Chemical Equilibria

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

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 = 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.