Which of the following substances has a standard enthalpy of formation of zero at 298 K?

• A. CO2(g)
• B. N2O(g)
• C. H2O(l)
• D. O2(g)

Answer: (D)

Verified step by step guidance

1. Understand the definition of standard enthalpy of formation: it is 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.
3. Identify the substances given and determine if they are elements or compounds: CO2(g), N2O(g), and H2O(l) are compounds, while O2(g) is an element in its standard state.
4. Recognize that O2(g) is the elemental form of oxygen at 298 K and 1 atm, so its standard enthalpy of formation is zero by definition.
5. Conclude that among the options, only O2(g) has a standard enthalpy of formation of zero at 298 K.