In the reaction `2H_(2)(g) + O_(2)(g) rarr 2H_(2)O (l), " "Delta H = - xkJ`

Options

A. x kJ is the heat of formation of `H_2O`
B. ` x kJ` is the heat of reaction
C. x kJ is the heat of combusion of `H_2`
D. `x/2 kJ` is the heat of formation of `H_2O`

Explanation

To solve the question regarding the reaction \( 2H_2(g) + O_2(g) \rightarrow 2H_2O(l) \) with the enthalpy change represented as \( \Delta H = -x \) kJ, we will analyze each option provided in the context of thermochemistry.

### Step-by-Step Solution:

1. **Understanding the Reaction**:
- The reaction involves the combustion of hydrogen gas to form water. The enthalpy change \( \Delta H \) is negative, indicating that the reaction is exothermic (releases heat).

2. **Analyzing Option 1**:
- The first option states that \( x \) kJ is the heat of formation of \( H_2 \).
- **Heat of formation** refers to the heat change when one mole of a compound is formed from its elements in their standard states. Since \( H_2 \) is an element in its standard state, this option is incorrect.

3. **Analyzing Option 2**:
- The second option states that \( x \) kJ is the heat of reaction.
- Since the reaction releases \( -x \) kJ, this means that \( x \) kJ is indeed the heat of reaction for the formation of water from hydrogen and oxygen. This option is correct.

4. **Analyzing Option 3**:
- The third option states that \( x \) kJ is the heat of combustion of \( H_2 \).
- The heat of combustion refers to the heat released when one mole of a substance is completely burned in oxygen. In this reaction, 2 moles of \( H_2 \) are combusted, so \( x \) kJ does not represent the heat of combustion of \( H_2 \) alone. This option is incorrect.

5. **Analyzing Option 4**:
- The fourth option states that \( \frac{x}{2} \) kJ is the heat of formation of \( H_2 \).
- If we rewrite the reaction to form one mole of water, we have:
\[ H_2(g) + \frac{1}{2}O_2(g) \rightarrow H_2O(l) \]
- The enthalpy change for this reaction would be \( -\frac{x}{2} \) kJ, which corresponds to the heat of formation of one mole of \( H_2O \). This option is correct.

### Conclusion:
- The correct answers are:
- Option 2: \( x \) kJ is the heat of reaction.
- Option 4: \( \frac{x}{2} \) kJ is the heat of formation of \( H_2O \).