Database  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:

logKp° = log [HCO3] − pH − log PCO2

P is in units of bar. In the Earth’s atmosphere log PCO2−3.5

For constant volume:

log Kv° = 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:

log Kp° = log Kv° + 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).

During the calculations to make a diagram, the concentration of a gas is not evaluated. The fugacity is calculated and 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