Final answer:
The thermodynamic equilibrium constant (K) for a reaction is determined by the ratio of activities of products to reactants at equilibrium, with Kc and Kp being its expressions in terms of concentration and partial pressures, respectively. Activities are used instead of concentration or pressure for non-ideal conditions.
Step-by-step explanation:
The thermodynamic equilibrium constant (K) is a dimensionless value that represents the ratio of product activities to reactant activities at equilibrium. For gas-phase reactions, the equilibrium constant may be expressed as Kc, using molar concentrations, or as Kp, using partial pressures. The relationship between Kc and Kp can be derived using the ideal gas law (PV = nRT).
In systems where non-ideal behavior is significant, such as gases at high pressures or concentrated solutions, activities or fugacities are used instead of concentration or pressure. This is because ideal assumptions no longer hold true under these conditions, and using activities allows for a more accurate representation of the equilibrium state.
Take the reaction CO2(g) + C(s) 2 CO(g) as an example. The equilibrium constant, expressed as Kp, is derived from the partial pressures of CO2 and CO. For solids and liquids, their activity is considered to be 1, as shown in cases like CaF2(s) where the equilibrium expression would be [CaF2]/[CaF2], thus simplifying to unity.