Which reaction involves neither oxidation nor reduction?

Options

A. `CrO_(4)^(2-) rarr Cr_(2)O_(7)^(2-)`
B. `Cr rarr CrCl_(3)`
C. `KMnO_(4) rarr MnO_(2)`
D. `Fe rarr Fe_(2)O_(3)*xH_(2)O`

Answer

To determine which reaction involves neither oxidation nor reduction, we need to analyze the oxidation states of the elements involved in the reactions. Oxidation is defined as an increase in oxidation state (loss of electrons), while reduction is a decrease in oxidation state (gain of electrons).

Let's break down the solution step by step:

### Step 1: Identify the Reactions
We need to identify the reactions provided in the options and analyze each one for changes in oxidation states.

### Step 2: Calculate Oxidation States
1. **For CrO4^2- (Chromate ion)**:
- Let the oxidation state of Cr be \( X \).
- The oxidation state of oxygen is -2.
- The equation becomes:
\[ X + 4(-2) = -2 \implies X - 8 = -2 \implies X = +6 \]

2. **For Cr2O7^2- (Dichromate ion)**:
- Let the oxidation state of Cr be \( X \).
- The equation becomes:
\[ 2X + 7(-2) = -2 \implies 2X - 14 = -2 \implies 2X = 12 \implies X = +6 \]

### Step 3: Compare Oxidation States
- In both CrO4^2- and Cr2O7^2-, the oxidation state of chromium is +6. Since there is no change in the oxidation state, this reaction does not involve oxidation or reduction.

### Step 4: Analyze Other Options
1. **For Cr (0) to CrCl3 (+3)**:
- The oxidation state changes from 0 to +3 (oxidation occurs).

2. **For KMnO4 (+7) to MnO2 (+4)**:
- The oxidation state changes from +7 to +4 (reduction occurs).

3. **For Fe2O3 (+3) to Fe (0)**:
- The oxidation state changes from +3 to 0 (reduction occurs).

### Conclusion
The only reaction that does not involve a change in oxidation state is the conversion of CrO4^2- to Cr2O7^2-. Therefore, the answer is:

**Option A: CrO4^2- to Cr2O7^2- involves neither oxidation nor reduction.**

---