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

2. **Identify the Key Criteria**:
- The reaction must produce exactly one mole of the compound.
- The reactants must be the elements in their standard states.

3. **Examine the Given Reactions**:
- **Reaction 1**: Produces 2 moles of HI.
- This does not meet the criteria since it does not produce one mole of product.
- **Reaction 2**: Also produces 2 moles of a substance.
- This again does not meet the criteria for the same reason.
- **Reaction 3**: Produces 1 mole of CH₄ from its elements (C in graphite and H₂ in gaseous state).
- This meets both criteria: it produces one mole of CH₄ and uses elements in their standard states.

4. **Conclusion**:
- The correct reaction that gives the enthalpy of formation directly is the one that produces 1 mole of CH₄ from its elements in their standard states.

### Final Answer:
The heat of the reaction that gives the enthalpy of formation directly is the reaction that forms 1 mole of CH₄ from carbon (graphite) and hydrogen (gas).

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