Which of the following reactions doesn't represent the standard state enthalpy of formation reaction?

Options

A. `1/2 H_(2)(g)+1/2 Cl_(2)(g) to HCl(g)`
B. `Xe(g)+2F_(2)(g) to XeF_(4) (g)`
C. `1/2Cl_(2)(g)+3/2 F_(2)(g) to ClF_(3)(g)`
D. `Kr(g)+F_(2)(g) to KrF_(2)(s)`

Answer

To determine which of the following reactions does not represent the standard state enthalpy of formation reaction, we first need to understand what an enthalpy of formation reaction is.

### Step-by-Step Solution:

1. **Definition of Enthalpy of Formation**:
The standard state enthalpy of formation (ΔH_f°) is defined as the heat change that results when one mole of a compound is formed from its elements in their standard states (most stable form at 1 atm and 298 K).

2. **Identify the Reactions**:
We have four reactions (A, B, C, D) to analyze. We need to check each reaction to see if it meets the criteria for being an enthalpy of formation reaction.

3. **Analyze Reaction A**:
- Example: H2(g) + Cl2(g) → 2 HCl(g)
- Here, HCl is formed from its elements (H2 and Cl2) in their standard states (gaseous).
- **Conclusion**: This is a valid enthalpy of formation reaction.

4. **Analyze Reaction B**:
- Example: Xe(g) + 2 F2(g) → XeF4(s)
- Here, XeF4 is formed from Xenon (Xe) and Fluorine (F2). However, the product XeF4 is in solid state, while the reactants are in gaseous state.
- **Conclusion**: This does not represent a standard state enthalpy of formation reaction because the product should be in its standard state, which for XeF4 is solid, but the reaction shows it as a gas.

5. **Analyze Reaction C**:
- Example: Cl2(g) + 3 F2(g) → 2 ClF3(g)
- Here, ClF3 is formed from its elements (Cl2 and F2) in their standard states (gaseous).
- **Conclusion**: This is a valid enthalpy of formation reaction.

6. **Analyze Reaction D**:
- Example: Kr(g) + F2(g) → KrF2(g)
- Here, KrF2 is formed from its elements (Kr and F2) in their standard states (gaseous).
- **Conclusion**: This is a valid enthalpy of formation reaction.

7. **Final Conclusion**:
Based on the analysis, the reaction that does not represent the standard state enthalpy of formation is **Reaction B**.

### Answer:
**Option B** does not represent the standard state enthalpy of formation reaction.