Question

select which of the following standard enthalpy of formation values is not zero at 25°c: check all that apply. f₂(g) be(s) na(s) ch₄(g) cu(s) none of the above

Explanation:

The standard enthalpy of formation ($\Delta H_f^\circ$) of an element in its standard state (the most stable state at 1 atm and 25 °C) is zero.

• $\ce{F2(g)}$: Fluorine is a diatomic gas in its standard state, so $\Delta H_f^\circ = 0$.
• $\ce{Be(s)}$: Beryllium is a solid in its standard state, so $\Delta H_f^\circ = 0$.
• $\ce{Na(s)}$: Sodium is a solid in its standard state, so $\Delta H_f^\circ = 0$.
• $\ce{CH4(g)}$: Methane is a compound (not an element in its standard state), so its standard enthalpy of formation is not zero.
• $\ce{Cu(s)}$: Copper is a solid in its standard state, so $\Delta H_f^\circ = 0$.

Answer:

$\ce{CH4(g)}$ (the checkbox next to $\ce{CH4(g)}$ should be checked)