Which of the following changes requires a reducing agent ?

Options

A. `H_(3)AsO_(3)rarrHAsO_(4)^(2-)`
B. `BrO_(3)^(ө)rarrBrO^(ө)`
C. `CrO_(4)^(2-)rarrCr_(2)O_(7)^(2-)`
D. `Al(OH)_(3)rarr[Al[OH)_(4)]^(ө)`

Explanation

To determine which of the given changes requires a reducing agent, we need to analyze the oxidation states of the elements involved in each reaction. A reducing agent is required when a substance undergoes reduction, which is the gain of electrons or a decrease in oxidation state.

### Step-by-Step Solution:

1. **Analyze Reaction A: H3AsO3 to HAsO4^2-**
- Determine the oxidation state of As in H3AsO3:
- H: +1 (3 H = +3)
- O: -2 (3 O = -6)
- Let the oxidation state of As be x.
- Equation: x + 3 - 6 = 0 → x = +3
- Determine the oxidation state of As in HAsO4^2-:
- H: +1
- O: -2 (4 O = -8)
- Let the oxidation state of As be y.
- Equation: y + 1 - 8 = -2 → y = +5
- Change in oxidation state: +3 to +5 (oxidation occurs).
- **Conclusion**: An oxidizing agent is required, not a reducing agent.

2. **Analyze Reaction B: BrO3^- to BrO^-**
- Determine the oxidation state of Br in BrO3^-:
- O: -2 (3 O = -6)
- Let the oxidation state of Br be z.
- Equation: z - 6 = -1 → z = +5
- Determine the oxidation state of Br in BrO^-:
- O: -2
- Let the oxidation state of Br be w.
- Equation: w - 2 = -1 → w = +1
- Change in oxidation state: +5 to +1 (reduction occurs).
- **Conclusion**: A reducing agent is required.

3. **Analyze Reaction C: CrO4^2- to Cr2O7^2-**
- Determine the oxidation state of Cr in CrO4^2-:
- O: -2 (4 O = -8)
- Let the oxidation state of Cr be a.
- Equation: 2a - 8 = -2 → 2a = +6 → a = +3
- Determine the oxidation state of Cr in Cr2O7^2-:
- O: -2 (7 O = -14)
- Let the oxidation state of Cr be b.
- Equation: 2b - 14 = -2 → 2b = +12 → b = +6
- Change in oxidation state: +6 to +6 (no change).
- **Conclusion**: No reducing agent is required.

4. **Analyze Reaction D: Al(OH)3 to Al(OH)4^-**
- Determine the oxidation state of Al in Al(OH)3:
- O: -2 (3 O = -6)
- H: +1 (3 H = +3)
- Let the oxidation state of Al be c.
- Equation: c + 3 - 6 = 0 → c = +3
- Determine the oxidation state of Al in Al(OH)4^-:
- O: -2 (4 O = -8)
- H: +1 (4 H = +4)
- Let the oxidation state of Al be d.
- Equation: d + 4 - 8 = -1 → d = +3
- Change in oxidation state: +3 to +3 (no change).
- **Conclusion**: No reducing agent is required.

### Final Answer:
The change that requires a reducing agent is **BrO3^- to BrO^-**.