Ferric iodide is very unstable but ferric chloride is not

Answer

To understand why ferric iodide (FeI3) is very unstable while ferric chloride (FeCl3) is stable, we can break down the explanation into clear steps:

### Step-by-Step Solution:

1. **Identify the Compounds**: We are comparing ferric iodide (FeI3) and ferric chloride (FeCl3).

2. **Understand the Oxidation States**: In both compounds, iron (Fe) is in the +3 oxidation state (Fe^3+).

3. **Examine the Iodide Ion (I-)**: The iodide ion (I-) is known to be a strong reducing agent. This means it has a tendency to donate electrons easily.

4. **Reduction Reaction**: When I- acts as a reducing agent, it can reduce Fe^3+ to Fe^2+. The reaction can be represented as:
\[ \text{Fe}^{3+} + \text{I}^- \rightarrow \text{Fe}^{2+} + \text{I}_2 \]
This reaction indicates that FeI3 will not remain stable because the I- ions will reduce Fe^3+ to Fe^2+.

5. **Stability of Ferric Iodide**: Due to the strong reducing nature of I-, FeI3 is unstable and tends to decompose into FeI2 and iodine (I2).

6. **Examine the Chloride Ion (Cl-)**: In contrast, the chloride ion (Cl-) is a much weaker reducing agent compared to I-.

7. **Lack of Reduction**: The Cl- ion does not have the same ability to reduce Fe^3+ to Fe^2+. Therefore, the stability of FeCl3 is maintained because it does not undergo a similar reduction reaction.

8. **Conclusion**: The instability of ferric iodide (FeI3) is due to the strong reducing nature of the iodide ion, which reduces Fe^3+ to Fe^2+, leading to its decomposition. In contrast, ferric chloride (FeCl3) is stable because the chloride ion does not reduce Fe^3+.

### Summary:
- Ferric iodide (FeI3) is unstable because I- is a strong reducing agent that reduces Fe^3+ to Fe^2+.
- Ferric chloride (FeCl3) is stable because Cl- is a weak reducing agent and does not reduce Fe^3+.