The `Delta_(r )H^@` (enthalpy of formation) is positive in :

Options

A. `O_(3)(g)`
B. `NO(g)`
C. `Hl(g)`
D. All of these

Explanation

To determine which of the given compounds has a positive enthalpy of formation (ΔrH°), we need to analyze the formation reactions of each compound from their standard elemental forms. A positive ΔrH° indicates that the reaction is endothermic, meaning energy is absorbed during the formation of the compound.

### Step-by-Step Solution:

1. **Identify the Compounds**: The question asks about the enthalpy of formation for three compounds: O3 (ozone), NO (nitric oxide), and HI (hydrogen iodide).

2. **Write the Formation Reactions**:
- For O3:
\[ \frac{3}{2} O_2(g) \rightarrow O_3(g) \]
- For NO:
\[ \frac{1}{2} N_2(g) + \frac{1}{2} O_2(g) \rightarrow NO(g) \]
- For HI:
\[ H_2(g) + \frac{1}{2} I_2(s) \rightarrow HI(g) \]

3. **Analyze Each Reaction**:
- **O3 Formation**: O2 is more stable than O3, so energy must be supplied to form O3 from O2. Therefore, ΔrH° for O3 formation is positive (endothermic).
- **NO Formation**: NO is a highly reactive species, and forming it from N2 and O2 requires energy input. Thus, ΔrH° for NO formation is also positive (endothermic).
- **HI Formation**: The formation of HI from H2 and I2 involves sublimating solid I2 to gas and then dissociating it. This process requires significant energy, making ΔrH° for HI formation positive (endothermic).

4. **Conclusion**: Since all three reactions (O3, NO, and HI) have a positive enthalpy of formation, the answer is that ΔrH° is positive for all the given compounds.

### Final Answer:
The enthalpy of formation (ΔrH°) is positive for O3, NO, and HI.

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