Gases in Chemical Equilibria
Any species with a name ending with “(g)” is considered to be a gas in this software.
logK°-value: for an equilibrium between a gas phase and an aqueous
solution, it must be assumed either
- constant pressure, or
- constant volume.
The value of logK° will be different for these
two cases. For example,
CO2(g) + H2O
HCO3− + H+
For constant pressure:
logKp = log[HCO3−]
− pH − logPCO2
where P is in units of “bar”.
In the Earth’s atmosphere logPCO2
≈ −3.5
For constant volume:
logKv = log[HCO3−]
− pH − log[CO2(g)]
where the concentrations of gases are given in mol/L.
Using the eqn. for perfect gases (P V =
n R T) it is found that:
PCO2 =
[CO2(g)] 0.01 R T
The factor 0.01 is needed to convert the units m3 to L and Pa to bar.
Finally one obtains:
logKp = logKv
+ log(0.01 R T)
Important: DATABASE and SPANA are based on
logKp values for gases, with a standard state of 1 bar
(≈1 atm).
Note: in diagrams only aqueous species and solids are considered,
and the concentration of gases is set to zero.
For example, CO2(g) does not appear in
fraction or
logarithmic diagrams.
However, the fugacity (≈ partial pressure) of gases is calculated and
it is displayed in log activity diagrams.
