The electrochemical series can be used to check if a particular element has a greater activity than the other. Firstly, on the electrochemical series, it can be seen that lead is at a higher position compared to mercury. All things being equal, it will show a higher activity than mercury and hence can knock it off its salt.
Although it is the Overall standard electrode potential that tells us if a reaction is feasible or not, we can use the activity series to quickly discern if the reaction is possible or not. The overall standard electrode potential of both is positive which makes the reaction to be feasible and hence lead can display mercury off its salt
