Which statement about the standard enthalpy of formation (ΔH_f^ext{∘}) is true?

• A. ΔH_f^ext{∘} values are independent of temperature and pressure.
• B. ΔH_f^ext{∘} for any element in its most stable form is zero.
• C. ΔH_f^ext{∘} is the enthalpy change when one mole of a compound decomposes into its elements.
• D. ΔH_f^ext{∘} for all compounds is always positive.

Answer: (B)

Verified step by step guidance

1. Understand the definition of standard enthalpy of formation (\$\Delta H_f^{\circ}\$): it is the enthalpy change when one mole of a compound is formed from its elements in their standard states under standard conditions (usually 1 atm pressure and 25°C).
2. Recall that the standard enthalpy of formation for any element in its most stable form (its standard state) is defined as zero. This is a reference point used to calculate enthalpy changes for compounds.
3. Recognize that \$\Delta H_f^{\circ}\$ values can depend on temperature and pressure, but standard values are typically given at 25°C and 1 atm, so they are not strictly independent of these conditions.
4. Note that \$\Delta H_f^{\circ}\$ refers to formation (making a compound from elements), not decomposition. The enthalpy change for decomposition would be the reverse process and have the opposite sign.
5. Understand that \$\Delta H_f^{\circ}\$ values for compounds can be either positive or negative depending on whether the formation reaction is endothermic or exothermic, so it is not always positive.