Standard enthalpy of formation is zero for

Options

A. Diamond
B. `O_3(g)`
C. `Brbar`(aq)
D. `Cl_2(g)`

Answer

To determine which substance has a standard enthalpy of formation equal to zero, we need to understand the concept of standard enthalpy of formation (ΔH_f°). The standard enthalpy of formation of a compound is defined as the change in enthalpy when one mole of the compound is formed from its elements in their standard states.

**Step-by-step solution:**

1. **Identify the standard states of elements:**
- The standard state of an element is its most stable form at 1 atm pressure and a specified temperature (usually 25°C).
- For example:
- Carbon (C) is in the form of graphite (solid).
- Bromine (Br) is in the form of Br2 (liquid).
- Iodine (I) is in the form of solid iodine (I2).
- Chlorine (Cl) is in the form of Cl2 (gas).
- Oxygen (O) is in the form of O2 (gas).
- Sulfur (S) is in the form of rhombic sulfur (solid).

2. **Evaluate the options:**
- **Option 1: Diamond (C)** - This is an allotrope of carbon, and since the standard state of carbon is graphite, the standard enthalpy of formation for diamond is not zero.
- **Option 2: Ozone (O3)** - The standard state of oxygen is O2, so the formation of O3 from O2 does not have a ΔH_f° of zero.
- **Option 3: Bromide ion (Br-)** - The standard state of bromine is Br2 (liquid), and the formation of Br- from Br2 does not have a ΔH_f° of zero.
- **Option 4: Chlorine gas (Cl2)** - This is the standard state of chlorine, and thus the standard enthalpy of formation for Cl2 is zero.

3. **Conclusion:**
- Among the options, only Cl2 gas is in its standard state, and therefore, the standard enthalpy of formation for Cl2 is zero.

**Final Answer:**
The standard enthalpy of formation is zero for Cl2 (option 4).

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