Standard state
The standard state of a chemical compound is the state in which one meets it under the pressure of 1 bar. Generally the temperature of reference of 298 K (25 °C) is selected to constitute thermodynamic tables of sizes.
The standard state corresponds to the pure Substance in its stable physical status at the temperature T, under the standard pressure: p0 = 1 bar = 105 Pa. There are thus no a standard state but standard states at each temperature T for which the pure substance is stable.
- For example, the standard state of water is the liquid state at temperatures ranging between 0° (C) and 100° (C), under the standard pressure.
If the pure substance is gas, one chooses the Perfect gas, if it is solid it is the the allotropic variety most stable in a crystalline state perfect.
Attention: in certain old tables, the data refer to the normal pressure of 1 atm = 1,01325.105 Pa. This pressure is not any more the standard pressure, even if the difference is tiny.
Table of the standard states of the principal chemical compounds with T 298K.
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