The species which by definition has zero standard molar enthalpy of formation at 298 K is

Options

A. `Br _(2)(g)`
B. `CI_(2)(g)`
C. `H_(2)O(g)`
D. `CH_(4)(g)`

Explanation

To solve the question regarding which species has a standard molar enthalpy of formation of zero at 298 K, we can follow these steps:

### Step-by-Step Solution:

1. **Understand Standard Molar Enthalpy of Formation**:
The standard molar enthalpy of formation (ΔH_f°) of a substance is defined as the change in enthalpy when one mole of a compound is formed from its elements in their standard states. By definition, the standard molar enthalpy of formation for any element in its standard state is zero.

2. **Identify Elements in Their Standard States**:
At 298 K (25 °C), we need to identify which elements are in their standard states. For example:
- Gases: O2, N2, Cl2, etc.
- Liquids: Br2, Hg, etc.
- Solids: C (graphite), Fe, etc.

3. **Check the Given Options**:
The question does not provide specific options, but we can infer that common diatomic gases such as O2, N2, and Cl2 are candidates.

4. **Select the Correct Species**:
Among the diatomic gases, Cl2 is indeed a gas at room temperature and is in its standard state. Therefore, it has a standard molar enthalpy of formation of zero.

5. **Conclusion**:
Based on the definitions and the identification of the species, we conclude that the species which has a standard molar enthalpy of formation of zero at 298 K is Cl2 (chlorine gas).

### Final Answer:
The species which by definition has zero standard molar enthalpy of formation at 298 K is **Cl2 (chlorine gas)**.

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