The rusting of iron is catalysed by which of the following ?

Options

A. Fe
B. `O_2`
C. Zn
D. `H^+`

Answer

To solve the question regarding what catalyzes the rusting of iron, we can break down the process step by step.

### Step-by-Step Solution:

1. **Understanding Rusting**:
- Rusting of iron is an electrochemical process that occurs in the presence of moisture (water) and oxygen.

2. **Formation of Carbonic Acid**:
- When water (H2O) and carbon dioxide (CO2) from the air combine, they form carbonic acid (H2CO3).
- The reaction can be represented as:
\[ H2O + CO2 \rightarrow H2CO3 \]

3. **Anodic Reaction**:
- At the anode, carbonic acid dissociates into hydrogen ions (H+) and carbonate ions (CO3^2-).
- The iron (Fe) oxidizes to iron ions (Fe^2+) by losing electrons:
\[ H2CO3 \rightarrow 2H^+ + CO3^{2-} \]
\[ Fe \rightarrow Fe^{2+} + 2e^- \]

4. **Cathodic Reaction**:
- At the cathode, hydrogen ions (H+) gain electrons to form hydrogen gas (H2):
\[ 2H^+ + 2e^- \rightarrow H2 \]
- Additionally, oxygen reacts with hydrogen ions to form water:
\[ O2 + 4H^+ + 4e^- \rightarrow 2H2O \]

5. **Formation of Rust**:
- The iron ions (Fe^2+) can further react with oxygen and water to form rust (Fe2O3·nH2O).
- The overall rusting process can be summarized as:
\[ 4Fe + 3O2 + 6H2O \rightarrow 4Fe(OH)3 \rightarrow 2Fe2O3·3H2O \text{ (rust)} \]

6. **Identifying the Catalyst**:
- In this process, the presence of hydrogen ions (H+) is crucial as they facilitate the oxidation of iron. Thus, H+ ions act as a catalyst in the rusting of iron.

7. **Conclusion**:
- Therefore, the rusting of iron is catalyzed by hydrogen ions (H+).

### Final Answer:
The correct option is **H+ ions** (Option 4).

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