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.