A displacement reaction is a reaction: in which an element reacts with a compound,displacing an element from it. Since these reactions involve an element and one of its compoud,these must be oxidaion-reduction reaction.For example:
Cu (s) + 2AgNO3 (aq) -------------> Cu(NO3)2 (aq) + 2Ag (s)
From this molecular equation is appears that copper displaces silver in silver nitrate,to produce silver metal and a greenish-blue solution of copper (II) nitrate.However, the net ionic equation shows that the reaction involves the transfer of electrons from copper metal to silver ion:
Cu (s) + 2Ag + (aq) -----------> Cu^2+ (aq) + 2Ag (s)
So, the whole point is whether a reaction occurs between a given element and a monoatomic ion depends on the relative ease with which the two species gain or lose electrons.
The table given here shows the activity series of the elements:
But magnesium metal can remove zinc ions from solution:
Mg(s) + Zn2+(aq) Zn(s) + Mg2+(aq)
The reaction goes nearly to completion.
"The activity series is a useful guide for predicting the products of metal displacement reactions. For example, placing a strip of zinc metal in a copper(II) sulfate solution will produce metallic copper and zinc sulfate, since zinc is above copper on the series. A strip of copper placed into a zinc sulfate solution will not produce an appreciable reaction, because copper is below zinc on the series and can't displace zinc ions from solution".