At the standard state conditions, the `DeltaH_f^(@)` of which halogen is taken to be zero ?

Options

A. `Cl_2` (s)
B. `I_2` (g)
C. `Br_2` (l)
D. `Br (g)`

Answer

To determine which halogen has a standard enthalpy of formation (ΔH_f^0) of zero at standard state conditions, we can follow these steps:

### Step-by-Step Solution:

1. **Understand Standard State Conditions**: The standard enthalpy of formation (ΔH_f^0) of an element in its standard state is defined as zero. The standard state refers to the physical state of a substance at 1 bar (or 1 atm) of pressure and a specified temperature (usually 25°C or 298 K).

2. **Identify the Halogens**: The halogens in question are chlorine (Cl), bromine (Br), and iodine (I). We need to determine their standard states:
- Chlorine (Cl) is a gas at room temperature.
- Bromine (Br) is a liquid at room temperature.
- Iodine (I) is a solid at room temperature.

3. **Determine the Standard States**:
- For chlorine (Cl), since it is a gas, its ΔH_f^0 is zero.
- For bromine (Br), as it is a liquid, its ΔH_f^0 is also zero.
- For iodine (I), being a solid, its ΔH_f^0 is zero.

4. **Conclusion**: All elements in their standard states have a ΔH_f^0 of zero. However, the question asks for the specific halogen that is commonly referenced in standard state conditions. In most contexts, the gaseous form of chlorine is often used as a reference point for halogens.

5. **Final Answer**: The halogen whose ΔH_f^0 is taken to be zero at standard state conditions is **chlorine (Cl)** in its gaseous state.