In a displacement reaction ie Mg in CuSO4 the copper matal is deposited on the magnesium. Would their ever be a point where the Cu biuld up completly encases the Mg preventing the displacement reation from occuring after that point.
Logically, it would stop the reaction completely because since the reaction involves copper deposting onto Mg, if there is no copper left, no reaction would occur.
Though, I predict that it wouldn't prevent the reaction from occurring. The copper produced is pure copper, which is an excellent conduct of electricity, so I would imagine that electrons would continue being delivered to the Cu2+ ions.
Eventually the reaction will stop as the cell also relies on concentration of reactants....once the Mg concentration becomes too high the displacement reaction will no longer be favoured so it will stop.
morgan is completely right. However, presume the concentration of Cu2+ and Mg2+ always stay effectively unchanged, theroetically, there can be a point upon the deposition of copper metal can effectively cover the entirety of the magnesium surface.
Since oxidised magnesium need to be in solution, but the newly formed copper layer prevents the metal ion from going into the solution, no further displacement can occur.
Though in practice, this observation can be said to be very rare.