which substance has $delta h_f$ defined as 0 kj/mol?\no $h_2o$ (s)\no $ne$ (l)\no $f_2$ (g)\no $co_2$ (g)

Explanation:

Step1: Recall definition of standard - enthalpy of formation

The standard enthalpy of formation ($\Delta H_f$) of an element in its standard state is defined as 0 kJ/mol.

Step2: Identify standard states of given substances

• $H_2O(s)$ is water in solid - state (ice). Water's standard state is $H_2O(l)$.
• $Ne(l)$: Neon's standard state is $Ne(g)$ (neon is a noble gas and is gaseous at standard conditions).
• $F_2(g)$: Fluorine is a di - atomic gas at standard conditions (25°C and 1 atm), so $F_2(g)$ is the standard state of fluorine.
• $CO_2(g)$ is a compound, not an element.

Answer:

C. $F_2(g)$