Which of the following represents redox reactions?

Options

A. `Cr_(2)O_(7)^(2-)+2overset(ө)OHrarr2CrO_(4)^(2-)+H_(2)O`
B. `SO_(3)^(2-)+H_(2)O+I_(2)rarrSO_(4)^(2-)+2I^(ө)+2H^(o+)`
C. `Ca(OH)_(2)+Cl_(2)rarrCa(Ocl)_(2)+CaCl_(2)`
D. `PCl_(5)rarrPCl_(3)+Cl_(2)`

Explanation

To determine which of the given reactions represent redox reactions, we need to analyze each option based on the changes in oxidation states of the elements involved. A redox reaction involves both oxidation (increase in oxidation state) and reduction (decrease in oxidation state) occurring simultaneously.

### Step-by-Step Solution:

1. **Identify the Reactions**: We have four reactions to analyze. Let's label them as Reaction 1, Reaction 2, Reaction 3, and Reaction 4.

2. **Analyze Reaction 1**:
- **Reaction**: Cr2O7^2- + 2 OH^- → 2 CrO4^2- + H2O
- **Oxidation States**:
- Chromium (Cr) in Cr2O7^2- is in +6 state.
- Chromium (Cr) in CrO4^2- is also in +6 state.
- **Conclusion**: There is no change in oxidation state. Therefore, this is **not a redox reaction**.

3. **Analyze Reaction 2**:
- **Reaction**: SO3^2- + H2O + I2 → SO4^2- + 2 I^- + 2 H^+
- **Oxidation States**:
- Sulfur (S) in SO3^2- is in +4 state and in SO4^2- is in +6 state (oxidation).
- Iodine (I) in I2 is in 0 state and in I^- is in -1 state (reduction).
- **Conclusion**: Both oxidation and reduction occur. Therefore, this is a **redox reaction**.

4. **Analyze Reaction 3**:
- **Reaction**: Ca(OH)2 + Cl2 → Ca(OCl)2 + CaCl2
- **Oxidation States**:
- Chlorine (Cl) in Cl2 is in 0 state and in Ca(OCl)2 is in -1 state (reduction).
- Chlorine (Cl) in CaCl2 is in +1 state (oxidation).
- **Conclusion**: Both oxidation and reduction occur. Therefore, this is a **redox reaction**.

5. **Analyze Reaction 4**:
- **Reaction**: PCl5 → PCl3 + Cl2
- **Oxidation States**:
- Phosphorus (P) in PCl5 is in +5 state and in PCl3 is in +3 state (reduction).
- Chlorine (Cl) in Cl2 is in 0 state and in PCl3 is in -1 state (oxidation).
- **Conclusion**: Both oxidation and reduction occur. Therefore, this is a **redox reaction**.

### Final Conclusion:
The reactions that represent redox reactions are:
- **Reaction 2**: SO3^2- + H2O + I2 → SO4^2- + 2 I^- + 2 H^+
- **Reaction 3**: Ca(OH)2 + Cl2 → Ca(OCl)2 + CaCl2
- **Reaction 4**: PCl5 → PCl3 + Cl2

Thus, the correct options are **2nd, 3rd, and 4th**.