What is dynamic equilibrium?

To illustrate dynamic equilibrium, let's take a look at this reaction:

#N_2(g) + 3H_2(g) rightleftharpoons 2NH_3(g)#

In this reaction, nitrogen gas and hydrogen gas are in a dynamic equilibrium with ammonia gas.

• When #N_2# and #H_2# are first placed into a reaction vessel, they will begin to react to form #NH_3#. The rate of the forward reaction, #N_2(g) + 3H_2(g) -> 2NH_3(g)#, is high.
• However, eventually, #NH_3# will start to reform #N_2# and #H_2#.
  The rate of the backward reaction, #2NH_3(g) -> N_2(g) + 3H_2(g)#, begins to rise.
• Eventually, the rates of the two reactions will be the same. Equilibrium has been reached.

We should remember, though, that this is a dynamic equilibrium!

This means that, although it may seem like nothing is happening (because the concentrations of the reactants and products essentially stay constant), #N_2# and #H_2# are still constantly forming #NH_3#. #NH_3# is also constantly reforming into #N_2# and #H_2#.
It's just that the rates at which they do this are the same.

So, although reactions are occurring, there is no net change.