which substance has △hf defined as 0 kj/mol?\no h₂o (s)\no ne (l)\no f₂ (g)\no co₂ (g)

Answer:

C. $F_2(g)$

Explanation:

Step1: Recall standard - state definition

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

Step2: Identify standard - state of each substance

• $H_2O(s)$ is ice. Water's standard - state at 1 atm and 298 K is $H_2O(l)$, so $\Delta H_f\neq0$ for $H_2O(s)$.
• $Ne(l)$: Neon's standard - state at 1 atm and 298 K is $Ne(g)$, so $\Delta H_f\neq0$ for $Ne(l)$.
• $F_2(g)$: Fluorine exists as a gas in its standard state (diatomic gas at 1 atm and 298 K), so $\Delta H_f = 0$ for $F_2(g)$.
• $CO_2(g)$ is a compound, not an element in its standard state, so $\Delta H_f\neq0$ for $CO_2(g)$.