Which of the following substances has a standard enthalpy of formation (ΔH_f^∘) of zero at 298 K?

• A. O_2(g)
• B. H_2O(l)
• C. N_2O(g)
• D. CO_2(g)

Answer: (A)

Verified step by step guidance

1. Understand that the standard enthalpy of formation, \(\Delta H_f^\circ\), is defined as the enthalpy change when one mole of a compound is formed from its elements in their standard states at 298 K and 1 atm pressure.
2. Recall that the standard enthalpy of formation for any element in its most stable form under standard conditions is defined as zero. This is a convention used to provide a reference point for measuring enthalpy changes.
3. Identify the substances given: \(O_2(g)\), \(H_2O(l)\), \(N_2O(g)\), and \(CO_2(g)\). Determine which of these are elements in their standard states and which are compounds.
4. Recognize that \(O_2(g)\) and \(N_2(g)\) are elements in their standard gaseous diatomic forms, while \(H_2O(l)\) and \(CO_2(g)\) are compounds formed from these elements.
5. Conclude that the substance with a standard enthalpy of formation of zero at 298 K is the element in its standard state, which is \(O_2(g)\) in this list.