Which statement correctly defines standard state conditions and standard enthalpy of formation?

• A. Standard state: 1 atm, 25°C; ΔHf° is the enthalpy change for formation of 1 mole of a compound from elements in their standard states.
• B. Standard state: 1 atm, 0°C; ΔHf° is energy to form bonds in a molecule.
• C. Standard state: 1 bar, 25°C; ΔHf° is energy to form 2 moles.
• D. Standard state: 1 atm, 100°C; ΔHf° is energy to ionize.

Answer: (A)

Standard state conditions are a reference set of conditions used in thermodynamics: pressure of 1 atmosphere and temperature of 25°C, with elements in their most stable forms at those conditions. The standard enthalpy of formation, ΔHf°, is defined as the enthalpy change when one mole of a compound is formed from its elements in their standard states under those same conditions. This makes the statement that describes 1 atm and 25°C and defines ΔHf° as the formation of one mole of a compound from elements in their standard states the correct choice. The other descriptions misstate either the temperature, the pressure, what ΔHf° represents (for example, bond formation energy or ionization energy), or the amount of substance formed (1 mole, not 2 moles). For example, ΔHf° for liquid water is the energy change when H2 and O2 in their standard states form one mole of H2O(l), which is a negative value, reflecting energy release.