Which of the following can not act as both oxidising and reducing agent ?

Options

A. `H_3PO_4`
B. `H_2 O_2`
C. `HNO_2`
D. `H_2SO_3`

Answer

To determine which of the following compounds cannot act as both an oxidizing and reducing agent, we need to analyze the oxidation states of the elements in each compound.

### Step-by-Step Solution:

1. **Understand Oxidizing and Reducing Agents**:
- An **oxidizing agent** gains electrons and gets reduced, while a **reducing agent** loses electrons and gets oxidized. A substance that can act as both is called an amphoteric agent.

2. **Analyze Each Compound**:
- **H3PO4 (Phosphoric acid)**:
- Oxidation state of phosphorus (P) can be calculated:
- H contributes +1 (3 H = +3)
- O contributes -2 (4 O = -8)
- Overall charge = 0
- Therefore, P = +5.
- Since phosphorus in H3PO4 is at its maximum oxidation state (+5), it cannot act as a reducing agent (cannot lose more electrons). Thus, it can only act as an oxidizing agent.

- **H2O2 (Hydrogen peroxide)**:
- Oxidation state of oxygen (O):
- H contributes +1 (2 H = +2)
- O contributes -2 (2 O = -4)
- Overall charge = 0
- Therefore, O = -1.
- Oxygen in H2O2 can be oxidized to 0 (by losing electrons) and can also be reduced to -2 (by gaining electrons). Hence, it can act as both.

- **HNO2 (Nitrous acid)**:
- Oxidation state of nitrogen (N):
- H contributes +1 (1 H = +1)
- O contributes -2 (2 O = -4)
- Overall charge = 0
- Therefore, N = +3.
- Nitrogen can be oxidized to +5 and reduced to 0. Hence, it can act as both.

- **H2SO3 (Sulfurous acid)**:
- Oxidation state of sulfur (S):
- H contributes +1 (2 H = +2)
- O contributes -2 (3 O = -6)
- Overall charge = 0
- Therefore, S = +4.
- Sulfur can be oxidized to +6 and reduced to 0. Hence, it can act as both.

3. **Conclusion**:
- Among the compounds analyzed, **H3PO4** is the only one that cannot act as both an oxidizing and reducing agent because phosphorus is already in its maximum oxidation state of +5.

### Final Answer:
**H3PO4 cannot act as both an oxidizing and reducing agent.**