Question

select all of the following substances that you expect to have a non - zero standard enthalpy of formation at 25°c.

h₂(g)

na(s)

ch₄(g)

o(g)

Explanation:

• The standard enthalpy of formation ($\Delta H_f^{\circ}$) of an element in its standard state is zero.
• $H_2(g)$ is the standard state of hydrogen, and $Na(s)$ is the standard state of sodium. So, their $\Delta H_f^{\circ}=0$.
• $CH_4(g)$ is a compound, not an element in its standard state. So, its $\Delta H_f^{\circ}

eq0$.

• $O(g)$ is not the standard state of oxygen (the standard state of oxygen is $O_2(g)$). So, its $\Delta H_f^{\circ}

eq0$.

Answer:

• $\text{CH}_4(g)$
• $O(g)$