CEE 680

 

16 December 2000

 

FINAL EXAM

 

Closed book, three pages of notes allowed.

Answer all questions.Please state any additional assumptions you made, and show all work.If you donít have time to complete a section, please describe how you would solve the problem (without using MINEQL).

 

 

 

A. Precipitation/Dissolution.(70%)

Magnesite (MgCO3 (s)) and Brucite (Mg(OH)2 (s)) are two important magnesium-bearing minerals.

 

1.      Prepare a solubility diagram (log C vs pH) for a water in equilibrium with magnesium hydroxide and magnesium carbonate.Assume the water has 10-2.5 M total carbonates.Show the MgT line and indicate where precipitation will occur and the type of precipitate.

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 (CT), the y-axis.Assume a total soluble magnesium concentration of 1 mM..

 

 

B. Redox (30%)

Ozone (O3) is one of the most powerful chemical oxidants used for the purification of waters and wastewaters.Its final reduced form is usually oxygen and water.Ozone is also a naturally-occurring oxidant that is produced in sunlit surface waters, in urban smog and in the upper atmosphere.The following questions concern the possible oxidation of aluminum by ozone.

1.      Write a balanced equation for the oxidation of aluminum metal to Al ion by ozone

2.      Determine the stoichiometry of this reaction (e.g., mg-O3/mg-Al).

3.      Determine the log K for this reaction

4.      Calculate the equilibrium ratio of trivalent Al ion to zero-valent Al solid at pH 7, and in the presence of 10-5 atm ozone.

5.      Discuss the implications of these calculations

 

 


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

 

 

 

 

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

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

+2.07

8

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

-1.68

9

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

+0.17

 

 

 

Properties of Selected Elements

Element

Symbol

Atomic #

Atomic Wt.

Valence

Electronegativity

Aluminum

Al

13

26.98

3

1.47

Calcium

Ca

20

40.08

2

1.04

Carbon

C

6

12.01

2,4

2.50

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

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, 25įC, 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

Ammonium ion

NH4+ = H++ NH3

9.24

Bicarbonate ion

HCO3- = H++ CO3-2

10.33

Monohydrogen phosphate

HPO4-2= H++ PO4-3

12.3