Electrochemistry & Batteries

Whenever electrons move from one substance to another, you can make them do work on the way. Track electrons with oxidation numbers and balance half-equations, race metals against each other's ions, then build galvanic cells from pairs of metals and read their voltage, stack them into batteries and see how concentration shifts the voltage. Run the process backwards to plate copper and make aluminium, look inside a lithium-ion cell, and save a ship's hull from rust.