In the equilibrium expression for a reaction, which species are included?

In the equilibrium expression for a reaction, which species are included?

• A. All species are included regardless of phase
• B. Only solids and liquids are included
• C. Only aqueous or gaseous species are included
• D. Only intermediates are included

Answer: (C)

In equilibrium expressions, you only include species whose activities can change with the reaction conditions. Pure solids and pure liquids have activities that are effectively constant and are taken as 1, so they do not appear in the expression. Gases and aqueous species, on the other hand, have activities that vary with pressure or concentration, and those are what determine the equilibrium constant. So the expression includes only species in the aqueous or gaseous phase (and uses concentrations for aqueous species or partial pressures for gases). This is why the correct understanding is that only aqueous or gaseous species are included.

In equilibrium expressions, you only include species whose activities can change with the reaction conditions. Pure solids and pure liquids have activities that are effectively constant and are taken as 1, so they do not appear in the expression. Gases and aqueous species, on the other hand, have activities that vary with pressure or concentration, and those are what determine the equilibrium constant. So the expression includes only species in the aqueous or gaseous phase (and uses concentrations for aqueous species or partial pressures for gases). This is why the correct understanding is that only aqueous or gaseous species are included.