Which of the following reactions is not correct ?

Options

A. `MnO_4^(-)+8H^(+)+5Fe^(2+)to5Fe^(3+)+Mn^(2+)+4H_2O`
B. `2MnO_2+4KOH+O_2to4KMnO_4+2H_2O`
C. `2Na_2CrO_4+2H^(+)toNa_2Cr_2O_7+2Na^(+)+H_2O`
D. `K_2Cr_2O_7+7H_2SO_4+6KIto4K_2SO_4+Cr_2(SO_4)_3+3I_2+7H_2O`

Explanation

To determine which of the following reactions is not correct, we will analyze each option step by step.

### Step 1: Analyze the First Reaction
**Reaction:**
\[ \text{MnO}_4^- + 8\text{H}^+ + 5\text{Fe}^{2+} \rightarrow 5\text{Fe}^{3+} + \text{Mn}^{2+} + 4\text{H}_2\text{O} \]

**Oxidation States:**
- In \(\text{MnO}_4^-\), the oxidation state of Mn is +7.
- In \(\text{Fe}^{2+}\), the oxidation state of Fe is +2, and in \(\text{Fe}^{3+}\), it is +3.
- In \(\text{Mn}^{2+}\), the oxidation state of Mn is +2.

**Analysis:**
- Mn is reduced from +7 to +2 (reduction).
- Fe is oxidized from +2 to +3 (oxidation).
- This is a redox reaction and is correct.

### Step 2: Analyze the Second Reaction
**Reaction:**
\[ 2\text{MnO}_2 + 4\text{KOH} + \text{O}_2 \rightarrow 4\text{KMnO}_4 + 2\text{H}_2\text{O} \]

**Oxidation States:**
- In \(\text{MnO}_2\), Mn is +4.
- In \(\text{KMnO}_4\), Mn is +7.
- In \(\text{O}_2\), the oxidation state of O is 0.

**Analysis:**
- Mn is oxidized from +4 to +7 (oxidation).
- O is reduced from 0 to -2 (reduction).
- This is a redox reaction and is correct.

### Step 3: Analyze the Third Reaction
**Reaction:**
\[ 2\text{Na}_2\text{CrO}_4 + 2\text{H}^+ \rightarrow \text{Na}_2\text{Cr}_2\text{O}_7 + 2\text{Na}^+ + \text{H}_2\text{O} \]

**Oxidation States:**
- In \(\text{Na}_2\text{CrO}_4\), Cr is +6.
- In \(\text{Na}_2\text{Cr}_2\text{O}_7\), Cr is also +6.

**Analysis:**
- There is no change in the oxidation state of Cr.
- This means there is no oxidation or reduction occurring.
- This reaction is not a redox reaction and is incorrect.

### Step 4: Analyze the Fourth Reaction
**Reaction:**
\[ \text{K}_2\text{Cr}_2\text{O}_7 + 7\text{H}_2\text{SO}_4 + 6\text{KI} \rightarrow 4\text{K}_2\text{SO}_4 + \text{Cr}_2\text{SO}_4 \cdot 3\text{SO}_4 + 3\text{I}_2 + 7\text{H}_2\text{O} \]

**Oxidation States:**
- In \(\text{K}_2\text{Cr}_2\text{O}_7\), Cr is +6.
- In \(\text{Cr}_2\text{SO}_4\), Cr is +3.
- In \(\text{KI}\), I is -1 and in \(\text{I}_2\), I is 0.

**Analysis:**
- Cr is reduced from +6 to +3 (reduction).
- I is oxidized from -1 to 0 (oxidation).
- This is a redox reaction and is correct.

### Conclusion
The incorrect reaction is the **third option**:
\[ 2\text{Na}_2\text{CrO}_4 + 2\text{H}^+ \rightarrow \text{Na}_2\text{Cr}_2\text{O}_7 + 2\text{Na}^+ + \text{H}_2\text{O} \]