The species which has zero standard molar enthalpy of formation is

Options

A. `Br_(2)(g)`
B. `CH_(4)(g)`
C. `CO_(2)(g)`
D. `CI_(2)(g)`

Answer

To determine which species has a standard molar enthalpy of formation equal to zero, we need to follow these steps:

### Step 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 the substance is formed from its elements in their standard states at a specified temperature (usually 298 K) and pressure (1 bar).

### Step 2: Identify the Most Stable Form of Elements
The standard molar enthalpy of formation is zero for elements in their most stable form at standard conditions. This means we need to identify the most stable physical state of the elements listed in the options.

### Step 3: Analyze the Given Options
1. **Br2 (Bromine)**: At standard conditions, bromine exists as a liquid. Therefore, its ΔH_f° is not zero.
2. **CH4 (Methane)**: Methane is a compound formed from carbon and hydrogen. Its formation involves a change in enthalpy, so ΔH_f° is not zero.
3. **CO2 (Carbon Dioxide)**: Carbon dioxide is also a compound formed from carbon and oxygen. Its formation from its elements involves a change in enthalpy, so ΔH_f° is not zero.
4. **Cl2 (Chlorine)**: Chlorine gas is an element and is in its most stable form at standard conditions. Therefore, the ΔH_f° for Cl2 is zero.

### Step 4: Conclusion
Based on the analysis, the species that has a standard molar enthalpy of formation equal to zero is **Cl2 (Chlorine gas)**.

### Final Answer
The species which has zero standard molar enthalpy of formation is **Cl2 (Chlorine gas)**.

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