Which of the following reaction is possible at anode ?

Options

A. `Fe_(2)+2e^-)rarr2F^(-)`
B. `2H^++1/2O_2+2e^-)rarrH_2O`
C. `2Cr^(3+)+7H_2OrarrCr_2O_7^2+14H^++6e^-`
D. `Fe^(2+)rarrFe^(3+)+e^-`

Answer

To determine which of the given reactions is possible at the anode, we need to understand that oxidation occurs at the anode in an electrochemical cell. This means that the oxidation state of the species involved in the reaction must increase.

Let's analyze each option step by step:

### Step 1: Understand the concept of oxidation
- Oxidation is defined as the loss of electrons or an increase in oxidation state. Therefore, at the anode, we expect to see reactions where the oxidation state of the reactants increases.

### Step 2: Analyze the first reaction
- **Reaction 1:** Fe²⁺ → Fe⁰
- Here, the oxidation state of iron is changing from +2 (Fe²⁺) to 0 (Fe⁰).
- Since the oxidation state is decreasing (from +2 to 0), this reaction represents a reduction, not oxidation.
- **Conclusion:** This reaction does not occur at the anode.

### Step 3: Analyze the second reaction
- **Reaction 2:** 2H⁺ + 2e⁻ → H₂O
- In this reaction, hydrogen ions (H⁺) are being reduced to water (H₂O).
- The oxidation state of hydrogen in H⁺ is +1, and in H₂O, it remains +1 (but the overall oxidation state of the water molecule is neutral).
- Since the oxidation state does not increase, this reaction also represents a reduction.
- **Conclusion:** This reaction does not occur at the anode.

### Step 4: Analyze the third reaction
- **Reaction 3:** Cr₂O₇²⁻ → Cr³⁺
- The oxidation state of chromium in Cr₂O₇²⁻ can be calculated as follows:
- Let the oxidation state of Cr be X.
- The equation is: 2X + 7(-2) = -2
- This simplifies to: 2X - 14 = -2 → 2X = 12 → X = +6
- Therefore, in Cr₂O₇²⁻, the oxidation state of chromium is +6, and in Cr³⁺ it is +3.
- The oxidation state is decreasing from +6 to +3, indicating a reduction.
- **Conclusion:** This reaction does not occur at the anode.

### Step 5: Analyze the fourth reaction
- **Reaction 4:** Fe²⁺ → Fe³⁺
- Here, the oxidation state of iron is changing from +2 (Fe²⁺) to +3 (Fe³⁺).
- This indicates an increase in oxidation state, which is characteristic of oxidation.
- **Conclusion:** This reaction occurs at the anode.

### Final Conclusion
- The reactions that occur at the anode are those where oxidation takes place. Based on our analysis:
- **Reactions possible at the anode:** Fe²⁺ → Fe³⁺ (Option 4).
- **Other reactions (1, 2, and 3) do not occur at the anode.**