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 options represents the heat of formation (ΔH_f), we need to understand the definition of heat of formation. The heat of formation is the change in enthalpy when one mole of a compound is formed from its elements in their standard states.

### Step-by-Step Solution:

1. **Understand the Definition**:
- The 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 Elements**:
- For a compound to represent heat of formation, it must be formed from its elemental forms. For example, the elemental form of carbon can be graphite or diamond, hydrogen is H2, nitrogen is N2, and oxygen is O2.

3. **Analyze Each Option**:
- **Option 1: C (diamond) + O2 → CO2**
- This reaction shows the formation of carbon dioxide (CO2) from its elemental forms (carbon in the form of diamond and oxygen). Since one mole of CO2 is formed from its elements, this represents the heat of formation.
- **Option 2: CO + O2 → CO2**
- Here, CO is already a compound, not an element. Therefore, this does not represent the heat of formation.
- **Option 3: H2 + O2 → H2O**
- Although this reaction shows the formation of water from hydrogen and oxygen, it does not represent the heat of formation for H2O since it involves elemental hydrogen and oxygen, but does not specify one mole of H2O being formed from its elements.
- **Option 4: N2 + H2 → NH3**
- This reaction shows the formation of ammonia from nitrogen and hydrogen, which are both elements. However, it does not specify that it is for one mole of NH3, so it is not a clear representation of heat of formation.

4. **Conclusion**:
- Based on the analysis, the only option that correctly represents the heat of formation is **Option 1**: C (diamond) + O2 → CO2.

### Final Answer:
The correct option that represents the heat of formation (ΔH_f) is **Option 1: C (diamond) + O2 → CO2**.

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