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For a pure substance, the most stable form of the substance at 1 atm and a specified temperature, usually 25°C: for a gas, 1 atm; for a solution, concentration of 1 molar.
standard enthalpy of formation
the enthalpy change that accompanies the formation of one mole of a compound at 25 C from its elements, with all substances in their standard states at that temperature
(thermodynamics) a thermodynamic quantity equal to the internal energy of a system plus the product of its volume and pressure
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