Question

Which of the following equations has AfH° and ∆Η° same?

• A. H$_{2(g)}$ + $\frac{1}{2}$O$_{2(g)}$ →H$_{2}$O$_{(l)}$
• B. N$_{2(l)}$ + O$_{3(g)}$ →N$_{2}$O$_{3(g)}$
• C. 2CO$_{(g)}$ + O$_{2(g)}$ → 2CO$_{2(g)}$
• D. CH$_{4(g)}$ + 2Cl$_{2(g)}$ →CH$_{2}$Cl$_{2(g)}$ + 2HCl$_{(g)}$

Answer 1

Answer: (A)

H$_{2(g)}$ + $\frac{1}{2}$O$_{2(g)}$ →H$_{2}$O$_{(l)}$

Answer 2

For a reaction, if all reactants are in their standard elemental states, then the enthalpy change (ΔH°) of the reaction is the standard enthalpy of formation (Δ_fH°) of the product (or products) formed. In option (A):

$$\text{H}_2\,(g) + \frac{1}{2}\text{O}_2\,(g) \rightarrow \text{H}_2\text{O}\,(l)$$

Both hydrogen and oxygen are in their standard states. Therefore, the reaction directly represents the formation of water from its elemental constituents, and hence ΔH° is exactly the standard enthalpy of formation of water.

The other options do not have all reactants in their standard states (or the reaction isn’t the direct formation reaction for a compound), and thus the enthalpy change does not equate to the standard enthalpy of formation.