`H(g) + O(g) rightarrow O - H(g), Delta H` for this reaction is

Options

A. Heat of formation of O - H
B. Bond energy of O - H
C. Heat of combustion of `H_(2)`
D. Zero at all temperatures

Answer

To determine the enthalpy change (ΔH) for the reaction \( H(g) + O(g) \rightarrow OH(g) \), we will analyze the formation of the hydroxyl radical (OH) from its constituent elements in the gaseous state.

### Step-by-Step Solution:

1. **Identify the Reaction**:
The reaction given is:
\[ H(g) + O(g) \rightarrow OH(g) \]
Here, hydrogen and oxygen in their gaseous forms combine to form the hydroxyl radical.

2. **Understand the Concept of ΔH**:
The enthalpy change (ΔH) for a reaction is the heat absorbed or released during the reaction at constant pressure. In this case, we are interested in the formation of the OH bond.

3. **Bond Formation**:
The formation of the OH bond releases energy. When bonds are formed, energy is released, which is reflected as a negative ΔH. This means that the reaction is exothermic.

4. **Relate to Bond Energy**:
The ΔH for the reaction can be related to the bond energy of the OH bond. The bond energy is the amount of energy required to break a bond, and when a bond is formed, the same amount of energy is released.

5. **Consider the Elements in Their Standard States**:
For the standard enthalpy of formation of OH, we would typically consider the elements in their standard states:
- Hydrogen in its standard state is \( \frac{1}{2} H_2 \)
- Oxygen in its standard state is \( \frac{1}{2} O_2 \)

However, since the reaction is given with \( H(g) \) and \( O(g) \), we are looking at the formation of OH from atomic hydrogen and oxygen, not from their diatomic forms.

6. **Conclusion**:
The ΔH for the reaction \( H(g) + O(g) \rightarrow OH(g) \) can be considered as the bond energy of the OH bond. Since energy is released when the bond is formed, ΔH will be negative.

### Final Answer:
\[ \Delta H \text{ for the reaction } H(g) + O(g) \rightarrow OH(g) \text{ is negative, representing the bond energy of the OH bond.} \]