Review section 4.4 of the textbook, especially the Oxidation numbers rules on page 137.
Chemistry is an experimental science where measurements are made. The theories and equations we see in the text are backed up by experimental results. The following example in the video below helped shape Electrochemistry.
20.1 Electrochemistry Observations
Electrochemistry is the study of chemical and electrical energy. The two processes we will discuss are the generation of an electrical current from a chemical (redox) reaction (this is a spontaneous process) and the use of an electric current to produce a chemical change (a nonspontaneous process).