action expression involves different powers of concentration or pressure depending on the
stoichiometry. However, the equilibrium constant is always dimensionless, with the understanding
that we are sticking to the units of molars and atmospheres for concentrations and pressures,
respectively.
Finally, to emphasize the equilibrium nature of a reaction, we often use two arrows, pointing in
opposite directions, to separate the reactants and the products. The reaction above could therefore
be written as an equilibrium of the form:
aA + bB cC + dD