaction. (Do NOT forget to write the
states of the species.)
2. What is the numerical value of K for the reaction?
3. Determine the molar solubility (mol/L) of aluminum hydroxide when [OH− ] is 1.0
x 10−3 M.
1
Chem 128, Exam III
April 23, 2004
Name
C. (15 points) You need to show your work for parts 1, 2, and 3. You do not need to do
so for parts 4 and 5.
A solution is made up by mixing 50.0 mL of 0.010 M AgNO3 with 50.0 mL of
0.0040 M HC. A precipitate of AgC (Ksp = 1.8 x 10−10 ) is obtained.
1. How many moles of precipitated AgC are obtained after equilibrium is established?
2. What is the concentration (in moles/L) of Ag+ when equilibrium is established?
3. What is the concentration (in moles/L) of C− when equilibrium is established?
4. To the solution at equilibrium described above, NaC is added. [Ag+ ]
increases,
decreases
remains the same
Circle the correct answer.
5. In a separate experiment, AuNO3 is added to the solution at equilibrium described
above. Ksp of AuC is 2.0 x 10−13 . [Ag+ ]
increases,
decreases
remains the same
Circle the correct answer.
2
Chem 128, Exam III
April 23, 2004
Name
II. (45 points)
A. (8 points) A solid and liquid are mixed at 25◦ C and 1 atm. Some observations are:
— The solid disappears completely.
— The beaker feels warmer to the touch after the compounds are mixed.
— A gas bubbles out.
One can state with a high degree of accuracy that
(Fill in the blanks with < , > , =, or MI .)
1. ∆H◦
0
2. ∆S◦
0
3. ∆G◦
0
4. K
1
B. (10 points) Sodium carbonate can be obtained from sodium hydrogen carbonate:
2 NaHCO3 (s) → Na2 CO3 (s) + CO2 (g) + H2 O ()
∆S◦ = 339 J/K
∆H◦ = 135.6 kJ
1. Calculate ∆G◦ at 85◦ C.
2. What is the molar entropy for NaHCO3 given the following molar entropies?
CO2 = 213.6 J/mol-K
H2 O () = 69.9 J/mol-K
Na2 CO3 = 136.0 J/mol-K
3. At what temperature is the reaction at equilibrium?
4. Does the reaction become spontaneous above or below that temperature?
3
Chem 128, Exam III
April 23, 2004
Name
C. (14 points) Answer the following questions by writing Y if the statement about the
diagram is true and N if the statement is false.
1.
2.
3.
4.
5.
6.
7.
Spontaneous reactions are always fast reactions.
∆S◦ is positive when dry ice sublimes.
∆G◦ is not temperature dependent.
If Q = 1, ∆G = ∆G◦
One can figure out the Ksp of AgCl using only tables of ∆H◦f and So .
At the normal melting point, ∆G◦ = 0.
In all reactions where ∆S◦ > 0, ∆G◦ < 0
D. (5 points) Given the following data for bromine at 25◦ C:
Br2 (): So = 152.2 J/mol-K
∆H◦f = 30.91 kJ/mol
Br2 (g): So = 245.4 J/mol-K
Estimate the normal boiling point for bromine. (Bromine is a liquid in its standard
state.)
E. (8 points) Acetic acid, HC2 H3 O2 , has a Ka of 1.8 x 10−5 .
1. What is ∆G◦ for acetic acid at 25◦ C?
2. Show by calculation whether the ionization of acetic acid at 25◦ C where the pH
is 4 and [HC2 H3 O2 ] = 0.100 M is spontaneous.
(Hint: [H+ ] = [C2 H3 O−
2 ])
4
Chem 128, Exam III
April 23, 2004
Name
III. ( 45 points)
A. (16 points) Use the following data to answer the questions below:
E◦red = + 0.339 V
Cu2+ (aq) + 2 e− → Cu (s)
2+
−
+
E◦red = + 0.161 V
Cu (aq) + e → Cu (aq)
E◦red = + 0.518 V
Cu+ (aq) + e− → Cu (s)
2+
−
E◦red = – 0.409 V
Fe (aq) + 2 e → Fe (s)
E◦red = – 0.762 V
Zn2+ (aq) + 2 e− → Zn (s)
2+
−
E◦red = – 0.236 V
Ni (aq) + 2 e → Ni (s)
E◦red = + 0.769 V
Fe3+ (aq) + e− → Fe2+ (aq)
For questions 1 – 6, consider the following species: Zn, Ni2+ , Fe2+ , Cu+
1. Using the table above, which are reducing agents? (You have more blanks than
needed.)
