Which of the following represents heat of formation `(Delta H_(f))` ?

Options

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

Answer

To determine which of the given reactions represents the heat of formation (ΔH_f), we need to understand the definition of standard heat of formation. The standard heat of formation is defined as the enthalpy change that occurs 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 heat of formation (ΔH_f) refers to the enthalpy change when one mole of a compound is formed from its elements in their standard states.

2. **Identify the Reactions**: We need to analyze the provided options to see which one fits the definition of ΔH_f.

3. **Analyze Each Option**:
- **Option 1**: C (diamond) + O₂ → CO₂
- This reaction produces CO₂, but it starts with carbon in the form of diamond. The standard state of carbon is graphite, not diamond. Therefore, this does not represent ΔH_f.

- **Option 2**: 2 CO₂ → (not applicable as it does not form one mole of a compound)
- This reaction does not form one mole of a substance, so it cannot represent ΔH_f.

- **Option 3**: H₂ + F₂ → HF
- This reaction forms one mole of HF from its constituent elements (H₂ and F₂) in their standard states. This matches the definition of ΔH_f.

- **Option 4**: N₂ + 3 H₂ → 2 NH₃
- This reaction produces two moles of NH₃, which does not fit the requirement of forming one mole of a compound. Thus, it cannot represent ΔH_f.

4. **Conclusion**: The only option that correctly represents the heat of formation is **Option 3: H₂ + F₂ → HF**.

### Final Answer:
The correct representation of heat of formation (ΔH_f) is given by **Option 3: H₂ + F₂ → HF**.