4. (20 points) Below is a pe-pH diagram for chromium in freshwater. a. If you we

Question

4. (20 points) Below is a pe-pH diagram for chromium in freshwater. a. If you were studying an alpine lake at pH-7, what form of dissolved Cr would you predict to be dominant in surface waters? Explain your reasoning, b. If the lake had a hypolimnion (deeper layer) that were anoxic, what form of dissolved Cr would you predict to be dominant? Explain your reasoning. c. What are the oxidation states ofthese two forms of Cr? Write a balanced half- reaction relating these two forms. d. Why are no Cr forms described for the regions labeled "A" and "B"? e. If the pe were 5, and the pH in the lake dropped from 7 to 4, would there be a redox change in the dominant Cr form? What if the pe were 15 and the same pH change occurred? 20 Cro. 15 Cro4 10 2- Cr(OH)2 CroH pe Cr(OH), -10 2 4 6 8 10 12 14 pH

Explanation / Answer

A-At pH =7 Cr is present as Cr(OH)2+ in surface water.Calculation of E for the conditions employed in this study(pH ~7) is complicated by the fact that the pKavalues of the CrVoxo anion are unknown .the relevent reaction is

Cr3+ + 2H2O---> Cr(OH)2+ +2H+

b-In dipper layer Cr is peresent as Cr(OH)3.the reaction is given as

Cr(OH)2+ + H2O--->Cr(OH)3 + H+

C-

Assuming chlorine functions as a two electron oxidant, CrVis produced. The redoxpotential for the CrV/CrIII half reaction in acidic solution is written as-

H3CrO4 (aq) + 2e- + 5H+---->Cr3+ +4H2O

Related Questions

Navigate

• Browse (All)
• Browse 4
• Subjects

---

---

Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.