To analyze the given situation, we need to consider how pH changes affect the equilibrium of the dissociation of iron thiocyanate (FeSCN²⁺) and the behavior of iron ions in solution.
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Dissociation Reaction: \[ \text{FeSCN}^{2+} (aq) \rightleftharpoons \text{Fe}^{3+} (aq) + \text{SCN}^- (aq) \]
- FeSCN²⁺ is red.
- Fe³⁺ is pale yellow.
- SCN⁻ is colorless.
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Effect of pH on Fe³⁺:
- When you decrease the pH (make it more acidic), you increase the concentration of H⁺ ions in solution.
- Fe³⁺ ions tend to form insoluble Fe(OH)₃ in basic or neutral conditions. As a result, lower pH (more acidic conditions) can prevent precipitation and keep Fe³⁺ in solution.
- Conversely, increasing the pH would promote the formation of insoluble Fe(OH)₃, which would lead to decreased concentration of Fe³⁺ ions as they precipitate out of solution.
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Color Change:
- If there are more Fe³⁺ ions in the solution, the color leans towards pale yellow. If FeSCN²⁺ dominates (due to lower Fe³⁺ concentration), the solution appears more red.
From this information, we can analyze the given response options:
- Response 1: Decreasing the pH will remove Fe³⁺ ions and cause the solution to become more yellow. (Incorrect — decreasing pH does not remove Fe³⁺; it helps keep it in solution.)
- Response 2: Decreasing the pH will remove Fe³⁺ ions and cause the solution to become more yellow. (Incorrect for the same reasons as above.)
- Response 3: Increasing the pH will remove Fe³⁺ ions and cause the solution to become more red. (Correct — as pH increases, Fe³⁺ will precipitate as Fe(OH)₃, reducing the concentration of Fe³⁺ and allowing FeSCN²⁺ to dominate, leading to a red solution.)
- Response 4: Increasing the pH will remove Fe³⁺ ions and cause the solution to become more red. (Correct by similar logic as Response 3; this statement is true.)
- Response 5: Decreasing the pH will release Fe³⁺ ions and cause the solution to become more red. (Incorrect — decreasing pH does not release Fe³⁺.)
- Response 6: Decreasing the pH will release Fe³⁺ ions and cause the solution to become more red. (Incorrect for the same reason as Response 5.)
- Response 7: Increasing the pH will release Fe³⁺ ions and cause the solution to become more yellow. (Incorrect — increasing pH decreases Fe³⁺ concentration, which would lead to a red solution due to dominance of FeSCN²⁺.)
Final Conclusion: The correct statement about the solution is: Increasing the pH will remove Fe³⁺ ions and cause the solution to become more red.