DATABASE / Databases / Create databases / logK-files

Reactions with Gases:

Any species with a name ending with “(g)” is considered to be a gas.

The fugacity for a gas is equivalent to the activity for an aqueous species.

logK°-value: for an equilibrium between a gas phase and an aqueous solution, it must be assumed either 1) constant pressure or 2) constant volume. The value of logK° will be different for these two cases. Example:

CO₂(g) + H₂O HCO₃⁻ + H⁺

For constant pressure:

logK_p° = log [HCO₃⁻] − pH − log P_CO2

P is in units of bar. In the Earth’s atmosphere log P_CO2≈−3.5

For constant volume:

log K_v° = 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:

P_CO2 = [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:

log K_p° = log K_v° + log (0.01 R T)

Important: DATABASE and SPANA are based on logK_p° values for gases, with a standard state of 1 bar (≈1 atm).

Note: in diagrams the concentration of gases is not evaluated. For example, CO₂(g) does not appear in fraction or logarithmic diagrams. The fugacity (≈ partial pressure) of gases is calculated and it is displayed in log activity diagrams.

Example of a redox gas reaction:

2 H₂O 4 H⁺ + 4 e⁻ + O₂(g)

line in database:

O2(g);-83.12;;;H+;-4;;;e-;-4;H2O;2;;;;;-4;Wateq4F