Question

Use the equation showing the dissociation of iron thiocyanate in water to answer the question.
FeSCN (aq) ⇄ Fe (aq) + SCN (aq)
In solution, FeSCN ions are red, Fe ions are pale yellow, and SCN ions are colorless.
Given that iron ions bind with hydroxide ions to produce solid Fe(OH) , which statement about the solution is correct?
(1 point)
2+ 3+ –
2+ 3+ –
3
Decreasing the pH will remove Fe ions and cause the solution to become more yellow.
3+
Increasing the pH will release Fe ions and cause the solution to become more yellow.
3+
Increasing the pH will remove Fe ions and cause the solution to become more red. 3+
Decreasing the pH will release Fe ions and cause the solution to become more red.

Answer 1

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:

1. **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.

2. **Color of Ions:**
- \(\text{FeSCN}^{2+}\) is red.
- \(\text{Fe}^{3+}\) is pale yellow.
- \(\text{SCN}^{-}\) is colorless.

3. **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 \(\text{FeSCN}^{2+}\).