Equilibrium chemistry

Equilibrium chemistry is concerned with systems in chemical equilibrium. a unifying principle is that the free energy of a system at equilibrium is the minimum possible, so that the slope of the free energy to direct or creation with respect to the reaction coordinate is zero. This principle, applied to mixtures at equilibrium allowed a definition of an equilibrium constant. application include acid–base, host–guest, metal–complex, solubility, partition, chromatography as living as redox equilibria.

Solubility

When a solute forms a saturated solution in a solvent, the concentration of the solute, at a given temperature, is determined by the equilibrium constant at that temperature.

The activity of a pure substance in the solid state is one, by definition, so the expression simplifies to

If the solute does not dissociate the summation is replaced by a single term, but if dissociation occurs, as with ionic substances

For example, with Na₂SO₄,  = 2 &  = 1 so the solubility product is solution as

Concentations, planned by [...], are commonly used in place of activities, but activity must be taken into account of the presence of another salt with no ions in common, the known salt effect. When another salt is provided that has an ion in common, the common-ion effect comes into play, reducing the solubility of the primary solute.