In which reaction `Delta S` is positive :-

Options

A. `H_(2)O(l)rarr H_(2)O(s)`
B. `3O_(2)(g)rarr 2O_(3)(g)`
C. `H_(2)O(l)rarr H_(2)O(g)`
D. `N_(2)(g)+3H_(2)(g)rarr 2NH_(3)(g)`

Answer

To determine in which reaction the change in entropy (ΔS) is positive, we need to analyze each reaction based on the concept of entropy, which is a measure of disorder or randomness in a system. A positive ΔS indicates an increase in disorder, while a negative ΔS indicates a decrease in disorder.

### Step-by-Step Solution:

1. **Identify the Reactions**:
- Reaction 1: H2O (liquid) → H2O (solid)
- Reaction 2: 3 O2 (gaseous) → 2 O3 (gaseous)
- Reaction 3: H2O (liquid) → H2O (gaseous)
- Reaction 4: N2 (gaseous) + 3 H2 (gaseous) → NH3 (gaseous)

2. **Analyze Reaction 1**:
- In this reaction, liquid water is converted to solid ice.
- **Entropy Change**: The transition from liquid to solid represents a decrease in disorder (as solids are more ordered than liquids).
- **ΔS**: Negative.

3. **Analyze Reaction 2**:
- Here, 3 molecules of gaseous oxygen are converted to 2 molecules of ozone.
- **Entropy Change**: The number of gas molecules decreases, leading to a more ordered state.
- **ΔS**: Negative.

4. **Analyze Reaction 3**:
- In this reaction, liquid water is converted to water vapor (gas).
- **Entropy Change**: The transition from liquid to gas represents an increase in disorder (gases have more randomness than liquids).
- **ΔS**: Positive.

5. **Analyze Reaction 4**:
- This reaction involves the combination of nitrogen gas and hydrogen gas to form ammonia gas.
- **Entropy Change**: Two or more gaseous reactants combine to form a single gaseous product, which typically results in a decrease in disorder.
- **ΔS**: Negative.

6. **Conclusion**:
- Among the given reactions, only Reaction 3 (H2O liquid → H2O gaseous) shows a positive change in entropy (ΔS is positive).

### Final Answer:
The reaction in which ΔS is positive is **H2O (liquid) → H2O (gaseous)**.

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