Which of the following does not represent heat of formation `(Delta H_f^@)` ?

Options

A. `C_("graphite") + O_(2)(g) to CO_(2)(g)`
B. `H_(2)(g) + 1/2O_(2)(g) to H_(2)O(l)`
C. `2CO(g) + O_(2)(g) to 2CO_(2)(g)`
D. All of these

Explanation

To determine which of the given reactions does not represent the heat of formation (ΔH_f°), we need to understand the definition of heat of formation. The heat of formation is defined as the heat change that results 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 heat of formation (ΔH_f°) refers to the heat change when one mole of a compound is formed from its elements in their standard states (at 1 atm pressure and a specified temperature, usually 25°C or 298 K).

2. **Identify the Reactions**:
- We need to analyze the provided reactions to see if they fit the criteria for heat of formation.

3. **Examine Each Reaction**:
- **Reaction A**: Formation of CO2 from carbon (graphite) and oxygen (O2).
- This reaction represents the formation of CO2 from its elements in their standard states. Thus, it represents ΔH_f°.

- **Reaction B**: Formation of CO2 from CO and O2.
- In this reaction, CO is not an element; it is a compound. Therefore, this reaction does not represent the heat of formation of CO2. This reaction is not valid for ΔH_f°.

- **Reaction C**: Formation of H2O from H2 and half O2.
- This reaction represents the formation of water from its elements (H2 and O2) in their standard states. Thus, it represents ΔH_f°.

- **Reaction D**: Formation of H2O from H2 and O2.
- This reaction also represents the formation of water from its elements in their standard states. Thus, it represents ΔH_f°.

4. **Conclusion**:
- The only reaction that does not represent the heat of formation is **Reaction B**, where CO is used instead of its constituent elements.

### Final Answer:
**The reaction that does not represent heat of formation (ΔH_f°) is Reaction B.**

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