Observations and Interpretations
1) Balance the chromate / dichromate equilibrium equation:
CrO42-(aq) + H+(aq) Cr2O72-(aq) + H2O(l) (unbalanced equation)
2) What was the original color of the K2CrO4 solution? If the above equilibrium is established in the original solution, which species, CrO42-(aq), or Cr2O72-(aq) appears to be present in greatest amount?
3) What color did you observe after adding the 3M H2SO4 solution?
4) What color did you observe after adding the 1M NaOH solution?
5) Assume that the dynamic equilibrium represented in the equation in 1 is present in the original solution you placed in the test tube. Based on LeChatelier’s Principle, as stated above on page one, you will interpret the observations made in 2, above: (Notethat H2SO4 produces H+ in solution.)
a) When sulfuric acid was added, which reaction (forward or reverse) was favored? What evidence supports this? Why would this reaction be favored?
b) Assume that the system rapidly returns to equilibrium. At this new position of equilibrium, is there more or less Cr2O72-present after the addition of the acid? Based on this, what happened to the amount of CrO42-?
6) Based on LeChatelier’s Principle, as stated above, you will interpret the observations made in 3, above: Note: NaOH produces OH- in solution.
a) When sodium hydroxide was added, which reaction in 1 (forward or reverse) was favored? What evidence supports this? Why would this reaction be favored? (Hint: What does OH- do with H+?)
b) Assume that the system rapidly returns to equilibrium. After the addition of the NaOH, at this new position of equilibrium, is there more or less Cr2O72- present? Based on this, what happened to the amount of CrO42-?
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