Question

select which of the following standard enthalpy of formation values is not zero at 25 °c:
check all that apply.

□ no (g)
□ s₈(rhombic)
□ h (g)
□ cd (s)
□ mno (s)
□ none of the above

Explanation:

1. Standard Enthalpy of Formation Concept: The standard enthalpy of formation ($\Delta H_f^\circ$) of an element in its most stable form at 25°C (298 K) and 1 atm is zero.
2. Analyzing Each Option:

• NO(g): Nitrogen monoxide is a compound (not an element in its stable form), so its $\Delta H_f^\circ

eq 0$.

• $S_8$(rhombic): Rhombic sulfur is the most stable form of sulfur at 25°C, so $\Delta H_f^\circ = 0$.
• H(g): The most stable form of hydrogen at 25°C is $H_2(g)$ (diatomic gas), so atomic H(g) is not the stable form, and $\Delta H_f^\circ

eq 0$.

• Cd(s): Cadmium is a metal, and its most stable form at 25°C is solid (Cd(s)), so $\Delta H_f^\circ = 0$.
• MnO(s): Manganese(II) oxide is a compound (not an element), so $\Delta H_f^\circ

eq 0$.

Answer:

A. NO(g), C. H(g), E. MnO(s)