Why Equilibrium Occurs

The molecules of water in a container travel about at random.  Occasionally some of them at the surface will have enough kinetic energy to overcome the attractive forces holding the molecules together in the liquid state.  They will escape into the surroundings.  Because the container is open, and they are moving in random directions, these molecules will likely move far enough away from the water's surface that they do not return to the liquid state.  Over time the water will evaporate from this open system, so the water level drops.

Suppose that water is added to replace what evaporates.  The level would no longer change.  If you did not know that the evaporated water was being replaced, you might think that the reaction had stopped.  Of course, the water molecules really keep on evaporating as they always have.  When an open system does not appear to change because those materials that react have been replaced, it has reached a steady state.

Now, let's see what happens if we stopper the container.  This makes our beaker of water into a closed system in which nothing can enter or leave.  Water molecules will continue to evaporate.  After all, water is not intelligent and does not know that the beaker has been sealed.  Sometimes a water vapor molecule, travelling at random, will return to the liquid state.  Eventually, just as many are condensing as are evaporating.  At this point, there will not appear to be any noticeable change in the beaker, and the system is said to have reached equilibrium.

Even with just a few molecules shown in the beaker, it is hard to see exactly what is happening, so let's look at a simplified representation.  Even after we stopper the beaker, molecules will evaporate from the liquid.  In fact, they always continue to evaporate at the same speed.  This is the rate of the reaction in the forward direction.  The vapor molecules cannot escape from the container though, so eventually there are enough that some condense back to liquid.  Condensation is called the reverse reaction, and at first its rate is low.

The more molecules of vapor there are, the greater the chances for a vapor molecule to change back to the liquid state.  At some point in time, there will be just as many molecules moving from liquid to vapor as there are from vapor to liquid.  Its a lot like walking one step forward, then one step back--you move a lot, but get nowhere!  In chemical terms, the rate of the forward and reverse reactions have become equal.

Of course, we can't see the molecules moving back and forth, so there will be no observable changes in the container.  In a closed system, we have reached an equilibrium.  The rate of the forward and reverse reactions are now the same.  Macroscopic properties--the water level in this case--are constant.  But, at the molecular level, equilibrium is a dynamic process.