All chemical reactions are, in principle, reversible, i.e., they can go in both directions.
Chemical equilibrium exists when the two opposite reactions occur simultaneously at the same rate.
At equilibrium the reaction quotient Q becomes constant. It is not in equilibrium and will proceed in the forward direction. It is not in equilibrium and will proceed in the reverse direction. The mixture is in equilibrium and will not change. Suddenly the vessel size is compressed to half its volume.