What substance is reduced in a lead storage battery?

In lead acid battery, chemical reactions are induced to do work on charge and produce a voltage difference between the two output terminals.

In a lead storage battery, the negative terminal is composed of lead electrode (Pb2+) while in the positive terminal it is composed of lead dioxide electrode (PbO2).

The chemical half reactions in each electrode are:

Negative Terminal: Pb2+  +  SO4(2-) ---> PbSO4 + 2e

            Lead electrode is oxidized to supply electrons.

Positive Terminal: PbO2 + H2SO4 + e ---> PbSO4

            Lead dioxide electrode is reduced by the addition of electrons.

Lead dioxide is reduced.

the independent variable, the one that is intentionally manipulated, is whether or not the car hits the barrier.

the dependent variable, the one that you measure the response in, is the distance that the washers travel.