Which statement best describes the standard enthalpy of formation, ${\mathrm{\Delta H}}_f\circ$, of a substance?

• A. The enthalpy change for forming $1$ mol of the substance from any reactants at equilibrium under standard conditions
• B. The enthalpy change for decomposing $1$ mol of the substance into its elements in their gaseous states at any pressure
• C. The enthalpy change for forming $1$ mol of the substance from its constituent elements in their standard states at specified conditions (typically $1$ bar and $298$ K)
• D. The enthalpy change for burning $1$ mol of the substance completely in $O_2$ under standard conditions

Answer: (C)

Verified step by step guidance

1. Understand that the standard enthalpy of formation, denoted as \(\Delta H_{f}^{\circ}\), refers to the enthalpy change when exactly 1 mole of a compound is formed from its elements.
2. Recognize that the elements must be in their standard states, which means their most stable physical form at 1 bar pressure and a specified temperature (usually 298 K).
3. Note that the reaction forming the compound from its elements is carried out under standard conditions, typically 1 bar pressure and 298 K temperature.
4. Remember that the standard enthalpy of formation is defined for the formation process, not decomposition or combustion, so it specifically involves building the compound from its elements, not breaking it down or burning it.
5. Summarize that the correct description of \(\Delta H_{f}^{\circ}\) is the enthalpy change for forming 1 mole of the substance from its constituent elements in their standard states at specified conditions.