All
reactions are essentially reversible,
but some lie so far to either extreme (only
reactants or products seem to be present)
that they seem to go to
completion in one direction only.
For
example, the reaction below favors the products
HCl (aq) + H2O (l) → Cl- (aq) + H3O+ (aq)
so much so that the concentration of HCl molecules in solution is essentially negligible.