Env/CE 543 Aquatic Chemistry. Sample Midterm Exam

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1 Env/CE 543 Aquatic Chemistry Sample Midterm Exam Note: The questions on this exam were taken from previous midterm and final exams. This sample exam is about 1.5 to 2.0 times longer than the midterm exam that will be given on November 21, Name: Exams must be turned in by the end of the class period at 10:00AM. The exam is closed book with the exception of a single sheet of paper (both sides) with hand-written notes. Show your work and explain any assumptions that you make. There are tables of acidity constants and hydrolysis constants and a periodic table at the end of this exam. Point totals for each question are given in brackets, and there are 100 total points on the exam. 1. Circle the appropriate terms in parentheses for the following statements: a. A reaction is favorable if the value of ( Δ / Δ G r ) is (positive / negative). b. A reaction with a positive value for increases. 0 G r 0 Δ H r becomes (more / less) favorable as the temperature 2. Circle the following properties that are conservative with respect to mixing of waters. alkalinity ph total inorganic carbon (C T ) 3. Anion exchange resins remove anionic species from solution. At ph 7, will arsenic(iii) or arsenic(v) be more effectively removed by anion exchange? Briefly (1-2 sentences) explain your answer. Note: As(III) is arsenous acid and As(V) is arsenic acid. 4. Without doing any detailed calculations involving equilibrium constants, estimate the alkalinity of the following solution: [NH 4 Cl] T = 10-4 M, [NaOH] T = 10-4 M, [Na 2 CO 3 ] T = 10-4 M.

2 5. Copper sulfate is sometimes added to ponds to control the growth of algae. What will happen to algae growth if ethylenediaminetetraacetic acid (EDTA) is added to a pond that had been treated with copper sulfate? Briefly (1-3 sentences) explain your answer. EDTA forms strong complexes with many metals, including copper(ii). 6. Denitrification is a process used to convert NO - 3 to N 2(g) in wastewater treatment plants. Methanol (CH 3 OH) is added to the system as an electron donor and is converted to CO 2(g). a. Write half reactions for the reduction of NO - 3 and the oxidation of methanol. b. Combine these half reactions to give a balanced net reaction for the overall process. c. Identify the oxidation states of nitrogen and carbon in both the reactants and products.

3 7. Label the regions on the following pe-ph plot with the dominant species for that region (from Morel and Hering, 1993).

4 8. The ph of the water in a laboratory-scale reactor is to be controlled by adjusting the partial pressures of CO 2(g) and NH 3(g). If P CO2 = 10-2 bar, what P NH3 is required to maintain the ph at 7.0? Give your answer in bar. Note: K H,CO2 = (M/bar) and K H,NH3 = (M/bar).

5 9. A wastewater at ph 7.5 contains 50 mg/l total zinc, 1000 mg/l total sulfate, and a negligible amount of carbonate. (25 points total) Zn 2+ + H 2 O = ZnOH + + H + K = Zn H 2 O = Zn(OH) 2(aq) + 2H + K = Zn H 2 O = Zn(OH) H + K = Zn H 2 O = Zn(OH) H + K = Zn 2+ + SO 2-4 = ZnSO 4(aq) K = Zn SO = Zn(SO 4 ) 2 K = Zn(OH) 2(s) + 2H + = Zn H 2 O K = a. Verify that the system is undersaturated with respect to Zn(OH) 2(s). b. Calculate the concentration of all dissolved zinc species in the system. c. If the ph is increased, a critical ph will be reached at which the solution is saturated with respect to Zn(OH) 2(s). Determine this critical ph. Hint: ph crit < 9.

6 (extra space for answer to Question 2)

7 10. Uranium(VI), which is present as UO 2+ 2 and associated complexes, is much more soluble than U(IV), and the reduction of U(VI) is a potential remediation strategy for contaminated groundwater. Certain microorganisms can use U(VI) species as an electron acceptor in conjunction with the oxidation of organic carbon (CH 2 O) to CO 2(g). Use the following reactions and speciation plot in answering the questions below. (20 points total) UO e - = UO 2(s) pe = 6.95 CO 2(g) + 4H + + 4e - = CH 2 O + H 2 O pe = UO 2 2+ UO 2 (CO 3 ) UO 2 (CO 3 ) 2 2- Percent UO 2 OH + (UO 2 ) 3 (OH) (UO 2 ) 2 (OH) 2 2+ UO 2 (OH) (aq) UO 2 CO 3(aq) ph Speciation of dissolved uranium(vi) for [U] diss = 5 μm and P CO2 = bar. a. (15 points) How much energy can be gained from the reduction of U(VI) at ph 4 for M CH 2 O, M dissolved U(VI), P CO2 = bar, and the presence of UO 2(s). Give your answer in kj/mol on a basis of moles U(VI) reduced. b. (5 points) How will the energy gained from U(VI) reduction be affected by increasing ph? Briefly (1-3 sentences) explain your answer.

8 11. A solution contains 10-3 M benzoic acid and M HEPES (pk a = 7.5) in its fully protonated form. Abbreviate benzoic acid and benzoate as HB and B -, and abbreviate HEPES and its conjugate base as HA and A -. a. On the following page, draw a LogC-pH plot for all species in the system. b. Determine the initial ph of the solution. c. On the second following page, sketch the curve for the titration of the solution with NaOH. d. On the third following page, sketch the curve for the buffering capacity of the solution. Note: that the buffering capacity of H 2 O is already included on the plot.

9 LogC (M) ph

10 ph NaOH Added (M)

11 5.0E E E-03 Buffer Intensity ((eq/l)/ph) 3.5E E E E E E E E ph

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