Which of the following reactions represents the enthalpy of formation of water?

Options

A. `H^(+)(aq) + OH^(-)(aq) rarr H_(2)O(l)`
B. `H_(2)(g) + 1/2 O_(2)(g) rarr H_(2)O(l)`
C. `2H_(2)(g) + O_(2)(g) rarr 2H_(2)O(l)`
D. `2H^(+)(aq) + 2OH^(-)(aq) rarr 2H_(2)O(l)`

Answer

To determine which reaction represents the enthalpy of formation of water, we need to follow these steps:

### Step 1: Understand the Definition
The enthalpy of formation of a substance is defined as the heat change when one mole of that substance is formed from its elements in their standard states under specific conditions of temperature and pressure.

### Step 2: Identify the Elements of Water
Water (H₂O) is composed of two elements: hydrogen (H) and oxygen (O). In their standard states, hydrogen exists as diatomic gas (H₂) and oxygen also exists as diatomic gas (O₂).

### Step 3: Write the Formation Reaction
To write the formation reaction for water, we need to show how one mole of water is formed from its elements:
- We start with hydrogen gas (H₂) and oxygen gas (O₂).
- The balanced chemical equation for the formation of water is:
\[ \text{H}_2(g) + \frac{1}{2} \text{O}_2(g) \rightarrow \text{H}_2\text{O}(l) \]

### Step 4: Balance the Reaction
In the reaction, we have:
- 2 hydrogen atoms from H₂
- 1 oxygen atom from \(\frac{1}{2} O_2\)
This results in 2 hydrogen atoms and 1 oxygen atom, which combine to form 1 molecule of water (H₂O).

### Step 5: Identify the Correct Option
From the options provided, the correct reaction that represents the enthalpy of formation of water is:
\[ \text{H}_2(g) + \frac{1}{2} \text{O}_2(g) \rightarrow \text{H}_2\text{O}(l) \]
This matches the balanced equation we derived.

### Conclusion
Thus, the correct option is:
**Option 2: H₂(g) + ½ O₂(g) → H₂O(l)**

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