Which of the following defines `DeltaH_(f)^@` ?

Options

A. `CO(g) + 1/2 O_(2)(g) rarr CO_(2)(g)`
B. `1/2 H_(2)(g) + 1/2 F_(2)(g) rarr HF (g)`
C. `N_(2)(g) + 2H_(2)(g) rarr 2NH_(3)(g)`
D. `C_("diamond") + O_(2)(g) rarr CO_(2)(g)`

Explanation

To determine which option defines the standard enthalpy of formation (ΔH_f^°), we need to understand the definition of ΔH_f^° clearly. The standard enthalpy of formation is the change in enthalpy when one mole of a compound is formed from its constituent elements in their standard states.

### Step-by-Step Solution:

1. **Understand the Definition**:
- The standard enthalpy of formation (ΔH_f^°) is defined as the enthalpy change when one mole of a compound is formed from its elements in their standard states.

2. **Identify Key Points**:
- Formation of **one mole** of a substance.
- The reactants must be **elements** in their **natural standard state**.

3. **Evaluate Each Option**:
- **Option 1**: Formation of CO2 from O2 and C.
- CO2 is formed from O2 (elemental state) and C (not in elemental state).
- **Conclusion**: This does not define ΔH_f^° (not correct).

- **Option 2**: Formation of HF from H2 and F2.
- HF is formed from H2 (elemental state) and F2 (elemental state).
- **Conclusion**: This correctly defines ΔH_f^° (correct).

- **Option 3**: Formation of 2 moles of NH3.
- This involves the formation of 2 moles, not 1 mole.
- **Conclusion**: This does not define ΔH_f^° (not correct).

- **Option 4**: Formation of carbon in diamond form.
- Diamond is not the standard state of carbon; graphite is.
- **Conclusion**: This does not define ΔH_f^° (not correct).

4. **Final Conclusion**:
- The only option that correctly defines the standard enthalpy of formation (ΔH_f^°) is **Option 2**.