For which of the following reactions does ΔH_{rxn} represent an enthalpy of formation?

• A. 2H_2(g) + O_2(g) → 2H_2O(l)
• B. N_2(g) + 3H_2(g) → 2NH_3(g)
• C. C(s) + O_2(g) → CO_2(g)
• D. CO_2(g) + H_2O(l) → H_2CO_3(aq)

Answer: (C)

Verified step by step guidance

1. Recall the definition of enthalpy of formation (ΔH_f): it is the enthalpy change when one mole of a compound is formed from its elements in their standard states under standard conditions.
2. Identify the reactants and products in each reaction and determine if the reactants are elements in their standard states and the product is a compound.
3. Check the first reaction: 2H_2(g) + O_2(g) → 2H_2O(l). Although H_2 and O_2 are elements in their standard states, the reaction forms 2 moles of H_2O, not 1 mole, so this is not a standard enthalpy of formation reaction as written.
4. Check the second reaction: N_2(g) + 3H_2(g) → 2NH_3(g). Again, elements in standard states form NH_3, but 2 moles of NH_3 are produced, so this is not the enthalpy of formation for 1 mole of NH_3.
5. Check the third reaction: C(s) + O_2(g) → CO_2(g). Carbon (graphite) and oxygen gas are elements in their standard states, and 1 mole of CO_2 is formed. This matches the definition of an enthalpy of formation reaction.