PLEASE HELP ME!! How is the Activity Series of Metals related to single replacement reactions

I just need to know this for chemistry homework :D and also could you possibly tell me why do all chemical reactions have to be balanced? Much appreciated :D

LOVE YOU TO PLUTO AND BACK :D <3

i'm pretty sure that in single replacement reactions, a metal has to be above the metal that it is replacing on the activity series. (not 100% sure though)

and chemical reactions need to be balanced b/c atoms are never gained or lost in a reaction according to the law of conservation of mass.