Question

For which one of the following reactions is Δh = Δh°f?

1) N₂(g) + 3 H₂(g) → 2 NH₃(g)
2) 2 c(s) + 2 h(g) → C₂H₂(g)
3) 3 fe(s) + O₂(g) → fe3O₂(s)
4) 2 KCl(aq) + O₂(g) → 2 KClO₃(aq)
5) 2 H₂(g) + O₂(g) → 2 H₂O(l)

Answer 1

The reaction where ΔH is equal to ΔH°f is the formation of water from hydrogen gas and oxygen gas: 2 H₂(g) + O₂(g) → 2 H₂O(l).

Step-by-step explanation:

The reaction for which ΔH is equal to the standard heat of formation, ΔH°f, is when a compound is formed directly from its elements in their standard states. For example, the formation of water from hydrogen gas and oxygen gas: 2 H₂(g) + O₂(g) → 2 H₂O(l). In this reaction, the reactants are in their standard states (H₂ and O₂ as gases), and water is being formed. Therefore, the enthalpy change for this reaction (ΔH) directly represents the standard heat of formation (ΔH°f) of liquid water.