At standard temperature and pressure (STP), which of the following substances has a standard enthalpy of formation (ΔH_f^ext{∘}) defined as 0 kJ/mol?

• A. Ne (l)
• B. H_2O (s)
• C. CO_2 (g)
• D. F_2 (g)

Answer: (D)

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 1 bar pressure and a specified temperature (usually 25°C or 298 K).
2. Recall that by convention, the standard enthalpy of formation of any element in its most stable form under standard conditions is defined as zero. This means that for elements in their standard states, \(\Delta H_f^{\circ} = 0\) kJ/mol.
3. Identify the substances given and determine which are elements and which are compounds. For example, \(\mathrm{Ne (l)}\) is neon in liquid form (an element but not in its standard state), \(\mathrm{H_2O (s)}\) is solid water (a compound), \(\mathrm{CO_2 (g)}\) is carbon dioxide gas (a compound), and \(\mathrm{F_2 (g)}\) is fluorine gas (an element in its standard state).
4. Recognize that the standard state of neon is the gaseous form, not liquid, so \(\mathrm{Ne (l)}\) does not have \(\Delta H_f^{\circ} = 0\). Similarly, \(\mathrm{H_2O (s)}\) and \(\mathrm{CO_2 (g)}\) are compounds, so their \(\Delta H_f^{\circ}\) values are not zero.
5. Conclude that \(\mathrm{F_2 (g)}\), being an element in its standard state (diatomic fluorine gas at 1 bar and 25°C), has a standard enthalpy of formation defined as zero, \(\Delta H_f^{\circ} = 0\) kJ/mol.