Which of the following substances has a standard enthalpy of formation equal to zero under standard conditions?

• A. CO_2(g)
• B. CH_4(g)
• C. O_2(g)
• D. H_2O(l)

Answer: (C)

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 under standard conditions (usually 1 atm and 25°C).
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 reference point for measuring enthalpy changes.
3. Identify the substances given: CO_2(g), CH_4(g), O_2(g), and H_2O(l). Determine which of these are elements in their standard states.
4. Recognize that O_2(g) is an element in its standard state (oxygen gas at 1 atm and 25°C), so its standard enthalpy of formation is zero by definition.
5. Note that CO_2(g), CH_4(g), and H_2O(l) are compounds formed from elements, so their standard enthalpies of formation are not zero but have specific values measured experimentally.