Which species are left out from the Keq expression, and why?
Solids, and liquids (pure or nearly pure). They are not really left out. They are assigned the value of "1" (one). Example:
CaCO3(s) <=> CaO(s) + CO2(g)
Keq=[CO2](1) / (1)
Keq = [CO2]
Solids are omitted. By definition they are in their standard state and have a concentration of 1. Water is often omitted but not always.
Good. GK mentioned pure liquids.
The species that are left out from the equilibrium constant (Keq) expression are the species that are either pure solids (s) or pure liquids (l). This is because the concentration of pure solids and pure liquids is considered to be constant and does not change significantly during the course of a reaction.
In a chemical equilibrium, the equilibrium constant expression is written based on the concentrations of the reactants and products involved. The concentrations are usually expressed in terms of gases (g) or aqueous solutions (aq), as these are the species whose concentrations can vary during the reaction.
By excluding pure solids and pure liquids from the Keq expression, it simplifies the equilibrium expression and allows for a more accurate representation of the equilibrium concentrations of the reacting species. This way, only the significant changes in concentration of the gaseous or aqueous species are taken into account when calculating the equilibrium constant.
It's important to note that the Keq expression still includes all the gaseous and aqueous species involved in the chemical reaction, whether they are reactants or products. The pure solids and pure liquids are left out because their concentrations remain constant and they do not affect the equilibrium constant.