To study the kinetics of this reaction prepare a series of mixtures of CO and NO 2 and measure the reaction rate immediately after mixing the reactants. The units of k varies for each particular expression. When a reaction is complete, the concentrations of the reactants and products do not change, but molecules continue to react. Reactant molecules, R, continue to form product molecules, P, and product molecules revert back to reactant molecules. This dynamic steady state is called equilibrium .
Reactants are in the denominator raised to their stoichiometric coefficient.
The partial pressures or concentrations are raised to the power of the stoichiometric coefficient for the balanced reaction. By definition, the concentration of pure solids, liquids, and solvent is one in an equilibrium constant expression. This definition and example is for the ideal case where molar concentration and activity are equal. In general, the K expression is written in terms of activity.