CEE 680

 

18 December 2007

 

FINAL EXAM

 

Closed book, three pages of notes allowed.

Answer Question A and either B or C. Please state any additional assumptions you made, and show all work. If you dont have time to complete a section, please describe how you would solve the problem (without using a computer program such as MINEQL).

 

 

Part I: Answer Question A

A. Solubility & Predominance. (75%)

Zinc carbonate (ZnCO3(s)) and Zinc Hydroxide (Zn(OH)2 (s)) are two important solid phases that may control zinc solubility in water[1]. In the attached pages is a detailed solution leading to a zinc hydroxide solubility diagram[2]. Please use this to help solve the following problems.

 

1.      Prepare a solubility diagram (log C vs pH) for a water that is potentially in equilibrium with zinc hydroxide and zinc carbonate. Assume the water has 10-3 M total carbonates. Show all soluble species along with the ZnT line and indicate where precipitation will occur and the type of precipitate. Please feel free to use any of the hydroxide calculations in developing the answer to part 1 or part 2. It could save you some time.

2.      Prepare a predominance diagram, showing the precipitates and major soluble species (in areas where there are no precipitates). As would be typical for a problem of this type, make pH the x-axis, and log total carbonate (CO3T), the y-axis. Assume a total soluble zinc concentration of 10-3 M (1 mM). Again you may find that using the solubility diagram provided or the one you did for part 1 can help in directing your work for part 2.

 

Equilibrium[3]

Log K

ZnOH+ = Zn+2 + OH-

-5.04

Zn(OH)2o = ZnOH+ + OH-

-6.06

Zn(OH)3-1 = Zn(OH)2o + OH-

-2.50

Zn(OH)4-2 = Zn(OH)3-1 + OH-

-1.20

Zn(OH)2 (s) = Zn+2 + 2OH-

-15.55

ZnCO3 (s) = Zn+2 + CO3-2

-10.26

 

 

 

Part II:. Answer either B or C

B. Redox (25%)

Galvanized pipe is still quite common in home plumbing in the US and Europe. The zinc coating on these pipe materials can corrode and undergo oxidation from the zero-valent metal to the soluble divalent aqueous zinc.

Residual active chlorine (hypochlorous acid) is a powerful oxidant. It can readily cause the oxidation of zinc metal to the divalent cation. However, in many drinking water systems, chlorine is not used as a disinfectant (e.g., groundwater systems) or it becomes substantially dissipated. The major residual oxidant is dissolved oxygen. After reaction, xxygens final reduced form is usually hydroxide and water. You have been asked to comment on the possibility that zinc can be oxidized by oxygen, and whether oxidized zinc can affect other metals. Specifically, your tasks are:

 

1.      Write a balanced equation for the oxidation of metallic zinc to Zn+2 by dissolved oxygen

2.      Determine the stoichiometry of this reaction (e.g., mg-O2/mg-Zn).

3.      Determine the equilibrium constant (K) for this reaction and comment on what thermodynamics tells you about whether this is a favorable reaction.

4.      Make a similar determination about the abililty of oxidized zinc (Zn+2 ) to oxidize metallic iron to Fe+2.

 

 

C. Redox True/False (25%) Mark each one of the following statements with either a "T" or an "F"

 

a.  ______ Lead is toxic partly because it has the same redox potential as calcium .

 

b. ______ As metals become more oxidized, they tend to be more prone to hydrolysis

 

c.  ______ Most Redox reactions essentially result in complete conversion to products or no conversion at all when at equilibrium

 

d. ______ Phosphate helps control corrosion by chemically reducing certain oxidized metal species

 

e.  ______ Environmental systems are far more likely to be at redox (e.g., electron) equilibrium than at acid/base (e.g., proton) equilibrium.

 

f.  ______ Oxidation state is almost always changes with ligand number.

 

g. ______ The oxidation state of phosphorus in the environment is almost always +V.

 

h. ______ As pe increases, so does the electron activity

 

i.   ______ The easiest way to measure half cell potentials is with a standard hydrogen electrode.

 

j.   ______ For the ferrous/ferric redox couple, the value of ao plus a1 must always equal one.


Some important equilibrium constants:

Equilibria

Log K

Mg(OH)2 (s) = Mg+2 + 2OH-

-11.6

Fe+3 + H2O = FeOH+2 + H+

-2.19

Mg+2 + H2O = MgOH+ + H+

-11.44

MgCO3 (s) = Mg+2 + CO3-2

-7.5

CaCO3(s) = Ca+2 + CO3-2

-8.34

Ca(OH2)(s) = Ca+2 + 2OH-

-5.19

CaSO4.2H2O(s) = Ca+2 + SO4-2 + 2H2O

-4.62

CaOH+ = Ca+2 + OH-

-1.15

AlOH+2 = Al+3 + OH-

-9.01

CdOH+ = Cd+2 + OH-

-3.92

CoOH+ = Co+2 + OH-

-4.80

CuOH+ = Cu+2 + OH-

-6.00

FeOH+ = Fe+2 + OH-

-4.50

HgOH+ = Hg+2 + OH-

-10.60

NiOH+ = Ni+2 + OH-

-4.14

PbOH+ = Pb+2 + OH-

-6.29

ZnOH+ = Zn+2 + OH-

-5.04

 

