Values of thermodynamic quantities are commonly expressed for standard state conditions, so it is a good idea to understand what the standard state conditions are.
A superscript circle is used to denote a thermodynamic quantity that is under standard state conditions:
ΔH = ΔH°
ΔS = ΔS°
ΔS = ΔS°
Standard State Conditions
Certain assumptions apply to standard state conditions. Standard temperature and pressure commonly is abbreviated as STP.
- The standard state temperature is 25°C (298 K). It is possible to calculate standard state values for other temperatures.
- All liquids are pure.
- The concentration of all solutions is 1 M (1 molar).
- All gases are pure.
- All gases are at 1 atm pressure.
- The energy of formation of an element in its normal state is defined as zero.
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