Specifying Standard States
For a gas, the standard state is 1 atm; ideal gas behavior is
assumed.
For a substance in aqueous solution, the standard state is
1 M concentration (1 mol/liter solution).
For a pure substance (element or compound), the standard state is usually the most stable form of the substance at 1 atm and the temperature of interest (usually 25 oC (298 K).
DHorxn = standard heat of reaction
(enthalpy change determined with all
substances in their standard states)
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