Homework answers / question archive / Please show all steps you are using Not just Direct answers

Please show all steps you are using Not just Direct answers

Chemistry

Share With

---

Please show all steps you are using Not just Direct answers.

1. (a) What is the saturation state of calcite in the surface waters of the Southern Ocean around Antarctica, where surface water temperatures average 5°C? Is the precipitation or dissolution of calcite favored?

 

Assume that:

Alk = 2.2 mM

DIC = 2.1 mM

                [Ca] = 10.2 mM

                Activity coefficient (γ) for doubly charged ion at seawater ionic strength = 0.2

Note: In seawater, a reasonable estimate of [CO₃^2-] = Alk – DIC

Use equilibrium constants from table 4.3 in Stumm and Morgan.              

 

 

• Ignore the effect of ion pairing.

 

(b) In these surface waters, phytoplankton form shells made out of calcite. What effect does precipitating calcium carbonate have on the alkalinity and pH of these surface waters?

 

2. Complexation in aquaculture: You are managing algal ponds used for the production of biofuel feedstock. After the algae die off suddenly, you notice that the concentration of total dissolved copper in the intake water of the ponds has increased to 1µM. The water contains 5e-6 M of CO₃^2- and is buffered to a pH of 7.6. The algal strain experiences Cu toxicity at a free Cu²⁺ concentration of 10^-8 M. You decide to treat the pond with EDTA in order prevent toxicity. What total concentration of EDTA do you need to add?

 

Consider the following complexation reactions in your answer (assume that the pond is at 25°C):

                Cu²⁺ + EDTA^4- = CuEDTA^2-                                                                     log K = 20.5

                Cu²⁺ + CO₃^2- = CuCO₃⁰                                                     log K = 6.8

                Cu²⁺ + OH^- = CuOH⁺                                                         log K = 6.3

HEDTA^3- = EDTA^4- + H⁺                                                       log K = -10.2

 

 

 

 

3. Redox in hydrothermal vents. At the mid-ocean ridge, hot and metal rich hydrothermal vent fluids mix with seawater, initiating rapid redox reactions. Consider the mixing of hydrothermal fluid and seawater at a ratio of 1:100

 

Species	Seawater concentration (mM)	Hydrothermal fluid (mM)
Reductants:
Fe(II)	0	8
Mn(II)	0	0.3
H2S	0	2
Oxidants:
O2	0.12	0

 

                Consider the following reactions:

                1/8 SO₄^2- + 5/4 H⁺ + e^- = 1/8 H₂S + ½ H₂O                 pe°_w = -3.6

 

Fe(OH)_3(s) + 3H⁺ + e^- = Fe²⁺ + 3H₂O                             pe°_w = +1.0

               

                ½ MnO_2(s) + 2H⁺ + e^- = ½ Mn²⁺ + H₂O                        pe°_w = +9.8

 

                ¼ O_2(g) + H⁺ + e^- = ½ H₂O                                                pe°_w = +13.75

 

               

1. Write out the balanced whole reactions for the three possible reductant and oxidant pairs. Rank the reactions in order of thermodynamic favorability?

 

2. What will be the final concentration of O₂?

 

3. What is the expected change in alkalinity of the seawater/hydrothermal fluid mixture once the redox pairs have equilibrated?