2. Which of the reducing agents is the strongest?
3. Which of the reducing agents is the weakest?
4. Using the table above, which are oxidizing agents? (You have more blanks than
needed.)
5. Which of the oxidizing agents is the strongest?
6. Which of the oxidizing agents is the weakest?
7. What is E◦ for the cell
Fe | Fe2+ || Cu2+ | Cu
8. Is the cell described in (7) a voltaic cell?
5
Chem 128, Exam III
April 23, 2004
Name
B. (12 points) Consider a voltaic cell in which the following reaction takes place
−
E◦ = 0.688 V
2 NO−
3 (aq) + 3 H2 (g) → 2NO (g) + 2 OH (aq) + 2 H2 O
1. What is n ?
2. What is ∆G◦ ?
3. What is Q (reaction quotient) under the following conditions:
[NO−
3 ] = 0.315 M, PNO = 0.922 atm, PH2 = 0.437 atm, pH = 11.50.
4. What is E under the conditions of (4) ?
6
Chem 128, Exam III
April 23, 2004
Name
C. (8 points) Consider a cell with the following reaction and standard reduction electrode
potentials:
Pb (s) + 2 H+ (aq) → Pb2+ (aq) + H2 (g)
E◦red = – 0.127 V
Pb2+ (aq) + 2 e− → Pb (s)
E◦red = 0.000 V
2 H+ (aq) + 2e− → H2 (g)
1. Calculate E◦ for this cell.
2. Chloride ions are added to the Pb | Pb2+ half cell to precipitate PbC2 . The voltage
is measured to be +0.210 V. If [H+ ] = 1.0 M and PH2 = 1.0 atm, what is [Pb2+ ]?
3. Taking [C− ] in (2) to be 0.10 M, calculate Ksp for PbC2
D. (9 points) A solution containing a 3+ metal ion (M3+ (aq)) is electrolyzed by a current
of 5.0 A for 10.0 min. A metal (M (s)) is obtained.
1. How many coulombs are supplied by the battery?
2. How many moles of electrons pass through the cell in 10 minutes?
3. What is the metal if 1.19 g of metal was plated out in this electrolysis.
7
Chem 128, Exam III
April 23, 2004
Name
IV. (25 points)
A. (4 points) Answer the following questions on the blanks provided.
1. What is the precipitating agent for group I ions?
2. At what pH do Group II ions precipitate as sulfides?
B. (11 points) Write a balanced net ionic equation using smallest whole number coefficients for the following reactions. Do not forget to write the physical states of all the
species. A point will be subtracted for each extra specie.
1. The reaction of the silver ammonia complex with a chloride ion and a strong acid.
2. The reaction between iron(III) ions and thiocyanate ions.
3. The precipitation of antimony (III) ions as a sulfide.
C. (10 points) When A+ ions react with B2− ions a precipitate forms.
2 A+ (aq) + B2− (aq) → A2 B (s)
In an experiment, 10.0 mL of 0.0200 M ACl are mixed with 10.0 mL of 0.0300 M Na2 B.
A green precipitate forms. When equilibrium is established, it is determined (using a
spectrophotometer) that [B2− ] is 0.0125 M.
1. Fill in the table below. Note that all answers must be expressed in MOLARITY.
Do not write x’s on the table. Only numerical values will be graded.
A+ (aq)
B2− (aq)
[ ]o
∆
[ ]eq
0.0125
2. Using the values from the table, calculate Ksp for A2 B
8
Chem 128, Exam III
April 23, 2004
Name
BONUS (15 points) All or nothing.
The bonus should be done only after you have completed the main part of this exam and
checked your work for errors. The time alloted for this exam does not include time for the
bonus. Trial and error solutions will not be accepted.
When CaF2 is dissolved in an HCHO2 /CHO−
2 buffer, the following reaction takes place:
+
2+
CaF2 (s) + 2 H (aq) Ca (aq) + 2 HF (aq)
What is the molar solubility of CaF2 in a buffer solution containing 0.30 M HCHO2 and
0.20 M NaCHO2 ?
The following information may be useful in solving the problem:
Ksp for CaF2 = 1.5 x 10−10
Ka for HF = 6.9 x 10−4
Ka for HCHO2 = 1.9 x 10−4
9