Below is a pe-pH diagram for chromium in freshwater. If you were studying an alp

Question

Below is a pe-pH diagram for chromium in freshwater. 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. 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. What are the oxidation states of these two forms of Cr? Write a balanced half-reaction relating these two forms. Why are no Cr forms described for the regions labeled "A" and "B"? 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?

Explanation / Answer

Cr(OH)3 + 3H+ + 3e- Cr + 3 H2O               (in acidic condition)
Cr(OH)4- + 3e- Cr + 4 OH-                          (in basic condition)

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