Heat of which of the following reaction gives enthalpy of formation directly?

Options

A. `H_(2) + I_(2)(g) rightarrow 2HI_(g)`
B. `N_(2)(G) + 3H_(2)(g) rightarrow 2NH_(3)(g)`
C. `C_(graphite) + 2H _(2)(g) rightarrow CH_(4)(g)`
D. All of these

Answer

To determine which reaction gives the enthalpy of formation directly, we need to understand the definition of enthalpy of formation. The enthalpy of formation (\( \Delta H_f^\circ \)) is defined as the heat change that occurs when one mole of a compound is formed from its elements in their standard states.

### Step-by-Step Solution:

1. **Identify the Definition**:
The enthalpy of formation is the heat change when one mole of a compound is formed from its elements in their standard states.

2. **Consider the Elements in Standard States**:
For a compound, we need to consider the elements that make up the compound in their most stable form at standard conditions (1 atm and 25°C). For example, for glucose (\( C_6H_{12}O_6 \)), the elements are carbon (solid graphite), hydrogen (gas), and oxygen (gas).

3. **Write the Formation Reaction**:
The formation reaction for glucose can be written as:
\[ 6C_{(s)} + 6H_2(g) + 3O_2(g) \rightarrow C_6H_{12}O_6(s) \]
This reaction shows the formation of one mole of glucose from its elements in their standard states.

4. **Identify the Correct Reaction**:
To find the reaction that gives the enthalpy of formation directly, we need to look for a reaction that represents the formation of one mole of a compound from its elements in their standard states.

5. **Conclusion**:
The reaction that directly gives the enthalpy of formation is the one that shows the formation of the compound from its elemental forms. Therefore, if you have multiple reactions to choose from, select the one that represents this formation process.