CHAP. 8: CHEMICAL EQUILIBRIUM [CONTENTS] 236
Note:If minimum is not reached inside the concentration interval, i.e. if the Gibbs energy
of the system monotonously decreases or increases with the extent of reaction , only the
products or only the reactants are present in equilibrium [see8.2.8and Figure8.1].8.2.5 Overview of standard states
The standard states of individual components may be chosen arbitrarily within certain limits.
The first limitation is that the standard temperature is always the temperature of the system.
Further on, the standard state of individual components of a system is usually chosen at fixed
composition. The standard pressure is either constant, or it changes with the system pressure.
Compare with6.5.3.- Standard state: the state of a pure substance in a given state of matter or modification
at the temperature of the system and a standard pressure.
S Symbols: For this standard state, the superscriptstis replaced with the superscript◦ifpst=
101.325 kPa.
Note: For gases, the superscript◦ is used to denote the standard state of the gaseous
component in the state of an ideal gas at the temperature of the system and a standard
pressure (pst= 101.325 kPa). The standard fugacity in this case equals the standard
pressure [see3.2.6].
- Standard state: the state of a pure substance in the same state of matter or modification
as in the mixture at the temperature and pressure of the system.
S Symbols:For this standard state, the superscriptstis replaced with the superscript•.
- Standard state: the state of a hypothetical pure substance at the temperature and
pressure of the system obtained by extrapolation from the behaviour of a component in
a mixture at infinite dilution.
S Symbols:For this standard state, the superscriptstis replaced with the superscript[x].
- Standard state: the state a given component would have in a hypothetical ideal mixture
whose properties would be extrapolated from infinite dilution at the temperature and
pressure of the system to the concentration 1 mol/dm^3 ;cst= 1 mol/dm^3.