Choose the reaction, for which the standard enthalpy of reaction is equal to the standard enthalpy of formation:

Options

A. `2H_(2)(g) + O_(2)(g) rarr 2H_(2)O (l)`
B. `2C(g) +3H_(2) (g) rarr C_(2)H_(6)(g)`
C. `1/8 S_(8)(s) + O_(2)(g) rarr SO_(2)(g)`
D. `3/2 O_(2)(g) rarr O_(3)(g)`

Explanation

To determine which reaction has a standard enthalpy of reaction equal to the standard enthalpy of formation, we need to understand the definitions involved:

1. **Standard Enthalpy of Formation (ΔH_f°)**: This is the heat change that results when one mole of a compound is formed from its elements in their standard states at 1 atm pressure and a specified temperature (usually 25°C or 298 K).

2. **Standard Enthalpy of Reaction (ΔH°)**: This is the heat change associated with a given reaction under standard conditions.

### Step-by-Step Solution:

**Step 1: Identify the requirements for standard enthalpy of formation.**
- The reaction must produce one mole of a compound.
- The compound must be formed from its constituent elements in their standard states.

**Step 2: Analyze the options provided.**
- **Option 1**: Produces 2 moles of H2O. This does not meet the requirement of forming one mole of a substance. **(Incorrect)**

- **Option 2**: Produces 1 mole of C2H6 from carbon and hydrogen. However, carbon must be in its standard state (graphite), and here it is assumed to be in a different form. **(Incorrect)**

- **Option 3**: Produces 1 mole of SO2 from sulfur (solid, standard state) and oxygen (gas, standard state). This meets both criteria. **(Correct)**

- **Option 4**: Produces 1 mole of O3 from oxygen (gas, standard state). This also meets both criteria. **(Correct)**

**Step 3: Conclusion**
- The reactions that have a standard enthalpy of reaction equal to the standard enthalpy of formation are options 3 (SO2) and 4 (O3).

### Final Answer:
**Options 3 and 4 are correct.**