To understand the effect of pH on the equilibrium of the iron thiocyanate reaction and the color of the solution, let's analyze the relevant components:
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Dissociation Reaction:
- \( \text{FeSCN}^{2+} (aq) \rightleftharpoons \text{Fe}^{3+} (aq) + \text{SCN}^{-} (aq) \)
- The solution is red due to the presence of \(\text{FeSCN}^{2+}\) ions.
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Color of Ions:
- \(\text{FeSCN}^{2+}\) is red.
- \(\text{Fe}^{3+}\) is pale yellow.
- \(\text{SCN}^{-}\) is colorless.
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Impact of pH:
- As the pH decreases (i.e., becomes more acidic), \(\text{Fe}^{3+}\) ions can react with hydroxide ions, leading to the formation of solid \(\text{Fe(OH)}_3\). This would reduce the concentration of \(\text{Fe}^{3+}\) in solution and potentially shift the equilibrium to the right, producing more \(\text{FeSCN}^{2+}\) ions, hence the solution would appear more red.
- As the pH increases (i.e., becomes more basic), the formation of \(\text{Fe(OH)}_3\) consumes \(\text{Fe}^{3+}\), which would decrease the concentration of \(\text{FeSCN}^{2+}\) as well (shifting the equilibrium to the left), leading to a yellowing of the solution.
Given this analysis, the correct statement is:
Decreasing the pH will release Fe ions and cause the solution to become more red.
The choice indicates that decreasing the pH removes \(\text{Fe}^{3+}\) ions (by precipitation as \(\text{Fe(OH)}_3\)), shifting the equilibrium to produce more red \(\text{FeSCN}^{2+}\).
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