Standard State Conditions - Standard Temperature and Pressure

Know the Standard State Conditions
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 zer