Gases in Chemical Equilibria
Any species with a name ending with (g) is considered to be a gas.
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]
where the concentrations are given in mol/L.
Using the eqn. for perfect gases (P V =
n R T) it is found that:
PCO2 =
[CO2] 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 also that only the aqueous and solid phases are considered when making
a diagram, and the concentration (in mol/L) for gases is set to zero.
For example, CO2 does not appear in
fraction or
logarithmic diagrams.
The fugacity (≈ partial pressure) of gases is calculated and
it is displayed in log activity diagrams.