If metal X is lower then metal Y in the activity series, then what would happen?
In order to determine whether a single replacement (displacement) reaction will occur, we look at the activity series for metals. If metal X will replace (displace) metal Y, then metal X must be above metal Y in the activity series for metals. If metal X is lower than metal Y, there will be no reaction.
For example, copper (Cu) is higher on the reactivity series than silver (Ag). Therefore, copper will replace (displace) silver in a single replacement (displacement) reaction.
However, the reverse reaction will not take place because silver is below copper in the reactivity series.