tasapainovakio

Tasapainovakio, or the equilibrium constant, is a fundamental concept in chemical thermodynamics that describes the relative amounts of products and reactants present at chemical equilibrium for a given reaction and temperature. In Finnish, the term often appears in chemistry texts and education as tasapainovakio.

For a general reaction aA + bB ⇌ cC + dD, the equilibrium constant is defined as K_eq = (a_C^c a_D^d) / (a_A^a a_B^b), where a_i denotes the activity of species i. Activities account for interactions in the system; in dilute solutions they are approximated by concentrations. Thus K_c = [C]^c [D]^d / ([A]^a [B]^b) for solutions. In gas phase, the analogous expression uses partial pressures: K_p = (P_C^c P_D^d) / (P_A^a P_B^b). When activities are used, K_eq is dimensionless; using concentrations or pressures can introduce units unless standard states are applied (1 M or 1 bar).

The value of tasapainovakio depends on temperature, not on initial amounts. It relates to thermodynamics through ΔG° = -RT ln K_eq, where ΔG° is the standard Gibbs free energy change. The temperature dependence is described by the van’t Hoff relation: d ln K_eq / dT = ΔH°/(R T^2). A large K_eq (K_eq > 1) indicates products are favored at equilibrium; a small K_eq (K_eq < 1) indicates reactants are favored. In practice, tasapainovakio is determined experimentally and used to predict yields, optimize reaction conditions, and relate thermodynamics to kinetics and process design.