Question

For which of the following reactions is Δh°rxn equal to Δh°f of the product(s)?

1) so(g) + 1/2O₂(g) → SO₂(g)
2) 2Na(s) + Cl₂(g) → 2NaCl(s)
3) S(s) + O₂(g) → SO₂(g)
4) SO₃(g) → 1/2O₂(g)
5) SO₂(g) + Na(s) + 1/2Cl₂(g) → NaCl(s)
6) Na(s) + 1/2Cl₂(l) → NaCl(s)

Answer 1

The ΔH°rxn equals ΔH°f for reaction 2 (2Na(s) + Cl2(g) → 2NaCl(s)) and reaction 3 (S(s) + O2(g) → SO2(g)), as these are formation reactions from elements in their standard states.

Step-by-step explanation:

For which of the following reactions is ΔH°rxn equal to ΔH°f of the product(s)? Among the listed reactions, ΔH°rxn is equal to the ΔH°f for the formation of the products in the synthesis reactions where elements in their standard states combine to form a compound. This is because the enthalpy change of the reaction (ΔH°rxn) for the formation of a compound from its elements in their standard states is defined as the standard enthalpy of formation (ΔH°f) for that compound.

Option 2: The reaction 2Na(s) + Cl2(g) → 2NaCl(s) is the formation of sodium chloride from its constituent elements, sodium and chlorine, both in their standard states. Therefore, the ΔH°rxn is equivalent to the ΔH°f for NaCl(s).

Option 3: The reaction S(s) + O2(g) → SO2(g) is the formation of sulfur dioxide from its constituent elements, sulfur in its solid standard state and oxygen as a gas. Thus, the ΔH°rxn is equivalent to the ΔH°f for SO2(g).

Other reactions listed involve either decomposition, not just formation from standard states (such as option 4), formation of compounds from species that are not in their standard states, or reactions that are not formation reactions from elements (such as option 5).