Which of these species has a standard enthalpy of formation equal to zero

Options

A. `F_(2)(g)`
B. F(g)
C. HF(aq)
D. `F^(-)`(aq)

Answer

To determine which species has a standard enthalpy of formation equal to zero, we need to understand the concept of standard enthalpy of formation.

### Step-by-Step Solution:

1. **Understanding Standard Enthalpy of Formation**:
The standard enthalpy of formation (ΔH°f) of a substance is defined as the change in enthalpy when one mole of the compound is formed from its elements in their standard states. For elements in their standard states, the standard enthalpy of formation is defined to be zero.

2. **Identifying Standard States**:
The standard state of an element is its most stable form at 1 bar (or 1 atm) pressure and a specified temperature (usually 25°C). Common examples include:
- Carbon (C) in the form of graphite
- Oxygen (O2) as a gas
- Hydrogen (H2) as a gas
- Sulfur (S) in the form of S8
- Phosphorus (P) in the form of P4

3. **Evaluating the Options**:
Now, we need to evaluate the given options to see which one is an element in its standard state:
- **Option 1: F2** - Fluorine gas is in its standard state, so its standard enthalpy of formation is zero.
- **Option 2: F** - Fluorine in any other form is not in its standard state, so it does not have a ΔH°f of zero.
- **Option 3: HF** - Hydrogen fluoride is a compound formed from H2 and F2, so it does not have a ΔH°f of zero.
- **Option 4: F-** - Fluoride ion is not an element in its standard state, so it does not have a ΔH°f of zero.

4. **Conclusion**:
Among the options provided, the only species that has a standard enthalpy of formation equal to zero is **F2** (fluorine gas).

### Final Answer:
**F2 (fluorine gas)** has a standard enthalpy of formation equal to zero.

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