A superscript degree symbol follows the thermodynamic symbol to indicate standard state of one atmosphere. If the temperature is not the standard SATP temperature, 298 K, we indicate the temperature with a subscript. Examples: AHO means change of enthalpy at 298 K and 1 atm AH273 means change of enthalpy at 273 K and 1 atm 0 If a reaction indicated an enthalpy change of AH300 = 220 kJ it would mean O 220 kJ is the value for enthalpy change when the reaction occurs at 1 atm and 300 C O 220 kJ is the value for enthalpy change when the reaction occurs. at 1 atm and 300 K O 220 kJ is the value for enthalpy change when the reaction occurs at 0 atm and 300 K