Which one of the following reactions is not a redox reaction ?

Options

A. `H^(+)+OH^(-)toH_(2)O`
B. `Zn+2HCl toZnCl_(2)+H_(2)`
C. `(1)/(2)H_(2)+(1)/(2)Cl_(2)toHCl`
D. `Zn+(1)/(2)O_(2)toZnO`

Answer

To determine which reaction is not a redox reaction, we need to analyze each of the provided equations for changes in oxidation states. A redox reaction involves both oxidation (loss of electrons) and reduction (gain of electrons). Let's go through the reactions step by step.

### Step-by-Step Solution:

1. **Understanding Redox Reactions:**
- Redox reactions are characterized by the transfer of electrons between species, resulting in changes in oxidation states.
- Oxidation is defined as the loss of electrons (increase in oxidation state), while reduction is the gain of electrons (decrease in oxidation state).

2. **Analyzing the First Reaction:**
- The first reaction involves the formation of water from hydrogen ions (H⁺) and hydroxide ions (OH⁻).
- **Oxidation States:**
- H⁺: +1
- OH⁻: O = -2, H = +1 (overall charge = -1)
- H₂O: O = -2, H = +1 (overall charge = 0)
- **Change in Oxidation States:**
- H⁺ remains +1, and O in OH⁻ remains -2.
- There is no change in oxidation states; hence, no oxidation or reduction occurs.
- **Conclusion:** This reaction is **not a redox reaction**.

3. **Analyzing the Second Reaction (Zn + HCl → ZnCl₂ + H₂):**
- **Oxidation States:**
- Zn (elemental): 0
- H⁺ in HCl: +1, Cl⁻: -1
- ZnCl₂: Zn = +2, Cl = -1 (two Cl atoms)
- H₂: 0 (elemental state)
- **Change in Oxidation States:**
- Zn goes from 0 to +2 (oxidation).
- H⁺ goes from +1 to 0 (reduction).
- **Conclusion:** This reaction is a **redox reaction**.

4. **Analyzing the Third Reaction (H₂ + Cl₂ → HCl):**
- **Oxidation States:**
- H₂: 0 (elemental state)
- Cl₂: 0 (elemental state)
- HCl: H = +1, Cl = -1
- **Change in Oxidation States:**
- H goes from 0 to +1 (oxidation).
- Cl goes from 0 to -1 (reduction).
- **Conclusion:** This reaction is a **redox reaction**.

5. **Analyzing the Fourth Reaction (Zn + O₂ → ZnO):**
- **Oxidation States:**
- Zn: 0 (elemental state)
- O₂: 0 (elemental state)
- ZnO: Zn = +2, O = -2
- **Change in Oxidation States:**
- Zn goes from 0 to +2 (oxidation).
- O goes from 0 to -2 (reduction).
- **Conclusion:** This reaction is a **redox reaction**.

### Final Conclusion:
The only reaction that is **not a redox reaction** is the first one (formation of water from H⁺ and OH⁻).

### Answer:
**The reaction that is not a redox reaction is the first one.**