Chemical Equilibrium

(and the Law of Mass Action)

Chemical reactions occur starting with initial concentrations of reactants and creating products in the course of time. The speed with which this occurs is called the rate of the reaction. This speed, or rate, is related to the concentrations of species. As the chemical reaction proceeds the product molecules react (in the example above they dissociate or break apart) to reform the reactant molecules. When the two speeds are equal the system has established equilibruim. That is, when the forward rate = the reverse rate equilibrium has occured.

The Law of Mass Action

The Law of Mass Action simply states that if the concentration of a species is increased (or decreased) the equilibrium will shift to maintain a constant ratio of concentrations. In our example, the forward rate might we written

Rate_f = k_f[C][O₂]

and the reverse rate as

Rate_r = k_r[CO₂]

Equilibrium is when Rate_f = Rate_r, or

k_f[C][O₂] = k_r[CO₂].

This may be rearranged to what is called an equilibrium constant expression

K = ( k_f/k_r )[CO₂]/([C][O₂]).

If the concentration of [C], for example, is increased then the concentrations [CO₂] must increase by some amount and the concentration of [O₂] must decrease by some amount in order for the ratio of concentrations to remain constant = K.

This chemical principle means that we can understand how equilibria shift on changes in concentrations of chemical species.