At `25^(@)` and 1 atm, which one(s) of the following has nonzero `Delta H^(@)._(f)` ?

Options

A. Fe
B. O
C. C(diamond)
D. Ne

Answer

To determine which of the given substances has a non-zero standard enthalpy of formation (ΔH°_f) at 25°C and 1 atm, we need to understand the concept of standard enthalpy of formation.

### Step-by-Step Solution:

1. **Understand the Definition of ΔH°_f**:
The standard enthalpy of formation (ΔH°_f) is defined as the change in enthalpy when one mole of a compound is formed from its constituent elements in their standard states.

2. **Identify Standard States**:
- The standard state of an element is its most stable form at 1 atm and 25°C.
- For example:
- Iron (Fe) is a solid at these conditions.
- Oxygen (O) exists as O₂ gas.
- Carbon (C) in its standard state is graphite, not diamond.
- Neon (Ne) is a noble gas and exists in its elemental form.

3. **Evaluate Each Substance**:
- **Iron (Fe)**: Since iron is in its standard state (solid), ΔH°_f(Fe) = 0.
- **Oxygen (O)**: Oxygen exists as O₂ in its standard state. Since O is not in its standard state, ΔH°_f(O) ≠ 0.
- **Carbon (C)**: Diamond is not the standard state for carbon; the standard state is graphite. Therefore, ΔH°_f(diamond) ≠ 0.
- **Neon (Ne)**: Neon is in its standard state (as a gas), so ΔH°_f(Ne) = 0.

4. **Conclusion**:
The substances with non-zero ΔH°_f are:
- Oxygen (O)
- Carbon (in diamond form)

### Final Answer:
The substances with non-zero ΔH°_f at 25°C and 1 atm are:
- Oxygen (O)
- Carbon (in diamond form)