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)

Brief Explanations:

The standard enthalpy of formation ($\Delta H_f$) is defined as 0 kJ/mol for an element in its standard state at 25°C and 1 - atm pressure. Fluorine ($F_2$) is a gas in its standard state. Water ($H_2O(s)$) is a compound, neon ($Ne(l)$) is not in its standard state (Ne is a gas at standard - state conditions), and carbon dioxide ($CO_2(g)$) is a compound.

Answer:

C. $F_2$(g)