The bond energy of an `O -H` bond is 109 K. cal `"mole"^(-1)`. When a mole of water is formed,heat involved in kilocalories is

Options

A. 109 kcal is released
B. 218 kcal is absorbed
C. 109 kcal is absorbed
D. 218 kcal is released

Explanation

To solve the problem, we need to determine the heat involved when one mole of water is formed, given the bond energy of the O-H bond.

### Step-by-Step Solution:

1. **Understand Bond Energy**:
The bond energy of an O-H bond is given as 109 kilocalories per mole. This means that to break one mole of O-H bonds, 109 kilocalories of energy is required.

2. **Formation of Water**:
The chemical reaction for the formation of water from its elements is:
\[ \text{H}_2(g) + \frac{1}{2} \text{O}_2(g) \rightarrow \text{H}_2\text{O}(l) \]
In this reaction, two O-H bonds are formed when one mole of water is produced.

3. **Calculate Total Bond Energy for Two O-H Bonds**:
Since one mole of water contains two O-H bonds, the total bond energy for the formation of these bonds is:
\[ \text{Total Bond Energy} = 2 \times \text{Bond Energy of O-H} = 2 \times 109 \text{ kcal} = 218 \text{ kcal} \]

4. **Determine the Heat Released**:
The formation of water is an exothermic process, meaning that energy is released when the bonds are formed. Therefore, the heat involved when one mole of water is formed is:
\[ \text{Heat Released} = -218 \text{ kcal} \]
The negative sign indicates that this is an exothermic reaction.

5. **Final Answer**:
The heat involved in kilocalories when one mole of water is formed is \(-218 \text{ kcal}\).