Some important half-cell reactions

Equ#

Half Cell Reaction

DEo (Volts)

1

O2(g) + 4H+ + 4e- = 2H2O

+1.23

2

Mn+3 + e- = Mn+2

+1.51

3

Mn+4 + e- = Mn+3

+1.65

4

MnO4- + 8H+ + 5e- = Mn+2 + 4H2O

+1.49

5

Fe+3 + e- = Fe+2

+0.77

6

Cu+2 + e- = Cu+

+0.16

7

HOBr + H+ + e- = Br- + H2O

+1.33

8

O3 (g) + 2H+ + 2 e- = O2 (g) + H2O

+2.07

9

Al+3 + 3e- = Al(s)

-1.68

10

S(s) + 2H+ + 2e- = H2S (g)

+0.17

11

NH2Cl + H+ +e- = Cl- + NH4+

+1.40

12

Zn+2 + 2e- = Zn(s)

-0.76

13

Ni+2 + 2e- = Ni(s)

-0.24

14

Pb+2 + 2e- = Pb(s)

-0.13

15

Cu+2 + 2e- = Cu(s)

+0.34

16

Hg2+2 + 2e- = 2Hg(l)

+0.91

17

Fe+2 + 2e- = Fe(s)

-0.44

 


Properties of Selected Elements

Element

Symbol

Atomic #

Atomic Wt.

Valence

Electronegativity

Aluminum

Al

13

26.98

3

1.47

Bromine

Br

35

79.904

1,3,5,7

2.74

Calcium

Ca

20

40.08

2

1.04

Carbon

C

6

12.01

2,4

2.50

Chlorine

Cl

17

35.453

1,3,5,7

2.83

Copper

Cu

29

63.54

1,2

1.75

Hydrogen

H

1

1.01

1

2.20

Magnesium

Mg

12

24.31

2

1.23

Manganese

Mn

25

54.94

2,3,4,6,7

1.60

Nitrogen

N

7

14.0047

3,5

3.07

Oxygen

O

8

16.00

2

3.50

Potassium

K

19

39.10

1

0.91

Sodium

Na

11

22.99

1

1.01

Strontium

Sr

38

87.62

2

0.99

Sulfur

S

16

32.06

2,4,6

2.44

 

 

Selected Acidity Constants

(Aqueous Solution, 25C, I = 0)

NAME

FORMULA

pKa

Perchloric acid

HClO4 = H+ + ClO4-

-7

Hydrochloric acid

HCl = H+ + Cl-

-3

Sulfuric acid

H2SO4= H+ + HSO4-

-3

Nitric acid

HNO3 = H+ + NO3-

0

Bisulfate ion

HSO4- = H+ + SO4-2

2

Phosphoric acid

H3PO4 = H+ + H2PO4-

2.15

o-Phthalic acid

C6H4(COOH)2 = H+ + C6H4(COOH)COO-

2.89

p-Hydroxybenzoic acid

C6H4(OH)COOH = H+ + C6H4(OH)COO-

4.48

Nitrous acid

HNO2 = H+ + NO2-

4.5

Acetic acid

CH3COOH = H+ + CH3COO-

4.75

Aluminum ion

Al(H2O)6+3 = H+ + Al(OH)(H2O)5+2

4.8

Carbonic acid

H2CO3 = H+ + HCO3-

6.35

Hydrogen sulfide

H2S = H+ + HS-

7.02

Dihydrogen phosphate

H2PO4- = H+ + HPO4-2

7.2

Hypochlorous acid

HOCl = H+ + OCl-

7.5

Hypobromous acid

HOBr = H+ + OBr-

8.71

Ammonium ion

NH4+ = H+ + NH3

9.24

Bicarbonate ion

HCO3- = H+ + CO3-2

10.33

Monohydrogen phosphate

HPO4-2 = H+ + PO4-3

12.3

 

Development of hydroxide solubility diagram.

 

a. Using the equilibria for zinc hydroxide from above:

we get the following for the free aquo ion:

 

From this we use the hydroxide equilibria to get hydroxide species concentrations:

 

and for the monohydroxide:

 

now for the dihydroxide:

 

now for the trihydroxide:

 

now for the tetrahydroxide:

 

 



[1] Hydrozincite is also an important solid, but it will not be considered here for the purpose of simplicity.

[2] Note that this considers only mononuclear species, as the known polynuclear species are never dominant

[3] For the purposes of this problem, we are ignoring all soluble zinc carbonate species.