The enthalpy change for the reaction 2C (graphite) + 3H2(g) → C2H6 (g) is called:

Question

Accepted Answer

The enthalpy change for the reaction 2C (graphite) + 3H2(g) → C2H6 (g) is called the "Enthalpy of formation." Let's explore why Option 1 is correct and why the other options are not applicable in this context.

1. **Enthalpy of formation (Correct Answer):**
   - The enthalpy of formation refers to the change in enthalpy when one mole of a compound is formed from its elements in their standard states. In this reaction, ethane (C2H6) is being formed from its constituent elements, carbon (in the form of graphite) and hydrogen gas (H2), both of which are in their standard states. Therefore, the enthalpy change associated with this reaction is indeed the enthalpy of formation of ethane.

2. **Enthalpy of hydrogenation:**
   - Enthalpy of hydrogenation is the enthalpy change when hydrogen is added to a compound, typically involving the conversion of unsaturated compounds (like alkenes or alkynes) to saturated compounds (like alkanes). In this reaction, we are not adding hydrogen to an unsaturated compound; instead, we are forming a compound from its elements. Thus, this option is incorrect.

3. **Enthalpy of combustion:**
   - Enthalpy of combustion refers to the enthalpy change when one mole of a substance is completely burned in oxygen. The given reaction does not involve combustion or burning in oxygen; it involves the formation of ethane from carbon and hydrogen. Therefore, this option is not applicable.

4. **Enthalpy of vaporization:**
   - Enthalpy of vaporization is the enthalpy change required to convert a substance from a liquid to a gas at its boiling point. The reaction in question does not involve a phase change from liquid to gas; it involves the formation of a gaseous compound from its elements. Hence, this option is incorrect.

In conclusion, the reaction 2C (graphite) + 3H2(g) → C2H6 (g) is best described by the enthalpy of formation, as it involves the formation of a compound from its elemental constituents in their standard states. This supports Option 1 as the correct answer.