Collisions, the law of mass action and Kc

The size of Kc says how far a reversible reaction goes before equilibrium. If Kc is much greater than 1, the products on top of the fraction are large: the reaction goes almost to completion. If Kc is much smaller than 1, the mixture holds mostly reactants, because the backward reaction wins. If Kc is near 1, reactants and products are both present in comparable amounts. Hydrogen with chlorine to give hydrogen chloride is an example of a huge Kc; we use it only as a direction, never to calculate. Kc says nothing about speed: a reaction with a large Kc may still be slow. Concentrations of pure solids and pure liquids do not appear, because they stay practically constant however much solid there is: CaCO₃(s) ⇌ CaO(s) + CO₂(g) gives Kc = [CO₂]. In dilute water solutions the solvent water is left out too. At one temperature Kc has one value, whatever the starting concentrations; a new temperature gives a new Kc (extra). Dissolving a sparingly soluble solid is treated in week 14 with its own constant, Ksp, because the solid is left out.