Which of the reactions defined `Delta_f H^@`?

Options

A. `C_("diamond") + O_2(g) to CO_2(g)`
B. `1/2H_2(g) + 1/2 F_2(g) to HF(g)`
C. `N_2(g) + 3H_2(g) to 2NH_3(g)`
D. `CO(g) + 1/2O_(2)(g) to CO_(2)(g)`

Answer

To determine which of the reactions defines the standard heat of formation (Δ_f H°), we need to understand the definition of standard heat of formation. It is the change in enthalpy when one mole of a compound is formed from its pure elements in their standard states.

Let's analyze the options step by step:

### Step 1: Understanding the Definition
- The standard heat of formation (Δ_f H°) is defined as the enthalpy change when one mole of a compound is formed from its constituent elements in their standard states.

### Step 2: Analyzing Each Option
- **Option A:** Check if one mole of a compound is formed from its elements.
- If the reaction produces more than one mole or does not involve pure elements, it cannot be the standard heat of formation.

- **Option B:** Check if one mole of a compound is formed from its elements.
- Ensure that the reactants are in their standard states (pure elements).

- **Option C:** Similar analysis as above.
- Check for the number of moles and the state of the reactants.

- **Option D:** Again, check the same criteria.
- Confirm if it produces one mole from pure elements.

### Step 3: Elimination of Incorrect Options
- Based on the analysis:
- If any option produces more than one mole of product or uses compounds instead of pure elements, eliminate that option.

### Step 4: Identifying the Correct Option
- After analyzing all options, the one that meets the criteria of producing one mole of a compound from pure elements in their standard states is the correct answer.

### Conclusion
- The correct option that defines Δ_f H° is **Option B**, as it satisfies all the conditions for standard heat of formation.

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