Question

select which of the following standard enthalpy of formation values is not zero at 25°c:
check all that apply.
hcl(g)
cd(s)
ch₄(g)
mn(s)
br₂(l)
none of the above

Explanation:

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

• $\text{Cd}(s)$: Cadmium is a solid metal in its standard state, so $\Delta H_f^\circ = 0$.
• $\text{Mn}(s)$: Manganese is a solid metal in its standard state, so $\Delta H_f^\circ = 0$.
• $\text{Br}_2(l)$: Bromine is a liquid in its standard state (its elemental form at 25 °C is liquid $\text{Br}_2$), so $\Delta H_f^\circ = 0$.
• $\text{HCl}(g)$: Hydrochloric acid is a compound, not an element in its standard state, so its $\Delta H_f^\circ$ is not zero.
• $\text{CH}_4(g)$: Methane is a compound, not an element in its standard state, so its $\Delta H_f^\circ$ is not zero.

Answer:

A. $\text{HCl}(g)$, C. $\text{CH}_4(g)$