For which one of the following reactions is ΔH°_rxn equal to the standard enthalpy of formation of the product?

• A. C(s) + 1/2 O2(g) → CO(g)
• B. CO2(g) → C(s) + O2(g)
• C. C(s) + O2(g) → CO2(g)
• D. 2CO(g) + O2(g) → 2CO2(g)

Answer: (C)

Verified step by step guidance

1. Recall the definition of the standard 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 standard states of the elements involved. For carbon, the standard state is solid graphite (C(s)), and for oxygen, it is O2 gas (O2(g)).
3. Look at each reaction and determine if it represents the formation of one mole of the product from its elements in their standard states.
4. Check the reaction: C(s) + O2(g) → CO2(g). This reaction forms one mole of CO2 from carbon in its standard state and oxygen in its standard state, matching the definition of ΔH°_f for CO2.
5. Conclude that the reaction for which ΔH°_rxn equals the standard enthalpy of formation of the product is the one where the product is formed directly from its elements in their standard states, which is C(s) + O2(g) → CO2(g).