ENVS 602 by Cl6ub0

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									Dr. Casey                                           Name: ______________________
Exam 1
Spring 2006                                     Signature: ______________________
ENVS 602

Honor Statement: By signing this exam, you indicate that you have neither given nor
received aid from any unauthorized source. For this exam you may use a general
chemistry book for reference. No other resources are allowed.

Answer each question in the space provided. If you need more space, use the back of the
sheet. All work must be shown to receive partial credit.

Useful Information
[G] = KH * PG         1 atm = 101,325 Pa            1 atm = 760 mm Hg = 760 torr


1. When Zn2+ ions are placed in a solution containing Cl-, both chloride and hydroxide
complexes can form. For example:

              Zn2+ + OH-             ↔      ZnOH+          log Kf1 = -9.00

              ZnOH+ + OH-            ↔      ZnOH2          log Kf2 = -8.80

              Zn2+ + Cl-             ↔      ZnCl+          log Kf3 = 0.40

              ZnCl+ + Cl-            ↔      ZnCl2          log Kf4 = 0.20

       a. Which type of complex do you expect to form to a greater extent? Why?




       b. Determine the value of the equilibrium constant for the following reaction:

                             ZnCl2          ↔       Zn2+ + 2Cl-
c. Assume that these reactions represent all of the Zn species that can form. Write an
expression for the total Zn in the system. Then write that same expression in a form
where the concentrations are expressed in terms of parent material, constants and
controlling variables.




d. Determine the concentration of ZnCl+ under the following conditions:
       total concentration of Zn in the system = 147 g/L
       pH = 6.5
       [Cl-] = 1200 mg/L
2. River water from two sites is assessed for Zn toxicity using a fish bioassay. Explain why the
LC50 differs so much between the two sites. Specify what the likely source of toxicity is to the
fish.

               Site A                                Site B
       pH      6.2                                   6.8
       [Cl-]   4.5 x 10-4 M                          2.1 x 10-3 M
       LC50    12.5 mg/L                             152 mg/L




3. Runoff water from a mine tailings pile is channeled through a shallow ditch toward a
constructed wetland. The ditch contains a gravel bottom and the water is well aerated. The
constructed wetland is vegetated with cattails (Typha latifola) and the sediment contains a thick
layer of decaying vegetation from the prior year’s growth. Water flows mainly through the
wetland subsurface and is then released as surface water into an adjacent stream. The inlet and
outlet of the wetland were monitored for total Cd and SO42-.

                              Cd (g/L)              SO42- (mg/L)

               Inlet                  473                   136
               Outlet                  34                    83


       a. Explain the factors affecting redox status in the channel and the wetland sediments.
       b. Explain why passage through the wetland caused the observed changes in these two
       constituents.




4. a. Dissolution of carbon dioxide in water affects the pH of water. Explain why a eutrophic
pond with large blooms of algae can oscillate between pH 6 and pH 9 from early morning to late
afternoon.




       b. If the pond has an aqueous CO2 concentration of 9.5 x 10-6 M, is it at equilibrium with
       atmospheric CO2, a source to the atmosphere or a sink for atmospheric CO2?
               KH = 3.3 x 10-7 mol/(L*Pa)
               PCO2 = 3.5 x 10-4 atm
5. Dissolved organic matter isolated from Site A is assessed for conditional binding constants
(Kf’) under several different conditions.

              pH 5.5         Kf’ = 3.1 x 104
              pH 8.3         Kf’ = 1.8 x 105


       a. What are conditional binding constants and why do they differ between the two pHs?




       b. The carboxylic acid functional group can be either protonated or deprotonated as
       shown below (where R indicates a larger organic molecule attached to this group).
       Explain how this reaction relates to your answer to part a above.

                       R-COOH        ↔         R-COO- + H+

								
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