Let us consider the following two reaction.
`2NaCl_((aq))+F_(2(g))to2NaF_((aq))+Cl_(2(g))` `1^(st)` reaction
`2NaF_((aq))+Cl_(2(g))to2NaCl_((aq))+F_(2(g))` `2^(nd)` reaction
Which reaction will not occur .Why?

Explanation

The first involves the displacement of chlorine from NaCl,by fluorine .In the second reaction chlorine displaces fluorine from NaF.Out of these two,the second reaction will not occur.Because fluorine is more active than chlorine and occupies the upper position in the periodic table.So, in displacement reactions, the activity of the elements and their relative position in the periodic table are the key factors to determine the feasibility of the reactions.More active elements readily displace less active elements from their aqueous solutions.