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(d) Is the groundwater sample described in part (a) at equilibrium with respect to transformations between CrO4? and Cr2O7²-? If not, which species of Cr(VI) will increase in concentration as the water proceeds towards equilibrium? (The groundwater has ionic strength of 10 mM and pH 8). (e) What is the answer to part (d) if the groundwater temperature was 8°C? Explain your answer by showing how the temperature will change the value for the equilibrium constant K.

(d) Is the groundwater sample described in part (a) at equilibrium with respect to transformations between CrO4? and Cr2O7²-? If not, which species of Cr(VI) will increase in concentration as the water proceeds towards equilibrium? (The groundwater has ionic strength of 10 mM and pH 8). (e) What is the answer to part (d) if the groundwater temperature was 8°C? Explain your answer by showing how the temperature will change the value for the equilibrium constant K.

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ISBN:9781305957404
Author:Steven S. Zumdahl, Susan A. Zumdahl, Donald J. DeCoste
Publisher:Steven S. Zumdahl, Susan A. Zumdahl, Donald J. DeCoste
Chapter1: Chemical Foundations
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I am not sure how to calculate part d and part e.

What does it mean by "as the water proceeds towards equilibrium?" How am I supposed to know which chemical species concentration is increased? Thank you.

1) Hexavalent chromium, Cr(VI), is a metal that is used for metal plating, wood preservation and corrosion inhibition. The improper disposal of industrial wastes in recent decades has led to Cr(VI) contamination in subsurface soils. At the pH of natural waters, Cr(VI) can exist in two major forms: the chromate anion CrO42- and the dimer compound Cr2O7². (a) A groundwater sample contains 10 µM CrO4? and 10 µM Cr2O,² at pH 8, 25°C. What is the total molar concentration of chromium (Cr) in the water sample? (b) Write a balanced chemical equation describing the transformation from CrO4² to the dimer compound Cr20,². Choose the reactants and products of your equation from the table below: Species (kJ/mol) -727.9 (kJ/mol) CrO42- Cr20,7- -881.1 Note: you do not necessarily need all species listed in the table. -1301 -1490 H2O H* -237.18 -285.83 ОН -157.3 -230 (c) Calculate the standard state Gibbs free-energy change, AG°r and equilibrium constant K for your reaction in part (b). (d) Is the groundwater sample described in part (a) at equilibrium with respect to transformations between CrO4? and Cr2O72-? If not, which species of Cr(VI) will increase in concentration as the water proceeds towards equilibrium? (The groundwater has ionic strength of 10 mM and pH 8). (e) What is the answer to part (d) if the groundwater temperature was 8°C? Explain your answer by showing how the temperature will change the value for the equilibrium constant К.
Transcribed Image Text:1) Hexavalent chromium, Cr(VI), is a metal that is used for metal plating, wood preservation and corrosion inhibition. The improper disposal of industrial wastes in recent decades has led to Cr(VI) contamination in subsurface soils. At the pH of natural waters, Cr(VI) can exist in two major forms: the chromate anion CrO42- and the dimer compound Cr2O7². (a) A groundwater sample contains 10 µM CrO4? and 10 µM Cr2O,² at pH 8, 25°C. What is the total molar concentration of chromium (Cr) in the water sample? (b) Write a balanced chemical equation describing the transformation from CrO4² to the dimer compound Cr20,². Choose the reactants and products of your equation from the table below: Species (kJ/mol) -727.9 (kJ/mol) CrO42- Cr20,7- -881.1 Note: you do not necessarily need all species listed in the table. -1301 -1490 H2O H* -237.18 -285.83 ОН -157.3 -230 (c) Calculate the standard state Gibbs free-energy change, AG°r and equilibrium constant K for your reaction in part (b). (d) Is the groundwater sample described in part (a) at equilibrium with respect to transformations between CrO4? and Cr2O72-? If not, which species of Cr(VI) will increase in concentration as the water proceeds towards equilibrium? (The groundwater has ionic strength of 10 mM and pH 8). (e) What is the answer to part (d) if the groundwater temperature was 8°C? Explain your answer by showing how the temperature will change the value for the equilibrium constant К.
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